Video signal correction apparatus which detects leading and trailing edges to define boundaries between colors and corrects for bleeding

A video signal correction apparatus has a memory for storing a luminance value and color difference values for each pixel in an image, a horizontal color difference correction arrangement and a vertical color difference correction arrangement. The horizontal color difference correction arrangement includes a luminance change point detector for detecting leading and trailing edges, in which an area defined between the leading and trailing edges is a boundary between two colors. The colors in a region outside the boundary and adjacent to the edges are bleeding. The color difference values further outside the bleeding region are sampled to make reference color difference value. The reference color difference value is used for replacing the color difference values in the bleeding region to eliminate or reduce the bleeding color.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a video signal correction apparatus for 
improving image quality by compensating for color bleeding in video 
devices such as video cassette recorders, video cameras, full-color image 
printers, and full-color facsimile machines used to manipulate color 
pictures, and in data devices for storing image information on magnetic 
disks, optical disks, and other media. 
2. Description of the Prior Art 
With the development of full-color hard copy output technologies in recent 
years, it is quickly becoming possible to faithfully reproduce original 
images using such printing technologies as subliminal thermal transfer. 
This capability is, in turn, accelerating demand for hard-copy still image 
output. The development of high definition television (HDTV) and other 
high resolution video signal technologies, color reproduction capabilities 
are also now nearly comparable to traditional silver halide photographic 
capabilities. 
When outputting hard-copy still images of current television signal formats 
(e.g., NTSC signals), however, the image resolution is limited by the 
limited bandwidth of the video signal. The resolution of the color 
difference signal in particular is less than 1/3 that of the luminance 
signal. As a result, images in which there should be a color change due to 
a change in luminance are recorded using the color before the luminance 
change after the luminance changes because the change in the color 
difference value cannot keep pace with the change in luminance. Simply 
stated, images with much color bleeding are often recorded. 
A method of removing this color bleeding by adding edge information from 
the luminance signal to the color difference signal has been proposed in 
Japanese patent laid-open No. 2-213282. This reference fails to teach the 
color bleeding correction by the use of color difference signal. 
FIG. 26 is a block diagram of a conventional video signal correction 
apparatus for correcting color bleeding. Referring to FIG. 26, the 
luminance signal Y and color difference signals R-Y, B-Y output from the 
digital signal source 100 pass through the interface 101 and are stored in 
the image memory 102. The edge correction circuit 103 performs the digital 
processing for both vertical and horizontal aperture correction based on 
the luminance signal read from the image memory 102. 
After edge blurring is corrected by the edge correction circuit 103, the 
luminance signal is output to the digital signal processor (DSP) 106, and 
the edge correction component is output as edge information to the adder 
105. The noise component of the color difference signals R-Y, B-Y read 
from the image memory 102 is then reduced by the low-pass filter (LPF) 
104, the edge information from the edge correction component of the 
luminance signal is then added by adders 105A and 105B, and the result is 
output to the DSP 106. 
Saturation enhancement, gamma correction, and other image enhancements 
based on the input luminance signal and color difference signals R-Y, B-Y 
are applied by the DSP 106 before the digital signal is output to the 
printer 109. When one frame comprises two fields as in the NTSC signal 
format, a motion detection circuit 107 is provided between the interface 
101 and image memory 102 for removal of image motion between the first and 
second fields. A scan line correction circuit 108 for extracting the frame 
image from the fields is connected to the motion detection circuit 107. 
The printer 109 prints the image to the desired paper or other medium 
based on the output signal from the DSP 106. 
With this configuration, however, when the bandwidth of the color 
difference signal is significantly narrower than the luminance signal 
bandwidth as in the NTSC format, adding the edge correction component of 
the luminance signal to the color difference signals does not 
significantly correct color difference signal bleeding. In fact, image 
areas of differing colors occur at the edge correction area of the 
luminance signal, resulting in further deterioration of image quality. In 
addition, pseudo-edges (ghosting) can easily occur because the edge 
correction component is added even where the color difference signal value 
is constant. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a video signal 
correction apparatus capable of improving correction of color bleeding in 
images, and of suppressing image deterioration during such correction. 
To achieve this object, a video signal correction apparatus according to 
the present invention comprises a storage means for storing the luminance 
value and color difference values for each pixel in an image; a luminance 
change point detection means to which the luminance signal generated from 
the stored luminance value is input for detecting the position of a pixel 
at at least one edge of a variable luminance area wherein the luminance 
value in a predetermined direction in the image increases or decreases a 
predetermined amount or more; a stable luminance area detection means for 
detecting, from the position of the detected edge pixel to the outside of 
the variable luminance area, a stable luminance area wherein the change in 
luminance is less than a predetermined luminance value; and a color 
difference correction means for detecting the variable color difference 
area wherein the color difference value in a predetermined direction of 
the image increases or decreases a predetermined amount within the 
detected stable luminance area, and correcting at least some of the color 
difference values in the detected variable color difference area and 
stored in the storage means. 
By means of this configuration, the correction applied to color bleeding in 
an image can be improved, and image deterioration resulting during 
correction can be suppressed.

DESCRIPTION OF PREFERRED EMBODIMENTS 
First Embodiment 
The first embodiment of a video signal correction apparatus according to 
the invention is described below with reference to the accompanying 
figures, of which FIG. 1 is a block diagram. 
As shown in FIG. 1, the video signal correction apparatus comprises an 
image memory 1, leading edge detector 2, trailing edge detector 3, leading 
edge color difference corrector 4, trailing edge color difference 
corrector 5, and vertical color difference corrector 6. 
Before the description of the first embodiment proceeds, a color picture, 
such as a picture of white paper on the brown desk, formed on a television 
screen will be explained. On a television screen, the color and the 
brightness of an image are expressed by color difference signal (color 
difference value) and luminance signal (luminance value), respectively. A 
boundary between two colors, particularly extending in a direction 
intercepting the horizontal scanning direction, has a width of several 
pixel pitches. For example, as shown in FIG. 11a, a boundary between the 
white paper area and the brown desk area has a width of four pixel 
pitches. 
Generally, the luminance signal makes a distinct change at the boundary 
area as shown by waveform W1 in FIG. 11a. More specifically, in waveform 
W1, the luminance signal representing the brightness of the white area 
makes a distinct change at a leading edge n1 of the boundary, then 
gradually decreases to a trailing edge n2 of the boundary, and starts 
representing the brightness of the brown area after the trailing edge n2. 
On the other hand, the color difference signal makes dull change at an 
area outspreading the boundary area. As shown in FIG. 11a, waveform W2, 
the color difference signal varies in a region from a point k1 to k2 for 
the change from white to brown. This region is much wider than the 
boundary, resulting in color bleeding around the boundary. In FIG. 11a, 
waveforms W3 and W4 show primary and completely corrected waveform of the 
color differences signal according to the present invention, as will be 
explained in detail later. 
Referring back to FIG. 1, the image memory 1 stores the luminance value and 
the two color difference values (R-Y, B-Y) of each consecutive pixel in 
the horizontal scanning direction (horizontal direction) of the video 
signal. These values are stored in a sequence corresponding to the pixel 
position on a raster screen. According to the preferred embodiment, the 
image memory 1 has three pages of storing areas 8a, 8b and 8c, as shown in 
FIG. 2, and each page having a size enough to store one frame data. The 
three page storing areas 8a, 8b and 8c store, respectively, luminance 
values of one frame, first color difference values (R-Y) of one frame and 
second color difference values (B-Y) of one frame. The image memory 1 
further has two pages of storing areas 10a and 10b, and each page having a 
size enough to store one vertical line data, such as shown in FIG. 11b. 
The two page storing areas 10a and 10b store, respectively, first color 
difference values (R-Y) of one vertical line and second color difference 
values (B-Y) of one vertical line. 
The leading edge detector 2 detects the leading edge n1 of the boundary by 
the use of the luminance signal. At the leading edge n1, the luminance 
value starts to increase or decrease for two or more continuous pixels in 
the horizontal direction of the image. 
The trailing edge detector 3 detects the trailing edge n2 of the boundary 
by the use of the luminance signal. At the trailing edge, the continuous 
increase or decrease in the luminance value ends. 
The leading edge color difference corrector 4 detects a pixel position k1, 
from which an increase or decrease in the color difference value begins. 
The leading edge color difference corrector 4 also changes the color 
difference value for at least a portion between pixel position k1 and the 
leading edge n1. Pixel k1 is located before the leading edge n1, 
specifically between a pixel position n1-a, at which a little change in 
the luminance values is observed from the luminance value at the leading 
edge n1, and the leading edge n1. 
The trailing edge color difference corrector 5 detects a pixel position k2, 
at which the increase or decrease in the color difference value ends. The 
trailing edge color difference corrector 5 also changes the color 
difference value for at least a portion between the trailing edge n2 and 
pixel position k2. Pixel k2 is located between the trailing edge n2 and a 
pixel position n2+b at which a little change in the luminance value is 
observed from the luminance value at the trailing edge n2. 
The vertical color difference corrector 6 reads the luminance values and 
color difference values for at least three vertically aligned consecutive 
pixels from the image memory 1. The data stored in the image memory 1 at 
this time is the color difference values corrected by leading edge color 
difference corrector 4 and trailing edge color difference corrector 5, 
i.e., the color difference values corrected in the horizontal direction. 
One of the read three pixels, particular the center pixel of the three 
vertical consecutive pixels, is then defined as the target pixel, and the 
luminance value Yh of the target pixel is read. Then, it is tested whether 
or not the other two read pixels have a luminance value within a 
predetermined range of Yh.+-..delta.. The read pixels which satisfies this 
test, as well as the target pixel, are selected. Then, the first color 
difference values (R-Y) of the selected pixels are averaged. The obtained 
average is stored in the first page vertical line storing area 10a at a 
position corresponding to the target pixel as a corrected first color 
difference value (R-Y). Thereafter, in a similar manner, the second color 
difference values (B-Y) of the selected pixels are averaged. The obtained 
average is stored in the second page vertical line storing area 10b at a 
position corresponding to the target pixel as a corrected second color 
difference value (B-Y). 
The operation of the above embodiment is described below. 
While reading the image luminance values in the horizontal direction form 
the image memory 1, the leading edge detector 2 detects the leading edge 
n1, at which a big change in the luminance value starts, as shown in 
waveform W1 in FIG. 11a, and trailing edge detector 3 detects the trailing 
edge n2, at which the change in the luminance value ends. If the color 
difference value before the leading edge n1 or after the trailing edge n2 
also varies continuously, the leading edge color difference corrector 4 
and the trailing edge color difference corrector 5 read the color 
difference values of the pixels in the horizontal direction from the image 
memory 1, and change the color difference values of the pixels in the area 
in which the color difference values vary, so as to produce waveform W3 
and in turn waveform W4. 
Three luminance values, i.e., the luminance value of the target pixel and 
the pixels immediately above and below the target pixel, are then read 
from the image memory 1. The vertical color difference corrector 6 then 
compares the luminance value of the two vertically adjacent pixels with 
the luminance value Yh of the target pixel, selects the pixel(s) for which 
the absolute value of the luminance difference is within a predetermined 
range Yh.+-..delta., and corrects the color difference of the target pixel 
using the color difference value of the selected pixel(s). After thus 
correcting the color difference of vertically adjacent pixels, the new 
color difference values are written back to the image memory 1. 
The storage means is achieved by the image memory 1, the luminance change 
point detection means is achieved by the leading edge detector 2 and the 
trailing edge detector 3, and the stable luminance area detection means 
and color difference correction means are achieved by the leading edge 
color difference corrector 4 and trailing edge color difference corrector 
5. 
By applying this operation to all data within one image, a processed image 
in which color bleeding is reduced can be obtained. 
To simplify reading the luminance values and reading and rewriting the 
color difference values, the image memory 1 preferably enables the pixel 
values (luminance and color difference) of horizontally consecutive pixels 
to be read continuously. If the image memory 1 does not enable the pixel 
values of horizontally consecutive pixels to be read continuously, a line 
memory can be used to temporarily store horizontally consecutive pixel 
values while reading the pixel values from the image memory 1. 
The image memory 1 is described as formed by a page memory device capable 
of storing at least one full image, but can alternatively be plural line 
memory devices with sufficient capacity to temporarily store image signals 
until each process is completed. 
Detection of a continuous increase or decrease in the pixel luminance 
values by the leading edge detector 2 and trailing edge detector 3 is 
possible by detecting, for example, whether the difference in the 
luminance values of adjacent pixels is positive or negative and is the 
same for two or more consecutive pixels, by detecting whether the 
difference in the luminance values of adjacent pixels is positive or 
negative and is the same for two or more consecutive pixels and the total 
difference is greater than a predetermined value, or by detecting whether 
the difference between the luminance values of consecutive pixels exceeds 
a predetermined threshold value for two or more consecutive pixels. 
Because there is a significant overlap in signal noise in most image 
signals, particularly in the NTSC format, false detection of a continuous 
increase or decrease in the luminance value is often caused by a small 
noise component in the signals. Such false detection can easily lengthen 
the processing time. It is therefore preferable to evaluate the increase 
or decrease in the luminance value using three or more consecutive pixels 
rather than just two, or to slightly increase the setting used with the 
difference total or the threshold value to the difference values in the 
above methods. 
It is also possible for the number of continuously increasing or decreasing 
pixels to be proportional to the magnitude of the difference in the 
luminance value of adjacent pixels. In this case it is possible to reduce 
color bleeding even when there is a gradual change in luminance rather 
than limiting this reduction to sudden luminance changes as above. 
The pixel position n1-a at which a little change in luminance value is 
observed can be recognized in the leading edge color difference corrector 
4 in several ways, including: reading the luminance values backward from 
the leading edge n1 to detect a pixel n1-a which has the absolute value of 
the variation of the luminance value from the leading edge n1 being 
greater than a predetermined threshold value; reading the luminance values 
backward from the leading edge n1 to detect a pixel n1-a which has a 
luminance value just before exceeding a predetermined range Y1.+-..alpha. 
(Y1 is the luminance value at the leading edge n1), as shown in FIG. 11a 
and employed in the present embodiment, or exceeding a predetermined 
percentage Z% from the reference luminance Y1; or defining the pixel 
located at the trailing edge of a boundary immediately preceding the 
currently detected leading edge as the pixel n1-a. 
Either the leading edge detector 2 or the leading edge color difference 
corrector 4 can be used to determine the area from pixel n1-a of little 
change in luminance to the leading edge n1. 
Recognition of pixel position n2+b at which there is little change in 
luminance after the trailing edge n2 can be detected by the trailing edge 
color difference corrector 5 using a process similar to that of the 
leading edge color difference corrector 4. Specifically, the trailing edge 
color difference corrector 5 reads the luminance values forward from the 
trailing edge n2 to detect a pixel n2+b which has the absolute value of 
the variation of the luminance value from the trailing edge n2 being 
greater than a predetermined threshold value; reads the luminance values 
forward from the trailing edge n2 to detect a pixel n2+b which has a 
luminance value just before exceeding a predetermined range Y2.+-..beta. 
(Y2 is the luminance value at the trailing edge n2), as shown in FIG. 11a 
and employed in the present embodiment, or exceeding a predetermined 
percentage Z% from the reference luminance Y2; or defines the pixel 
located at the leading edge of a boundary immediately following the 
currently detected trailing edge as the pixel n2+b. 
Either the trailing edge detector 3 or the trailing edge color difference 
corrector 5 can be used to determine the area from the trailing edge n2 to 
pixel n2+b of little change in luminance. 
Detection of a continuous increase or decrease in the color difference 
values by the leading edge color difference corrector 4 and trailing edge 
color difference corrector 5 is possible: by detecting, for example, 
whether the difference in the color difference values of adjacent pixels 
is positive or negative and is the same for two or more consecutive 
pixels; by detecting whether the difference in the color difference values 
of adjacent pixels is positive or negative and is the same for two or more 
consecutive pixels and the total difference is greater than a 
predetermined value; or by detecting whether or not the difference between 
the color difference values of consecutive pixels exceeds a predetermined 
threshold value for two or more consecutive pixels. 
When the band width of the color difference signal is extremely narrow 
relative to the luminance signal, e.g., when detecting the increase or 
decrease in the color difference values in the NTSC signal with the above 
methods, it is preferable to evaluate the difference between pixels 
separated by two or more pixel pitches because of the small change in the 
color difference of adjacent pixels. Because noise is also contained in 
most image signals, and particularly in NTSC format signal, false 
detection of a continuous increase or decrease in the color difference is 
often caused by a small noise component in the signals. Such false 
detection can easily lengthen the processing time. To assure processing is 
completed in the shortest possible time, it is therefore preferable to 
evaluate the increase or decrease in the color difference value using 
three or more consecutive pixels rather than just two, or to slightly 
increase the setting used with the difference total or the threshold value 
to the difference values in the above methods. 
The change in the color difference value by the leading edge color 
difference corrector 4 uses the color difference value of a pixel between 
pixel n1-a and the leading edge where the change in the color difference 
value is small. For example, the color difference value may be changed to 
the typical color difference value C1 of each pixel from k1 to k1-c 
(.gtoreq.n1-a) where the change in the color difference value is small, or 
to the average of the typical color difference value C1 and each color 
difference value. 
The change in the color difference value by the trailing edge color 
difference corrector 5 uses the color difference value of a pixel between 
the trailing edge n2 and pixel n2+b where the change in the color 
difference value is small. For example, the color difference value may be 
changed to the typical color difference value C2 of each pixel from k2 to 
k2+d (.ltoreq.n2+b) where the change in the color difference value is 
small, or to the average of the typical color difference value C2 and each 
color difference value. 
Typical values C1 and C2 can be any average value used in a common 
frequency distribution, including the additive mean, multiplicative mean, 
harmonic mean, median value, or modal value of the color difference values 
of plural selected pixels. The color difference value is preferably not 
changed if only one pixel is selected. 
Because the vertical color difference corrector 6 must reference 
unprocessed pixel values from the image memory 1 for pixel processing, the 
vertical color difference corrector 6 also comprises a function for 
temporarily storing the processed pixel color difference values so that 
these values are not immediately written back to the image memory 1 from 
which the pixel values required for the next pixel processing operation 
are referenced. This can be achieved by providing a line memory with 
capacity to temporarily store processing results in the vertical 
direction, or plural buffers with sufficient capacity to store processing 
results in the vertical direction for as long as the corresponding pixel 
position is within the processing range. 
The vertical color difference corrector 6 references three vertically 
consecutive pixels (including the target pixel) for color difference 
correction, but any other odd number of pixels, e.g., five or seven, can 
be alternatively referenced. In this case, however, only those pixels of 
which the luminance value is close to the luminance value of the target 
pixel and are positioned near the target pixel are selected. 
Color difference correction by the vertical color difference corrector 6 
may be executed more than once for the same pixel. Color noise originally 
occurring in the horizontal direction and color bleeding occurring when 
color difference correction is not completed in areas of horizontal color 
bleeding can be made inconspicuous by the vertical color difference 
corrector 6 dispersing the color difference vertically. This effect is 
enhanced as the number of times the same pixel is processed in the 
vertical direction increases, and each pixel is therefore preferably 
processed three or more times. 
In the color difference correction process of the vertical color difference 
corrector 6, the pixels for which the difference in the luminance value to 
the luminance value of the target pixel and are vertically adjacent above 
and below the target pixel are selected, and the color difference values 
of the selected pixels are replaced by the average of the selected color 
difference values and the color difference value of the number h pixel. It 
is possible, however, to add or subtract Z% of the difference with the 
color difference value of the number h pixel, and substitute this value as 
the new number h color difference value. 
By applying this color difference correction operation to all image data, 
the vertical color difference corrector 6 can also reduce the slight color 
noise created by the color bleeding process to obtain a processed image 
with color image quality. 
FIG. 2 is a block diagram of the video signal correction apparatus shown in 
FIG. 1 achieved by a microcomputer. 
In this microcomputer, the luminance signal Y, and color difference signals 
R-Y, B-Y obtained for horizontally consecutive pixels in the input video 
signal are input to the external interface 7 for conversion to 8-bit 
digital signals, and the 8-bit digitized luminance signal Y and color 
difference signals R-Y, B-Y are stored together with the corrected color 
difference values in the image memory 8. 
The image memory 8 comprises a three-page storage capacity to store these 
various values. The luminance value and two color difference values 
defining one pixel in memory are obtained using the same 
vertical-horizontal address coordinates common to all three pages. In this 
embodiment, the luminance signal Y is stored to page 0, color difference 
signal R-Y is stored to page 1, and color difference signal B-Y is stored 
to page 2. 
The microcomputer 9 executes the functions of the leading edge detector 2, 
trailing edge detector 3, leading edge color difference corrector 4, 
trailing edge color difference corrector 5, and vertical color difference 
corrector 6. Note that this microcomputer 9 comprises a CPU, ROM, RAM, and 
an input/output interface. The vertical color difference corrector 6 is an 
image line memory used as a temporary buffer for storing one vertical line 
of corrected color difference values. 
The operation of the video signal correction apparatus thus comprised is 
described below with reference to the pixel matrix diagram in FIG. 3 and 
the flow charts in FIGS. 4-10. 
FIG. 3 shows the relationship between the pixel position and the pixel 
value storage position in the image memory 8. This addressing matrix 
matches the horizontal (X) axis of the image memory 8 with the horizontal 
scanning direction of the image signal, and the vertical (Y) axis of the 
image memory 8 with the sub-scanning direction. Each (x,y) value therefore 
identifies the position of one pixel in the image, and is the address of 
that pixel in the image memory 8. 
FIG. 4 is a flow chart of the overall process executed by the video signal 
correction apparatus. As shown in FIG. 4, the first step s1 corrects the 
color difference values in the horizontal direction of the image to reduce 
color bleeding, and the second step s2 applies the process in the vertical 
direction, thereby reducing noise and improving the image quality in terms 
of picture color. Thus, the first step s1 corrects the color difference 
values in the horizontal direction, and the second step s2 corrects the 
color difference values in the vertical direction. 
The horizontal color difference correction process (step s1) is described 
below with reference to the flow charts in FIGS. 5-9, and the vertical 
color difference correction process (step s2) is described below with 
reference to the flow chart in FIG. 10. 
The overall horizontal color difference correction process is shown in FIG. 
5, each step of which is described below. 
In step s3, the position of the pixel at which processing in the vertical 
direction starts is set in the image memory 8. 
In step s4, the position of the pixel at which processing in the horizontal 
direction starts is set in the image memory 8. Steps s3 and s4 are 
necessary when a window is set for designating an area in which the 
boundary lines should be corrected. 
In step s5, the page number is set (to page 0 in this embodiment) for 
reading the luminance values from the image memory 8. 
In step s6, the luminance values are sequentially read from the image 
memory 8, and the position of pixel n1 at the start of the current 
luminance value edge, and the area of minimal change in the luminance 
value before the leading edge pixel n1 are obtained. This step corresponds 
to the process executed by the leading edge detector 2 in FIG. 1. 
In step s7, it is determined whether a pixel which is three pixels ahead of 
the detected leading edge n1 (i.e., n1+3) is ahead of the end point of the 
pixels in the horizontal direction, i.e., ahead of end of the H-line. If 
this pixel exceeds the horizontal end point, the end point cannot be 
detected, and the color difference value is therefore not corrected. 
In step s8, luminance values are again read from the image memory 8 to find 
the end of the luminance value edge, and the pixel position n2 of this 
edge and the area of minimal change in the luminance value after the 
trailing edge pixel are obtained. This step corresponds to the process 
executed by the trailing edge detector 3 in FIG. 1. 
In step s9, the page number is changed in the image memory 8 to read the 
color difference values. The new page number is set to read either of the 
color difference value R-Y or B-Y. 
In step s10, the color difference value of the pixels in the area of 
minimal change in the luminance value obtained in step s6 is read from the 
same address in the image memory 8. Pixel k1 (the pixel from which an 
increase or decrease in the color difference value continuous to the 
leading edge begins) is detected, and the color difference values from the 
pixel k1 to the leading edge n1 are corrected and the corrected color 
difference values are written back to the image memory 8. This step 
corresponds to the process executed by the leading edge color difference 
corrector 4 in FIG. 1. 
In step s11, the color difference value of the pixels in the area of 
minimal change in the luminance value obtained in step s8 is read from the 
same address in the image memory 8. Pixel k2 (the pixel at which the 
continuous increase or decrease in the color difference value ends) is 
detected, and the color difference values from the trailing edge point to 
pixel k2 are corrected and written back to the image memory 8. This step 
corresponds to the process executed by the trailing edge color difference 
corrector 5 in FIG. 1. 
In step s12, it is determined whether to change the page in the image 
memory 8 to correct each of the color difference values R-Y, B-Y. 
In step s13, it is determined whether processing of one complete horizontal 
line in the image has been completed. If one line has not been completely 
processed, the process loops back to step s5 to correct the color 
difference values for all pixels in one horizontal line. 
In step s14, it is determined whether processing of one frame, i.e., to the 
bottom horizontal line in the image has been completed. It is therefore 
determined whether the video signal correction process has been completed 
for the entire image by evaluating the y coordinate. If the entire image 
has not been processed, the vertical address coordinate y is incremented 
by one (step s15), and the process loops back to step s4. 
Referring to FIG. 6, the process executed to obtain pixel n1 at the leading 
edge of the luminance value (i.e., step s6 in the FIG. 5 flow chart) is 
shown. This process starts at step s20 by initializing variables c and a. 
Variable c is used to count the number of pixels through which there is a 
continuous increase or decrease in the luminance value. Variable a is used 
to count the number of pixel positions from the leading edge pixel n1 to a 
pixel which is traced backwards from the pixel n1 and the one located 
furthermost from the pixel n1, but within a predetermined luminance range 
Y1.+-..alpha., wherein Y1 is the luminance value of the leading edge pixel 
n1 and .alpha. is a predetermined value. Variable a therefore represents 
the number of pixels assumed to have a luminance value within a 
predetermined tolerance range, i.e., a stable luminance value. In the 
example shown in FIG. 11a, the variable a is 8 (=12-4). 
In step s21, a difference .DELTA.L (FIGS. 11a) in the luminance value of 
the process pixel at coordinate x and the adjacent pixel at coordinate x+1 
is obtained. If the absolute value of the difference 
.vertline..DELTA.L.vertline. is greater than a predetermined threshold 
value Th1, step s22 defines these pixels as pixels of increasing or 
decreasing luminance. If the difference is less than the threshold value 
Th1, step s22 loops to step s23 where the process pixel is redefined to 
pixel coordinate x+1. This loop is repeated until step s22 returns a TRUE 
result, that is until x=12 in FIG. 11a is obtained. 
If the evaluated pixels are determined in step s22 to have an increasing or 
decreasing luminance value, the sign of the difference is stored to an 
incline flag in step s24. For example, if the difference .DELTA.L is plus, 
indicating the increase of the luminance value, the incline flag is set to 
"1", and if the difference .DELTA.L is minus, indicating the decrease of 
the luminance value, the incline flag is set to "0". The incline flag is 
used to determine whether the luminance value is continuously increasing 
or decreasing. 
In step s25 the process pixel position is shifted one pixel to the right by 
incrementing x, and the difference .DELTA.L in the luminance values of the 
process pixel at coordinate x and the adjacent pixel at coordinate x+1 is 
obtained in step s26. The same comparison of .vertline..DELTA.L.vertline. 
with the threshold value Th1 performed in step s22 is executed in step 
s27. If the difference .vertline..DELTA.L.vertline. is less than the 
threshold value Th1, the process loops through step s28, which resets the 
variable c counting the number of continuously increasing or decreasing 
pixels to zero. The position of the process pixel is then incremented 
again in step s23, and step s21 is repeated. This process of counting the 
number of continuously increasing or decreasing pixels is thus repeated 
until the absolute value of the luminance value difference 
.vertline..DELTA.L.vertline. is greater than or equal to the threshold 
value Th1 in step s27. 
When step s27 returns TRUE, the sign of the difference .DELTA.L is obtained 
in step s29, and this sign is used for obtaining a new incline flag which 
is compared with the previously stored incline flag. 
If the new and old incline flags are different, it is known that the 
continuous increase or decrease in the luminance value has stopped, and 
the incline flag is therefore overwritten in step s30 to the new incline 
flag as obtained in step s29. The variable c counting the number of 
continuously increasing or decreasing pixels is therefore reset to zero, 
and the process loops back to step s25. 
If the new and old incline flags are the same, however, it is known that 
the luminance value continues to increase or decrease. The variable c 
counting the number of continuously increasing or decreasing pixels is 
therefore incremented one in step s32. 
When the value of c is greater than or equal to two (step s33), it is known 
that the increase or decrease in the luminance value continues through at 
least three pixels. The counting loop is therefore ended. If c&lt;2, however, 
the loop continues from step s25. To optimize the processing time, this 
embodiment thus detects whether the increase or decrease in the luminance 
value continues for three pixels. 
When c.gtoreq.2, the leading edge n1 is set to pixel position (x-2). The 
next step is to determine for how many pixels adjacent to this pixel the 
luminance value is stable. 
The luminance value Y1 of this leading edge pixel n1 is therefore obtained 
at step s35. 
In step s36, the process pixel n is set to n1-1 which is one previous pixel 
to the leading edge pixel n1. 
In step s37, the luminance value Yn of the pixel n is read from memory. 
In step s38, it is determined whether the read luminance value Yn is within 
the range Y1.+-..alpha.. 
If the luminance value Yn is within the range Y1.+-..alpha., variable a, 
which counts the number of pixels satisfying the test in step s38, is 
incremented one, the position of the process pixel is shifted back one 
pixel (n=n-1), and the loop counting the number of stable pixels a is 
repeated from step s37. 
If, however, the luminance value is not within the range Y1.+-..alpha., 
this process is terminated, and the leading edge pixel n1 and the pixel 
(n1-a) at the edge of area determined to be of a stable luminance value 
are set. 
Referring to FIG. 7, the process executed to obtain pixel n2 at the end of 
the variable luminance value range (i.e., step s8 in the FIG. 5 flow 
chart) is shown. This process starts at step s50 by initializing variable 
b, which is used to count the number of pixel positions from the trailing 
edge pixel n2 to a pixel which is traced forward from the pixel n2 and the 
located furthermost from the pixel n2, but within a predetermined 
luminance range Y2.+-..beta., wherein Y2 is the luminance value of the 
trailing edge pixel n2 and .beta. is a predetermined value. Variable b 
therefore represents the number of pixels assumed to have a luminance 
value within a predetermined tolerance range, i.e., a stable luminance 
value. In the example shown in FIG. 11a, the variable b is 6 (=22-16). 
In step s51, a difference .DELTA.L in the luminance values between the 
process pixel x and the adjacent pixel x+1 is obtained, and the incline 
flag representing the sign of the difference is stored at step s52. This 
flag is used to determine whether the luminance value is continuously 
increasing or decreasing. 
In step s53, the process pixel position is shifted one pixel to the right, 
and the difference .DELTA.L in the luminance values of the process pixel 
at coordinate x and the adjacent pixel at coordinate x+1 is obtained in 
step s54. 
If the absolute value of the difference .vertline..DELTA.L.vertline. is 
greater than or equal to the threshold value Th1, the process pixel is 
defined as one of the pixels with a continuously increasing or decreasing 
luminance value. If the difference is less than the threshold value Th1, 
the continuous increase or decrease in the luminance value is determined 
to have ended. 
When step s55 returns TRUE, the new incline flag obtained in step s56 is 
compared with the old incline flag stored in step s52. If the new and old 
incline flags are identical, the process loops back to step s53 to further 
determine if the increase or decrease in luminance value continues at the 
next pixel. If the old and new flags are different, it is known that the 
continuous increase or decrease in the luminance value has stopped, and 
the position of the current process pixel x is defined as the trailing 
edge point n2. 
The following process then determines the number of pixels b continuous 
from the trailing edge point n2 for which the luminance value is stable, 
i.e., within the predetermined range. 
This loop starts by obtaining the luminance value Y2 of the trailing edge 
point n2. 
At step s59, the current process pixel x is shifted one pixel to the right, 
and the luminance value Yx of the new pixel x is obtained in step s60. 
It is determined whether this luminance value Yx is within the range 
Y2.+-..beta. in step s61. If the result is TRUE, the counter b is 
incremented in step s62, and the loop returns to step s59. 
If, however, this luminance value Yx is not within the range Y2.+-..beta., 
the loop ends, and the trailing edge point n2 and number of pixels b 
considered to be of a stable luminance value are set. 
Referring to FIG. 8a, the process executed to correct the color difference 
signal at the leading edge portion (i.e., step s10 in FIG. 5) is shown. 
This process starts at step s70 by initializing variable c, which is used 
to count the number of pixels through which there is a continuous increase 
or decrease in the color difference value. 
At step s71, the pixel position k of this process is set to the leading 
edge pixel n1. 
In step s72, the difference .DELTA.C in the color difference signals 
between the pixel k (leading edge pixel n1) and the pixel k-2 which is two 
pixel spaced from pixel k in the direction towards n1-a is obtained. Here, 
the color difference signals from two pixels which are not adjacent to 
each other but are spaced two pixels are obtained for calculating the 
difference .DELTA.C therebetween, because the difference .DELTA.C would be 
too small to evaluate if the signals from adjacent two pixels are 
calculated. Then, in step s73, an incline flag is stored according to the 
sign of the difference .DELTA.C. The incline flag is used to determine 
whether the color difference value is continuously increasing or 
decreasing. 
In step s74, the process pixel position k is shifted one pixel. In step 
s75, it is determined whether the pixel range is within an area of slight, 
smooth change in the luminance value (n1&gt;k-2&gt;n1-a). If pixel k-2 is 
determined not to be within an area of slight, smooth change in the 
luminance value, the color difference value should not be corrected, 
because it will not be possible to determine the true color difference 
value. 
If pixel k-2 is within this range (step s75), however, the difference 
.DELTA.C in the color difference values of the process pixel k and the 
offset pixel k-2 is obtained in step s76. A new incline flag based on the 
newly obtained difference .DELTA.C is obtained in step s77, and is 
compared with the stored old incline flag. 
If the old and new incline flags are the same, there is a continuous 
increase or decrease in the color difference value. The variable c is 
therefore incremented (step s78), and the counting loop continues again 
from step s74 to determine the total number of increasing or decreasing 
color difference value pixels. 
If the old and new incline flags are different, however, or if the 
difference .DELTA.C is zero, the increase or decrease has ended. It is 
then determined whether variable c, which tracks the number of 
continuously increasing or decreasing pixels, is greater than or equal to 
3. If it is, it is known that the increase or decrease in the color 
difference value continues through at least three pixels and the process 
moves to the next stage. If c&lt;3, the color difference value correction 
process ends. This embodiment thus detects whether the increase or 
decrease in the color difference value continues for three pixels. 
When c.gtoreq.3, in step s80, currently obtained k is set to the pixel 
position k1 representing the end of the area in which the color difference 
value should be changed, and the color difference values from pixel 
position k1 to the leading edge n1 are corrected in step s81. In this 
example the color difference values of the pixels from pixel position k1 
to the leading edge n1 are changed to the average color difference value 
from pixel k1 at the end of the continuous increase or decrease in the 
color difference value to the end point n1-a of the smooth luminance area. 
In other words, the additive mean is used as the typical value C1 of the 
color difference values from pixel k1 to the end point n1-a of the smooth 
luminance area. 
Referring to FIG. 8b, a detail of step S81, i.e., the step for correcting 
the color difference values from pixel position k1 to the leading edge n1 
is shown. 
This process starts at step s141 by initializing variables c and d. 
Variable c is used to count the number of pixels from pixel k1 to the 
leading edge pixel n1. Variable d is used for storing the color difference 
value to be replaced. 
In step s142, k1 obtained in step s80 is set as m. In step s143, the color 
difference value Cm at pixel m is added to variable d. In step s144, the 
processing pixel m is shifted to one adjacent pixel to the left, and in 
step s145 variable c is incremented. In step s146, it is detected whether 
the processing pixel m is within a range of stable luminance 
(n1&gt;m.gtoreq.n1-a), or not. If the processing pixel m is within the 
luminance stable range (from n1-a to n1), step s143 is carried out to 
accumulate the color difference value in variable d. If, however, the 
processing pixel m is outside the luminance stable range, the accumulated 
color difference value in variable d is divided by variable c in step s147 
to obtain an average color difference value among the pixels from n1-a to 
k1. In step s148, the color difference values of the pixels in the range 
from k1+1 to n1 are replaced with the average color difference value 
obtained in step s147. 
By the above operation, the color bleeding on one side (left side) of the 
boundary can be eliminated or reduced. Thus, the color difference signal 
W3 (FIG. 11a), particularly in the left hand side of the boundary is 
obtained. 
According to the embodiment shown in FIG. 8b, the average color difference 
value is obtained by taking an average of the color difference values of 
the pixels from n1-a to k1, but such an average color difference value can 
be obtained by taking an average of the color difference values of three 
consecutive pixels from pixel k1 counted towards pixel n1-a. In this case, 
the processing time can be shortened. 
Referring to FIG. 8c, a modification of the operation shown in FIG. 8b is 
shown. Instead of step s148 in FIG. 8b, steps s150 and s151 are provided 
in FIG. 8c. Other steps are the same. In step s150, the color difference 
values of the pixels in the range from k1+1 to n1-1 (instead of n1) are 
replaced with the average color difference value obtained in step s147, 
and in step s150, the color difference value of the pixel n1 is replaced 
with an average of two color difference values: one color difference value 
C.sub.n1+1 is obtained from pixel n1+1; and the other one is the average 
color difference value obtained in step s147. 
By this modification of FIG. 8c, the change of the color difference signal 
at the left side of the boundary can be smoothed to eliminate abrupt 
change of the color difference signal. 
Referring to FIG. 9a, the process executed to correct the color difference 
value at the trailing edge (i.e., step s11 in FIG. 5) is shown. 
This process starts at step s90 by initializing variable c, which is used 
to count the number of pixels through which there is a continuous increase 
or decrease in the color difference value. 
At step s91, the trailing edge pixel n2 as obtained in step s57 (FIG. 7) is 
set as a pixel position k for processing. 
In step s92, a difference .DELTA.C between the color difference value of 
the trailing edge pixel n2 and that of the pixel offset two pixels in the 
direction towards n2+b is obtained, and an incline flag is set and stored 
in step s93 according to the sign of the obtained difference .DELTA.C. The 
incline flag is used to determine whether the color difference value is 
continuously increasing or decreasing. 
In step s94, the process pixel position is shifted one pixel. In step s95, 
it is determined whether the pixel range is within an area of slight, 
smooth change in the luminance value (n2&lt;k+2&lt;n2+b). If the pixel k+2 is 
determined not to be within this range, the color difference value should 
not be corrected, because it will not be possible to determine the true 
color difference value. 
If pixel k+2 is within this range (step s95), however, the difference 
.DELTA.C in the color difference values of the process pixel k and the 
offset pixel k+2 is obtained in step s96. A new incline flag based on the 
newly obtained difference .DELTA.C is obtained in step s97, and is 
compared with the stored old incline flag. 
If the old and new incline flags are the same, there is a continuous 
increase or decrease in the color difference value. The variable c is 
therefore incremented (step s98), and the counting loop continues again 
from step s94 to determine the total number of increasing or decreasing 
color difference value pixels. 
If the old and new incline flags are different, however, or if the 
difference .DELTA.C is zero, the increase or decrease has ended. It is 
then determined whether variable c, which tracks the number of 
continuously increasing or decreasing pixels, is greater than or equal to 
3. This embodiment thus detects whether the increase or decrease in the 
color difference value continues for three pixels. 
When c.gtoreq.3, in step s100, currently obtained k is set to the pixel 
position k2 representing the end of the area in which the color difference 
value should be changed, and the color difference values from pixel 
position k2 to the trailing edge n2 are corrected in step s101. 
If c&lt;3, the process ends. 
After correcting the color difference values in the horizontal direction, 
the color difference values in the vertical direction are thus corrected 
as described above. 
Referring to FIG. 9b, a detail of step S101, i.e., the step for correcting 
the color difference values from pixel position k2 to the trailing edge n2 
is shown. 
This process starts at step s160 by initializing variables c and d. 
Variable c is used to count the number of pixels from pixel k2 to the 
trailing edge pixel n2. Variable d is used for storing the color 
difference value to be replaced. 
In step s161, k2 obtained in step s100 is set as m. In step s162, the color 
difference value Cm at pixel m is added to variable d. In step s163, the 
processing pixel m is shifted to one adjacent pixel to the left, and in 
step s164 variable c is incremented. In step s165, it is detected whether 
the processing pixel m is within a range of stable luminance 
(n2&lt;m.ltoreq.n2+b), or not. If the processing pixel m is within the 
luminance stable range (from n2 to n2+b), step s162 is carried out to 
accumulate the color difference value in variable d. If, however, the 
processing pixel m is outside the luminance stable range, the accumulated 
color difference value in variable d is divided by variable c in step s166 
to obtain an average color difference value among the pixels from k2 to 
n2+b. In step s167, the color difference values of the pixels in the range 
from n2 to k2-1 are replaced with the average color difference value 
obtained in step s166. 
By the above operation, the color bleeding on one side (right side) of the 
boundary can be eliminated or reduced. Thus, the color difference signal 
W3 (FIG. 11a), particularly in the right hand side of the boundary is 
obtained. 
According to the embodiment shown in FIG. 9b, the average color difference 
value is obtained by taking an average of the color difference values of 
the pixels from k2 to n2+b, but such an average color difference value can 
be obtained by taking an average of the color difference values of three 
consecutive pixels from pixel k2 counted towards pixel n2+b. In this case, 
the processing time can be shortened. 
Referring to FIG. 9c, a modification of the operation shown in FIG. 9b is 
shown. Instead of step s167 in FIG. 9b, steps s170 and s171 are provided 
in FIG. 9c. Other steps are the same. In step s170, the color difference 
values of the pixels in the range from n2+1 (instead of n2) to k2-1 are 
replaced with the average color difference value obtained in step s166, 
and in step s170, the color difference value of the pixel n2 is replaced 
with an average of two color difference values: one color difference value 
C.sub.n2-1 is obtained from pixel n2-1; and the other one is the average 
color difference value obtained in step s166. 
By this modification of FIG. 9c, the change of the color difference signal 
at the right side of the boundary can be smoothed to eliminate abrupt 
change of the color difference signal. 
Referring to FIG. 10, a flow chart of the process for correcting the color 
difference values in the vertical direction of the image is shown. This 
process corresponds to step s2 in FIG. 4. 
In this operation, three vertical pixels are read at a time to process the 
center one of the three vertical pixels, and is repeated plural times to 
cover all vertically aligned pixels. 
The first step s120 is to count the number of cycles this loop is executed 
in the vertical direction. 
The y coordinate from which this process starts is then set in step s121. 
This step is useful when a window is designated to indicate an area to be 
processed. 
In step s122, the page number is set to page 0 for reading the luminance 
values from the image memory 8, and in step s123 the luminance values for 
the three vertically aligned pixels y-1, y, and y+1 are fetched from the 
image memory 8. 
In step s124, it is determined whether the luminance values of pixels y-1 
and y+1 are within the range Yh.+-..delta. where Yh is the luminance value 
of the center pixel (target pixel) y. Of the pixels y-1, y and y+1. The 
pixel(s) having the luminance value which falls within this range 
Yh.+-..delta. are selected. Needless to say that the target pixel y is 
always selected. 
In step s125, the image memory 8 is changed to a page that stores the color 
difference values, and the line memory page is turned to page one 10a (see 
FIG. 2) for storing the color difference value R-Y. In step s126, an 
average of the color difference value, e.g., for R-Y, of the selected 
pixels in step s124 is calculated and is stored in line memory 10a at an 
area corresponding to the target pixel y. As shown in FIG. 11b, if all 
three pixels y-1, y and y+1 are selected, three color difference values 
Ca, Cb and Cc of R-Y are read from the image memory 8b and an average 
Cav=(Ca+Cb+Cc)/3 is calculated and is stored in line memory 10a at an area 
corresponding to pixel y. 
If neither pixel y-1 nor y+1 was selected in step s124, the color 
difference value of pixel y is not changed. In this case, the existing 
color difference value for pixel y is directly stored in the corresponding 
area in the image line memory 10a. 
In step s127, the memory page is checked. If step s126 has only been 
executed for only page one 10a for the color difference values R-Y, the 
page is turned to page two 10b (FIG. 2) for storing the color difference 
value B-Y, and the procedure loops back through step s126. 
If step s126 has been executed for both of the two color difference values 
R-Y and B-Y, it is determined in step s129 whether the color difference 
correction process has been completed for all pixels in the vertical 
direction, i.e., to the bottom H-line, actually the penultimate H-line 
because the pixel in the bottom H-line can not be the center pixel of the 
three vertical pixels. 
If the process has not been completed, the process pixel coordinate y is 
incremented (step s130), and the process loops back to step s122. 
If the process has been completed for all pixels in the vertical direction 
(determined in step s129), the corrected color difference values R-Y and 
B-Y stored in the image line memories 10a and 10b are written back to the 
corresponding pixel position addresses in the image memories 8b and 8c 
(step s131), respectively. 
In step s132 it is determined whether this correction loop has been 
executed the predetermined number of cycles. If not, the loop returns to 
step s120 to repeat the same operation to do the vertical correction 
operation for the required number of cycles. If it has, it is determined 
whether the process has been executed for the entire image. If not, the 
horizontal pixel position is incremented one, and the loop returns to step 
s120. By thus repeating this loop until the entire image is processed, 
color noise in the image can be reduced. 
In this embodiment the color difference values from the trailing edge point 
n2 to pixel k2 are changed to the average color difference value 
calculated from pixel k2 to pixel n2+b. In other words, the additive mean 
is used as the typical value C2 of the color difference values from pixel 
k2 to the pixel n2+b. 
By using a minimum luminance value of 0 and a maximum of 255, there will be 
256 gradations available. Tests showed that a threshold value Th1 range 
from 3 to 10 is preferable in this case. This is because while the object 
is to find those pixels for which there is a continuous increase or 
decrease in the luminance value, noise typically contained in the image 
makes it difficult to obtain pixels of continuously increasing or 
decreasing luminance. To compensate for this noise component, the present 
embodiment defines pixels of continuously increasing or decreasing 
luminance as those for which the luminance value difference is outside of 
a predetermined value range. 
Specifically, if threshold value Th1.ltoreq.2, it is difficult to identify 
areas in which there is a continuous increase or decrease because of 
noise. If threshold value Th1.gtoreq.11, pixels within a range of 
continuously increasing or decreasing luminance values are falsely 
determined to be pixels of a constant luminance value, and reduction of 
color bleeding is greatly reduced. The appropriate range for the threshold 
value Th1 is therefore from approximately 1% to 4% of the maximum 
luminance value. 
It has also been shown that the appropriate range for .alpha. and .beta. is 
from 3 to 10. This is because while the object is to find those pixels for 
which there is a continuous increase or decrease in the luminance value, 
noise typically contained in the image makes it difficult to obtain a 
constant luminance value. To compensate for this noise component, the 
present embodiment defines pixels of constant luminance as those for which 
the luminance value is within of a predetermined value range. 
Specifically, if .alpha. and .beta. are less than or equal to 2, it is 
difficult to identify pixels of constant luminance because of noise, and 
if greater than or equal to 11, unrelated areas in which the luminance 
value changes are falsely determined to be stable, leading to image 
quality deterioration. The appropriate range for .alpha. and .beta. is 
therefore from approximately 1% to 4% of the maximum luminance value. 
The values of .alpha. and .beta. can also be changed proportionally to the 
magnitude of the luminance values Y1 and Y2. For example, .alpha. and 
.beta. can be set to Z% (&lt;50%) of the luminance value, or several 
constants can be distributed proportionally to the luminance value. 
In the correction of the color difference values, the difference between 
the color difference values of the target pixel and the pixels offset two 
pixels therefrom is obtained. This is because it is relatively difficult 
to discern the change in the color difference values when there is a 
smooth change in the color difference signal or the difference of adjacent 
pixels is obtained. The color difference value of the pixel offset two 
pixels is therefore used to make it easier to determine the increase or 
decrease in pixel values. It is to be noted that an offset of three pixels 
can also be used. 
The vertical color difference corrector 6 in this embodiment executes the 
same process plural cycles for pixels consecutive in the vertical 
direction, but can also execute this process only once. 
In addition, three vertically consecutive pixels are also processed at one 
time in this embodiment, but five or any other odd number of pixels can 
also be processed at a time. In addition, an asymmetrical number of 
vertical pixels, including an even number of pixels, may also be 
referenced at the top and bottom image edges because it is difficult to 
reference the same number of pixels above and below the target pixel. 
As stated above, the minimum and maximum luminance values are set at 0 and 
255, respectively, for 256 gradations, at which level a .delta. value 
range of 3-10 is preferable. This is because while the object is to find 
those pixels for which there is a continuous increase or decrease in the 
luminance value, if .delta. is less than 3, there will often be no pixels 
selected due to noise, making it difficult to efficiently smooth color 
noise. 
In addition, if .delta. is greater than 10, smoothing will be applied with 
an area of a different color without finding the color edge even if the 
color changes at the luminance edge, and it will not be possible to reduce 
color noise. It is therefore preferable for the value of .delta. to be 
from approximately 1% to 4% of the maximum luminance value. The value of 
.delta. in this embodiment is therefore set to 8. 
The number of cycles the vertical process loop is executed is from 3 to 10. 
This is because the object of repeating this loop is to increase the 
overall number of pixels referenced, and to reduce color noise. However, 
if the loop is executed only one or two cycles, the number of referenced 
pixels is small and it is difficult to reduce color noise; if the loop is 
executed more than ten cycles, the number of referenced pixels becomes too 
large, and the overall color becomes light. The number of cycles in this 
embodiment is therefore set to 8. 
Slight color noise produced by the color bleeding process can be reduced 
and a processed image with good image quality can be obtained by the 
vertical color difference corrector 6 applying this color difference 
correction process to all of the image data. 
In addition, the processed color difference values R-Y and B-Y are 
temporarily stored in the image line memories 10a and 10b, respectively, 
and all processed color difference values in the vertical direction are 
batch written to the image memories 8b and 8c after one complete line is 
processed in this embodiment. It is also possible, however, to use a 
temporary buffer to transfer the data to the image memory 8a or 8b after 
processing the target pixel position is completed. The image line memories 
10a and 10b are used in this embodiment to simplify the description of 
this process. 
Color bleeding can be significantly suppressed and a noticeable improvement 
in image quality can be obtained compared with the original image by this 
color difference value process. However, if the correction processes of 
the leading edge color difference corrector 4 and the trailing edge color 
difference corrector 5 are executed as above, the edge values will change 
more sharply than the smooth change in the color difference signal. This 
can result in parts of the image contours where the color changes being 
emphasized with a slightly unnatural image resulting. 
Second Embodiment 
This is compensated for in the second embodiment of the invention by using 
linearly interpolating the color difference values between the leading 
edge and the trailing edge, and substituting these interpolated values for 
the color difference values replaced by the average of the color 
difference values in the first embodiment above. 
Referring to FIG. 25, a flow chart is shown for correcting the color 
difference values by a linear interpolation method applied in the boundary 
area, i.e., between pixels n1 and n2. The flow chart of FIG. 25 is carried 
out after the operation of FIG. 8b or 9b is completed. 
In step s191, the color difference value at the leading edge pixel n1 is 
read and stored as CD1, and in step s192, the color difference value at 
the trailing edge pixel n2 is read and stored as CD2. In step s193, a 
counter CC for counting the pixel position between the leading and 
trailing pixels n1 and n2 is reset to 1, so that the first occurring pixel 
from the leading edge pixel n1 will be processed first. In step s194, an 
interpolation color difference value is calculated by the following 
equation: 
EQU CD1+(CD2-CD1)/(n2-n1).multidot.CC 
and the color difference value at the pixel (n1+CC) is replaced with the 
calculated result. In step s195, the counter CC is examined whether the 
counted value by the counter has reached the trailing edge pixel n2 or 
not, i.e., CC.gtoreq.(n2-n1)-1, or not. If not, counter CC is incremented 
in step s196. If it has, the operation stops. By this operation, the 
interpolation between the leading and trailing edges n1 and n2 is effected 
to linearly change the color difference value from the leading edge pixel 
to the trailing edge pixel. 
Using this linear interpolation, the edge area and the color difference 
edge are matched and color bleeding is eliminated, there is no partial 
emphasis of color edges, and a natural image with good image quality can 
be obtained. 
Table 1 shows the numeric data obtained by the correction processes of the 
first and second embodiments above immediately after processing the 
luminance signals and color difference signals of the source image in the 
horizontal direction. These results were obtained from tests conducted 
using the values Th1=.alpha.=.beta.=5. Each row in the table shows the 
pixel position, column A shows the number of the pixel position, column B 
the luminance value of the source image, column C the color difference 
value of the source image, column D the color difference value of the 
image after processing by the first embodiment above, column E the color 
difference value of the image after processing by the second embodiment 
above, and column F each pixel detected during processing. Note that while 
there are two color difference signals (R-Y and B-Y), only the values for 
color difference signal B-Y are shown because the same process is used for 
both color difference signals. 
TABLE 1 
______________________________________ 
A B C D E F 
Pixel Bright- Color 1st 2nd Relative 
position 
ness diff. embod. embod. position 
______________________________________ 
1 167 -9 -9 -9 
2 164 -8 -8 -8 
3 172 -10 -10 -10 n1 - a 
4 176 -9 -9 -9 
5 176 -9 -9 -9 k1 
6 176 -11 -9 -9 
7 175 -13 -9 -9 
8 175 -14 -9 -9 
9 175 -15 -9 -9 
10 175 -16 -9 -9 
11 174 -17 -9 -9 
12 173 -19 -9 -9 n1 
(start) 
13 163 -21 -21 -18 
14 144 -25 -25 -26 
15 131 -27 -27 -35 
16 116 -32 -32 -43 n2 (end) 
17 117 -36 -43 -43 
18 119 -39 -43 -43 
19 120 -42 -43 -43 
20 119 -43 -43 -43 k2 
21 117 -44 -44 -44 
22 117 -42 -42 -42 
23 116 -43 -43 -43 n2 + b 
24 110 -43 -43 -43 
25 109 -42 -42 -42 
______________________________________ 
In FIG. 11a, waveforms W1-W4 are the process results shown in Table 1. The 
horizontal axis shows the pixel position, and the vertical axis shows the 
luminance value or color difference value of each pixel. Waveform W1 
represents the luminance value of the source image where n1 is the leading 
edge of the luminance value, n2 is the trailing edge, and n1-a and n2+b 
are the end points of the luminance smoothing area. Waveform W2 represents 
the color difference values for the pixel position of a given luminance 
value in the source image where k1 and k2 are the ends of the color 
bleeding area for the color difference value. Waveform W3 shows the 
results of the process executed by the first embodiment, and waveform W4 
shows the results of linear interpolation effected according to the second 
embodiment to the color difference values between the leading edge n1 and 
the trailing edge n2. 
Rounding of the color difference values is improved in the process results 
shown by waveform W3, and elimination of color bleeding is demonstrated. 
However, there is a discontinuous change in the color difference values 
between the leading edge n1 and the trailing edge n2, and the color change 
is slightly emphasized. 
By linearly connecting the color difference values between the leading and 
trailing edges using linear interpolation as shown by waveform W4, the 
change in color difference values is clarified, the edge of the color 
difference values is clarified, and the color difference values do not 
change discontinuously. As a result, color bleeding is eliminated, and at 
the same time, an image free of unnecessary emphasis of color change is 
obtained. 
FIGS. 12a-12d show graphs of the experimental results of color difference 
correction by the vertical color difference corrector 6 after correction 
of the color difference values of a given image in the horizontal 
direction according to the second embodiment. The y-axis shows the 
luminance value or the color difference value of each pixel, the x-axis 
shows the horizontal pixel position, and the z-axis shows the vertical 
pixel position. 
FIG. 12a shows the luminance values of the source image. The trailing edge 
point of any given luminance value is where the luminance values are 
uniform (or within a specifically limited value range). 
FIG. 12b shows the color difference values for the pixel position 
corresponding to FIG. 12a before the correction. Note that the color 
difference values vary smoothly across the range of uniform luminance 
values in FIG. 12a. 
FIG. 12c shows the results of the horizontal color difference correction 
process removing this color bleeding area. The area in the center is the 
area that could not be processed for color bleeding because the area of 
continuously varying color difference values is larger than the area of 
smooth luminance values. 
FIG. 12d shows the results of the vertical color difference correction 
process after horizontal color difference correction. Note that the area 
of color bleeding that remained after just horizontal color difference 
correction (FIG. 12c) is removed. This is because the luminance values in 
the vertical direction are evenly distributed through these pixels and the 
color difference signals are assumed to be the same, and color bleeding 
that could not be removed by averaging the color difference values is 
distributed through the surrounding pixels and smoothed. As a result, 
color bleeding is removed, vertical variation of the color difference 
values is improved, and image quality is improved. 
The leading edge detector 2 and trailing edge detector 3 form a luminance 
change point detection means, the leading edge color difference corrector 
4 and trailing edge color difference corrector 5 form a stable luminance 
area detection means, color difference value variation range detection 
means, and color difference value corrector, and the vertical color 
difference corrector 6 forms a pixel selection means and vertical color 
difference corrector. Note also that the luminance variation range is the 
range from pixels n1 to n2, the stable luminance ranges are the ranges 
from pixels n1 to n1-a and n2 to n2+b, and the color difference variation 
ranges are the ranges from pixels n1 to k1 and n2 to k2. 
As described above, the edge of the pixel range of continuously varying 
luminance values in the horizontal direction is first detected by the 
leading edge detector 2 and the trailing edge detector 3. The color 
difference values varying along the edge of this detected luminance value 
range are then changed to values relative to the change in the luminance 
values by the leading edge color difference corrector 4 and trailing edge 
color difference corrector 5. The vertical color difference corrector 6 
then extracts those pixels that are consecutive in the vertical direction 
of the image, and corrects the color difference values. Color difference 
signal rounding is thus corrected and color bleeding is greatly reduced by 
correcting the color difference signals in two stages, in the vertical and 
horizontal directions of the image, so that the change in the color 
difference values matches the change in the luminance values. 
In most images, the video signal is output in the horizontal scanning 
direction, there is significant rounding of the color difference signal 
due to the limited band width of the video signal, and color bleeding is 
observed because the change in the color difference values cannot keep 
pace with the change in the luminance values. In other words, there is a 
correlation between the area in which color changes and the area in which 
the luminance values change, and where there is a significant change in 
the luminance values there is a correspondingly noticeable change in the 
color difference signal, which appears to the observer as pronounced color 
bleeding. 
Therefore, the area (i.e., the edge) of significant change in the luminance 
value where color bleeding is most conspicuous is first detected in the 
horizontal direction, and color bleeding is removed from this area as much 
as possible. However, if correction is only applied in the horizontal 
direction, there will be vertically adjacent areas where the color 
bleeding was removed and was not removed. This results in an even more 
unnatural image and image quality may actually deteriorate. By also 
correcting the color difference values in the vertical direction using the 
luminance values, the color difference values of vertically adjacent areas 
are averaged (smoothed). Discontinuous variations in the color difference 
values can thus be prevented, color bleeding can be uniformly reduced, and 
image quality can be significantly improved throughout a more natural 
image. 
Third Embodiment 
The third embodiment of a video signal correction apparatus according to 
the present invention is described below with reference to FIG. 13. 
Like parts in FIGS. 1 and 13 are identified by like reference numerals. 
This third embodiment differs from the first embodiment in the addition of 
an access direction selector 11. The access direction selector 11 reads 
and rewrites the luminance values and color difference values from the 
image memory 1, and selects either the forward horizontal direction or 
reverse horizontal direction for reading and rewriting the data with 
respect to the image memory 1. 
In this embodiment, the image luminance values are read from the image 
memory 1 in one access direction, i.e., the forward horizontal direction 
or reverse horizontal direction, as determined by the access direction 
selector 11. The leading edge pixel n1 where a significant change in the 
luminance value begins is detected by the leading edge detector 2, and the 
trailing edge pixel n2 where this change in luminance values ends is 
detected by the trailing edge detector 3. 
While reading the color difference values of the image from the image 
memory 1 in the same access direction, the trailing edge color difference 
corrector 5 then changes the color difference values for all data in the 
image if the color difference values change continuously immediately after 
the trailing edge. 
The same operations are then repeated by the leading edge detector 2, 
trailing edge detector 3, and trailing edge color difference corrector 5 
while reading the data from the image memory 1 in the access direction 
opposite to that previously set by the access direction selector 11 to 
correct the color difference values near the area of significant change in 
the luminance value. 
The image luminance values are then read from the image memory 1 in the 
vertical direction of the image, and the color difference values of smooth 
luminance values are averaged and the results are output to the image 
memory 1 by the microcomputer 9. This averaging process is repeated plural 
times. 
It is therefore possible by this embodiment to obtain an image of improved 
image quality, reduced color noise, and no color bleeding as described in 
the first embodiment above. Because the effects of this embodiment are 
obtained by simply changing the processing direction, the required 
hardware configuration is simpler and the cost is lower when compared with 
those of the first embodiment shown in FIG. 1. 
A processed image of corrected color difference values is obtained in this 
embodiment by correcting the color difference values of all image pixels 
in either the forward or reverse horizontal direction, reversing the 
access direction, and then repeating the correction process. Note, 
however, that the same effect can be obtained by alternating the forward 
and reverse access directions and correction process on a line by line 
basis. 
The configuration of an image data processing apparatus according to the 
present embodiment achieved in a microcomputer is the same as that shown 
in FIG. 2. In this embodiment, however, the video signal input to the 
microcomputer 9 is processed by the leading edge detector 2, trailing edge 
detector 3, trailing edge color difference corrector 5, access direction 
selector 11, vertical color difference corrector 6. 
A detailed description of the operation of the leading edge detector 2, 
trailing edge detector 3, trailing edge color difference corrector 5, and 
vertical color difference corrector 6 is omitted below because the 
operation of these in this embodiment is identical to that of the first 
embodiment described above. 
FIG. 14 shows the relationship between the pixel position and the pixel 
value storage position in the image memory 8. This addressing matrix 
matches the horizontal (X) axis of the image memory 8 with the forward 
horizontal scanning direction of the image signal, defines the reverse 
access direction as the opposite of this forward horizontal direction, and 
matches the vertical (Y) axis of the image memory 8 with the sub-scanning 
direction. Each (x,y) value therefore identifies the position of one pixel 
in the image, and is the address of that pixel in the image memory 8. 
The overall horizontal color difference correction process executed by the 
video signal correction apparatus in FIG. 13 is shown in FIG. 15, each 
step of which is described below. Note that identical processes are 
identified by identical step numbers in FIGS. 5 and 15. In addition, the 
referenced horizontal direction is the access direction set by the access 
direction selector 11. 
According to the flow chart shown in FIG. 15, when compared with the flow 
chart of FIG. 5, steps s180 and s181 are added, and step s10 is deleted. 
This process starts (step s180) by the access direction selector 11 setting 
the access direction of the image memory 8. 
In step s3, the position of the pixel at which processing in the vertical 
direction starts is set in the image memory 8. 
In step s4, the position of the pixel at which processing in the horizontal 
direction starts is set in the image memory 8. 
In step s5, the page number is set (to page 0 in this embodiment) for 
reading the luminance values from the image memory 8. 
In step s6, the luminance values are sequentially read from the image 
memory 8, and the position of pixel n1 at the start of the current 
luminance value edge is obtained. This step corresponds to the process 
executed by the leading edge detector 2. 
In step s7, it is determined whether the pixel which is three pixels ahead 
of the detected leading edge is ahead of the end point of the pixels in 
the horizontal direction. If this pixel exceeds the horizontal end point, 
the end point cannot be detected, and the color difference value is 
therefore not corrected. 
In step s8, luminance values are again read from the image memory 8 to find 
the end of the luminance value edge as pixel n2. The luminance value of 
the pixel n2+b pixels ahead of the pixel trailing edge n2 (where b is 
greater than or equal to 2) is also evaluated to determine whether the 
continuous increase or decrease in luminance continues for at least two 
pixels beyond pixel n2. This range from n2 to n2+b is obtained as the area 
of minimal change in the luminance value. This step corresponds to the 
process executed by the trailing edge detector 3. 
In step s9, the page number is changed in the image memory 8 to read the 
color difference values. The new page number is set to read either of the 
color difference values R-Y or B-Y. In step s9, the color difference value 
of the pixels in the area of minimal change in the luminance value 
obtained in step s8 is read from the same address in the image memory 8. 
Pixel k2 (the pixel at which the continuous increase or decrease in the 
color difference value ends) is detected, and the color difference values 
from the trailing edge point n2 to pixel k2 are corrected and written back 
to the image memory 8. This process is executed by the trailing edge color 
difference corrector 5. 
In step s12, it is determined whether both of the color difference values 
R-Y and B-Y have been corrected. 
In step s13, it is determined whether processing of one complete horizontal 
line in the image has been completed. If one H line has not been 
completely processed, the process loops back to step s5 to correct the 
color difference values for all pixels in one horizontal line. 
By step s14 it is detected whether or not the processing of the bottom 
horizontal line in the image has been completed. It is therefore 
determined whether the video signal correction process has been completed 
for the entire image by evaluating the y coordinate. If the entire image 
has not been processed, the vertical address coordinate y is incremented 
by one (step s15), and the process loops back to step s4. 
When step s14 returns TRUE and the image correction process has been 
executed for the entire image in one horizontal scanning direction, it is 
then determined in step s181 whether the image correction process has been 
executed in both the forward and reverse directions. If processing in both 
horizontal directions has not been completed, step s181 loops back to step 
s180, the pixel value access direction is reversed, and the same process 
from step s3 to step s14 is repeated. 
Color bleeding can be significantly suppressed and a noticeable improvement 
in image quality can be obtained compared with the original image by this 
vertical color difference value process. However, because the edge changes 
more sharply than the smooth change in the color difference signal in this 
horizontal correction process, parts of the image contours where the color 
changes may be emphasized with a slightly unnatural image resulting. 
Fourth Embodiment 
In the fourth embodiment of the invention, the color difference value of 
the trailing edge n2 is therefore changed to an average between the n2-1 
pixel color difference value and the typical color difference value C2 of 
the pixels in the range of minimal change in the color difference value 
during color difference correction. It is thus possible to reduce emphasis 
of some color edges in image edge areas and color difference edge areas, 
and a good recorded image in which any unnatural components are further 
reduced can be obtained. 
Table 2 shows the numeric data obtained by the correction processes of the 
third and fourth embodiments above immediately after processing the 
luminance signals and color difference signals of the source image in the 
horizontal direction. Note that in the third and fourth embodiments, the 
pixel at the leading edge immediately following the currently detected 
trailing edge point is used as the n2+b pixel for the trailing edge 
detector 3 to detect the n1+b pixel at the end of the area of minimal 
change in the luminance value continuous from the trailing edge. 
These results were obtained from tests conducted using the value Th1=5 in 
the leading edge detection means and the trailing edge detection means. In 
Table 2, each row in the table shows the pixel position, column A shows 
the number of the pixel position, column B the luminance value of the 
source image, column C the color difference value of the source image, 
column D the color difference value after processing in the forward 
direction, column E the color difference value after processing in the 
opposite horizontal direction after processing in the forward horizontal 
direction, column F the color difference value after processing in the 
forward direction by the fifth embodiment, and column G the color 
difference value after processing in the opposite horizontal direction 
after processing in the forward horizontal direction by the fifth 
embodiment. Note that while there are two color difference signals (R-Y 
and B-Y), only the values for color difference signal B-Y are shown 
because the same process is used for both color difference signals. 
TABLE 2 
______________________________________ 
F G 
For- Re- 
A B C D E ward versed 
Pixel Bright- Color Forward 
Reversed 
direc- 
direc- 
position 
ness diff. direction 
direction 
tion tion 
______________________________________ 
1 160 -9 -9 -9 -9 -9 
2 166 -8 -8 -8 -8 -8 
3 173 -10 -10 -10 -10 -10 
4 176 -9 -9 -9 -9 -9 
5 176 -9 -9 -9 -9 -9 
6 176 -11 -11 -9 -11 -9 
7 175 -13 -13 -9 -13 -9 
8 175 -14 -14 -9 -14 -9 
9 175 -15 -15 -9 -15 -9 
10 175 -16 -16 -9 -16 -9 
11 174 -17 -17 -9 -17 -9 
12 173 -18 -18 -9 -18 -15 
13 163 -21 -21 -21 -21 -21 
14 144 -25 -25 -25 -25 -25 
15 131 -27 -27 -27 -27 -27 
16 116 -32 -43 -43 -35 -35 
17 117 -36 -43 -43 -43 -43 
18 119 -39 -43 -43 -43 -43 
19 120 -42 -43 -43 -43 -43 
20 119 -43 -43 -43 -43 -43 
21 117 -44 -44 -44 -44 -44 
22 117 -42 -42 -42 -42 -42 
23 116 -43 -43 -43 -43 -43 
24 109 -43 -43 -43 -43 -43 
25 103 -42 -42 -42 -42 -42 
______________________________________ 
FIG. 16 shows waveforms W1, W2, W5-W8 of the process results shown in Table 
2. In FIG. 16, the horizontal axis represents the pixel position, and the 
vertical axis represents the luminance value or color difference value of 
each pixel. Waveform W1 shows the luminance value of the source image, and 
waveform W2 shows the color difference values for the pixel position of a 
given luminance value in the source image without the correction of the 
present invention. Waveform W5 shows the results of the process executed 
by the third embodiment in the forward horizontal direction, and waveform 
W6 shows the results obtained by the third embodiment after reversing the 
access direction for the second loop through the flow chart shown in FIG. 
15. Rounding of the color difference values is improved on only one side 
of the forward horizontal direction, resulting in the same improvement 
obtained by the first embodiment. In other words, by correcting the color 
difference values of pixels in which there is a continuous increase or 
decrease in the color difference value from the edge, color bleeding is 
eliminated and picture quality is significantly improved. However, there 
is a discontinuous change in the color difference values at the edges, and 
the color change is slightly emphasized in the edge area. 
Waveforms W7 and W8 show the results obtained by the fourth embodiment, 
specifically, waveform W7 shows the result obtained by the color 
difference value correction in the forward H-direction processing, and 
waveform W8 shows the result obtained by the color difference value 
correction in the reverse H-direction processing. By interpolating the 
discontinuous color difference values near the edges using an average 
value, a discontinuous change in the color difference values is prevented. 
Color bleeding is eliminated by the fourth embodiment in a similar manner 
to that of the third embodiment shown in waveform W6, and a corrected 
image with natural image quality free of unnecessary emphasis of color 
changes is obtained. 
As in the first embodiment, color bleeding can also be removed from areas 
which are incompletely processed in the horizontal direction only by also 
correcting the color difference values in the vertical direction. 
Fifth Embodiment 
The fifth embodiment of a video signal correction apparatus according to 
the present invention is described below with reference to FIG. 17. Like 
parts in FIGS. 13 and 17 are identified by like reference numerals. 
Referring to FIG. 17, the image luminance values are read from the image 
memory 1 in one access direction, i.e., the forward horizontal direction 
or reverse horizontal direction, as determined by the access direction 
selector 11. The leading edge pixel n1 where a significant change in the 
luminance value begins is then detected by the leading edge detector 2. 
If there is a continuous change in the color difference values immediately 
before the leading edge, the leading edge color difference corrector 4 
then changes the color difference values while reading the color 
difference values of the image from the image memory 1 in the same access 
direction. This operation is repeated until the color difference values 
are corrected in the horizontal direction for all image data. 
The access direction selector 11 then reverses the access direction and the 
same correction sequence is repeated to correct the color difference 
values in the area around any significant change in luminance value. 
The image luminance values are then read from the image memory 1 in the 
vertical direction of the image, and the color difference values of smooth 
luminance values are averaged and the results are output to the image 
memory 1 by the vertical color difference corrector 6. This process is 
executed for all image data, resulting in a processed image with reduced 
color noise. 
This embodiment features the most simplified construction of the video 
signal correction apparatuses described heretofore, and can be achieved at 
the lowest cost. 
A processed image of corrected color difference values is obtained in this 
embodiment by correcting the color difference values of all image pixels 
in either the forward or reverse horizontal direction, reversing the 
access direction, and then repeating the correction process. Note, 
however, that the same effect can be obtained by alternating the forward 
and reverse access directions and correction process on a line by line 
basis. 
The configuration of an image data processing apparatus according to the 
present embodiment achieved in a microcomputer is the same as that shown 
in FIG. 2. In this embodiment, however, the video signal input to the 
microcomputer 9 is processed by the leading edge detector 2, leading edge 
color difference corrector 4, access direction selector 11, and vertical 
color difference corrector 6. 
The operation of this video signal processing apparatus is described below, 
omitting further description of the operation of the leading edge detector 
2, leading edge color difference corrector 4, and vertical color 
difference corrector 6 the operation of these is identical to the 
operation executed by the same components in the first and second 
embodiments. Note also that the relationship between pixel position and 
image memory 8 addresses is also as shown in FIG. 14. 
The overall horizontal color difference correction process executed by the 
video signal correction apparatus is shown in FIG. 18, each step of which 
is described below. Note that identical processes are identified by 
identical step numbers in FIGS. 15 and 18. In addition, the referenced 
horizontal direction is the access direction set by the access direction 
selector 11. 
According to the flow chart shown in FIG. 18, when compared with the flow 
chart of FIG. 15, step s10 is added and steps s8 and s11 is deleted. 
This process starts (step s180) by the access direction selector 11 setting 
the access direction of the image memory 8. 
In step s3, the position of the pixel at which processing in the vertical 
direction starts is set in the image memory 8. 
In step s4, the position of the pixel at which processing in the horizontal 
direction starts is set in the image memory 8. 
In step s5, the page number is set (to page 0 in this embodiment) for 
reading the luminance values from the image memory 8. 
In step s6, the luminance values are sequentially read from the image 
memory 8, and the position of pixel n1 at the start of the current 
luminance value edge is obtained. This step corresponds to the process 
executed by the leading edge detector 2. 
In step s7, it is determined whether the pixel which is three pixels ahead 
of the detected leading edge is ahead of the end point of the pixels in 
the horizontal direction. If this pixel exceeds the horizontal end point, 
it is not possible to determine whether the change in luminance beginning 
from the leading edge is a continuously increasing or decreasing luminance 
edge, and the color difference value is therefore not corrected. 
In step s9, the page number is changed in the image memory 8 to read the 
color difference values. The new page number is set to read either of the 
color difference values R-Y or B-Y. 
In step s10, the color difference value of the pixels in the area of 
minimal change in the luminance value obtained in step s6 is read from the 
same address in the image memory 8. Pixel k1 (the pixel from which an 
increase or decrease in the color difference value continuous to the 
leading edge begins) is detected, and the color difference values from 
pixel k1 to the leading edge are corrected and written back to the image 
memory 8. This step corresponds to the process executed by the leading 
edge color difference corrector 4. 
In step s12, it is determined whether to change the page in the image 
memory 8 to correct each of the color difference values R-Y, B-Y. 
In step s13, it is determined whether processing of one complete horizontal 
line in the image has been completed. If one line has not been completely 
processed, the process loops back to step s5 to correct the color 
difference values for all pixels in one horizontal line. 
By step s14 processing of one complete horizontal line in the image has 
been completed. It is therefore determined whether the video signal 
correction process has been completed for the entire image by evaluating 
the y coordinate. If the entire image has not been processed, the vertical 
address coordinate y is incremented by one (step s15), and the process 
loops back to step s4. 
When step s14 returns TRUE and the image correction process has been 
executed for the entire image in one horizontal scanning direction, it is 
then determined in step s181 whether the image correction process has been 
executed in both the forward and reverse directions. If processing in both 
horizontal directions has not been completed, step s181 loops back to step 
s180, the pixel value access direction is reversed, and the same process 
from step s3 to step s14 is repeated. 
Color bleeding can be significantly suppressed and a noticeable improvement 
in image quality can be obtained compared with the original image by this 
vertical color difference value process executed after the horizontal 
correction process. 
In the leading edge color difference corrector 4 of this embodiment, the 
color difference values from the leading edge detected by the leading edge 
detector 2 to the n1-a pixel in the area of little change in the luminance 
value are sequentially read, pixel k1 at the end of the continuous 
increase or decrease in the color difference values is detected, and the 
median color difference value from pixel k1 to pixel n1-a is output to the 
image memory 8 as the color difference value from pixel k1 to the leading 
edge. 
Sixth Embodiment 
In the sixth embodiment, as in the fifth embodiment described above, by 
changing the color difference value of the leading edge pixel n1 to the 
average of the n1+1 pixel value and the typical value C1 of the color 
difference values in the area of minimal color difference change, partial 
color edge emphasis can be reduced at the edge areas and at the color 
difference value edge areas, and a good recorded image in which any 
unnatural components are further reduced can be obtained. 
Color bleeding can also be removed from areas which are incompletely 
processed in the horizontal direction only by also correcting the color 
difference values in the vertical direction, and a good recorded image can 
obtained. 
Table 3 shows the numeric data obtained by the correction processes of the 
fifth and sixth embodiments above immediately after processing the 
luminance signals and color difference signals of the source image in the 
horizontal direction. These results were obtained from tests conducted 
using the value Th1=5 in the leading edge detector 2. In Table 3, each row 
in the table shows the pixel position, column A shows the number of the 
pixel position, column B the luminance value of the source image, column C 
the color difference value of the source image. Column D shows the color 
difference value after processing in the forward direction, and column E 
shows the color difference value after processing in the opposite 
horizontal direction after processing in the forward horizontal direction 
by the fifth embodiment. Column F shows the color difference value after 
processing in the forward direction by the fifth embodiment, and column G 
the color difference value after processing in the opposite horizontal 
direction after processing in the forward horizontal direction by the 
sixth embodiment. Note that while there are two color difference signals 
(R-Y and B-Y), only the values for color difference signal B-Y are shown 
because the same process is used for both color difference signals. 
TABLE 3 
______________________________________ 
F G 
For- Re- 
A B C D E ward versed 
Pixel Bright- Color Forward 
Reversed 
direc- 
direc- 
position 
ness diff. direction 
direction 
tion tion 
______________________________________ 
1 167 -9 -9 -9 -9 -9 
2 164 -8 -8 -8 -8 -8 
3 172 -10 -10 -10 -10 -10 
4 176 -9 -9 -9 -9 -9 
5 176 -9 -9 -9 -9 -9 
6 176 -11 -9 -9 -9 -9 
7 175 -13 -9 -9 -9 -9 
8 175 -14 -9 -9 -9 -9 
9 175 -15 -9 - 9 -9 -9 
10 175 -16 -9 -9 -9 -9 
11 174 -17 -9 -9 -9 -9 
12 173 -18 -9 -9 -15 -15 
13 163 -21 -21 -21 -21 -21 
14 144 -25 -25 -25 -25 -25 
15 131 -27 -27 -27 -27 -27 
16 116 -32 -32 -43 -32 -35 
17 117 -36 -36 -43 -36 -43 
18 119 -39 -39 -43 -39 -43 
19 120 -42 -42 -43 -42 -43 
20 119 -43 -43 -43 -43 -43 
21 117 -44 -44 -44 -44 -44 
22 117 -42 -42 -42 -42 -42 
23 116 -43 -43 -43 -43 -43 
24 110 -43 -43 -43 -43 -43 
25 109 - 42 -42 -42 -42 -42 
______________________________________ 
FIG. 19 show waveforms W1, W2, W9-W12 showing the process results shown in 
Table 3. The horizontal axis shows the pixel position, and the vertical 
axis shows the luminance value or color difference value of each pixel. 
Waveform W1 shows the luminance value of the source image, and waveform W2 
shows the color difference values for the pixel position of a given 
luminance value in the source image without any correction by the present 
invention. Waveforms W9 and W10 show the results of the process executed 
by the fifth embodiment, specifically, waveform W9 is obtained by the 
processing in the forward H-direction, and waveform W10 is obtained by the 
processing in the reverse H-direction after reversing the access direction 
for the second loop through the flow chart shown in FIG. 18. Rounding of 
the color difference values is improved on only one side of the forward 
horizontal direction in waveform W9, but color difference value rounding 
is improved by reversing the horizontal direction as shown by the waveform 
W10, resulting in the same improvement obtained by the first embodiment. 
In other words, by correcting the color difference values of pixels in 
which there is a continuous increase or decrease in the color difference 
value from the edge, color bleeding is eliminated and picture quality is 
significantly improved. 
Waveforms W11 and W12 show the results obtained by the sixth embodiment, 
specifically, waveform W11 shows the result obtained by the color 
difference value correction in the forward H-direction processing, and 
waveform W12 shows the result obtained by the color difference value 
correction in the reverse H-direction processing. As in the fourth 
embodiment, a corrected image with natural image quality free of 
unnecessary emphasis of color changes is obtained. 
Seventh Embodiment 
The seventh embodiment of a video signal correction apparatus according to 
the invention is constructed as shown in FIG. 1. 
In this embodiment, however, the vertical color difference corrector 6 
reads the luminance values and color difference values for at least three 
vertically consecutive pixels from the image memory 1, which stores the 
color difference values corrected for the horizontal direction. One of the 
read pixels is then defined as the target pixel, and those pixels for 
which the absolute value of the difference in luminance values is less 
than Y% (where 5.ltoreq.Y.ltoreq.30) of the luminance value Yh of the 
target pixel are selected from among the remaining pixels read from the 
image memory 1. The color difference value of the target pixel is then 
changed using the color difference value of the selected pixels. 
The operation of the above embodiment is described below, omitting further 
description of the horizontal processing operation, which is identical to 
that in the first embodiment above. 
Three luminance values, i.e., the luminance value of the one target pixel 
and the pixels immediately above and below, are read from the image memory 
1, to which the pixel values have been rewritten after completion of the 
horizontal process as described above. The vertical color difference 
corrector 6 then compares the luminance value of the two vertically 
adjacent pixels with the luminance value Yh of the target pixel, selects 
the pixel(s) for which the absolute value of the luminance difference is 
Y% (where 5.ltoreq.Y.ltoreq.30) of luminance value Yh, and corrects the 
color difference of the target pixel using the color difference value of 
the selected pixel(s). After thus correcting the color difference of 
vertically adjacent pixels, the new color difference values are written 
back to the image memory 1. 
By applying this operation to all data within one image, a processed image 
in which color bleeding and color noise are reduced can be obtained. 
FIG. 20 is the flow chart of the process correcting the color difference 
values in the vertical direction of the image in the seventh embodiment of 
the invention. The flow chart of FIG. 20 differs from that of FIG. 10 in 
that step s124 is replaced by step s140. This process is described in 
detail below. 
It is to be noted that this operation processes three vertical pixels at a 
time, and is repeated plural times to cover all vertically aligned pixels. 
In addition, steps identified with the same numbers as in FIG. 10 
represent the same process. 
The first step s120 is to count the number of cycles this loop is executed 
in the vertical direction. 
The y coordinate from which this process starts is then set in step s121. 
In step s122, the page number is set to page 0 for reading the luminance 
values from the image memory 8, and in step s123 the luminance values for 
the three vertically aligned pixels y-1, y, and y+1 are fetched from the 
image memory 8. 
In step s140, it is determined whether the absolute value of deviations of 
the luminance values of pixels y-1 and y+1 are within Y% of Yh where Yh is 
the luminance value of the target pixel y. The pixel(s) within this range 
are selected. 
In step s125, the image memory 8a (page) is changed to image memory 8b 
storing the color difference values R-Y, and the image line memory 10a 
(see FIG. 2) is selected to temporarily store the color difference values. 
In step s126, the average of the color difference value of pixel y and the 
color difference value(s) of the pixel(s) selected in step s124 is output 
to the image line memory 10a at a position corresponding to the color 
difference value of pixel y, as diagrammatically shown in FIG. 11b. If no 
pixel was selected in step s124, the color difference value of pixel y is 
not changed. In this case, the existing color difference value for pixel y 
remains held in the image line memory 10a. 
In step s127, the memory page is checked. If step s126 has only been 
executed for one of the two color difference values (R-Y, B-Y) the page is 
reset to page 2, so that the data from image memory 8c and line memory 10b 
are activated and the procedure loops back through step s126. 
If step s126 has been executed for both of the two color difference values, 
it is determined in step s129 whether the color difference correction 
process has been completed for all pixels in the vertical direction. 
If the process has not been completed, the process pixel coordinate y is 
incremented one (step s130), and the process loops back to step s122. 
If the process has been completed for all pixels in the vertical direction 
(determined in step s129), the corrected color difference values stored in 
the image line memories 10a and 10b are written back to the corresponding 
pixel position addresses in the image memories 8b and 8c (step s131), 
respectively. 
In step s132 it is determined whether this correction loop has been 
executed the predetermined number of times. If it has, it is determined 
whether the process has been executed for the entire image. If not, the 
horizontal pixel position is incremented one, and the loop returns to step 
s120. By thus repeating this loop until the entire image is processed, 
color noise in the image can be reduced. 
It is difficult to discern changes in color because of the normal range of 
human visual perception when the luminance value of the target pixel is 
low. In high luminance value areas, it is therefore necessary to use a 
wide difference range in determining the value of Y (%) used by the 
vertical color difference corrector 6 in order to select the correct 
pixels. 
The absolute value of the difference in the luminance values of the target 
pixel (luminance value Yh) and the adjacent pixels is therefore Y% of Yh 
where 5.ltoreq.Y.ltoreq.30 (%). 
This is because the correction process has minimal effect when Y% is less 
than 5% because the pixels vertically bracketing the target pixel are 
rarely selected due to noise. When the luminance value difference is 
large, the change in the color difference values is also great. If Y% is 
greater than 30%, color bleeding may be caused in the vertical direction. 
The value of Y% in this embodiment is therefore set to 10%, three 
vertically adjacent pixels (the target pixel and the one pixel each above 
and below) are processed at one time, and the average of the selected 
pixel and target pixel color difference values is output as the new 
(corrected) color difference value of the target pixel. 
The vertical color difference corrector 6 references three vertically 
consecutive pixels (including the target pixel) for color difference 
correction, but any other odd number of pixels, e.g., five or seven, can 
be alternatively referenced. In this case, however, only those pixels of 
which the luminance value is within Y% of the luminance value of the 
target pixel and are positioned continuously to the target pixel are 
selected. 
The vertical color difference corrector 6 in this embodiment executes the 
same process plural times for pixels consecutive in the vertical 
direction, but can also execute this process only once. 
The number of times the vertical process loop is executed is from 3 to 10. 
This is because the object of repeating this loop is to increase the 
overall number of pixels referenced, and to reduce color noise. However, 
if the loop is executed only one or two times, the number of referenced 
pixels is small and it is difficult to reduce color noise; if the loop is 
executed more than ten times, the number of referenced pixels becomes too 
large, and the overall color becomes light. The number of loops in this 
embodiment is therefore set to 4. 
Slight color noise produced by the color bleeding process can be reduced 
and a processed image with good image quality can be obtained by the 
vertical color difference corrector 6 applying this color difference 
correction process to all of the image data. 
In addition, the processed color difference values are stored in the image 
line memories 10a and 10b and all processed pixel values in the vertical 
direction are batch written to the image memories 8b and 8c, respectively, 
after one complete line is processed in this embodiment. It is also 
possible, however, to use a temporary buffer to transfer the data to the 
image memory 8b or 8c after processing the target pixel position is 
completed. The image line memories 10a and 10b are used in this embodiment 
to simplify the description of this process. 
FIGS. 21a-21d are graphs of the experimental results of color difference 
correction by the present embodiment. The y-axis shows the luminance value 
or the color difference value of each pixel, the x-axis shows the 
horizontal pixel position, and the z-axis shows the vertical pixel 
position. 
FIG. 21a shows the luminance values of the source image. The trailing edge 
point of any given luminance value is where the luminance values are 
uniform (or within a specifically limited value range). 
FIG. 21b shows the color difference values for the pixel position 
corresponding to FIG. 21a. Note that the color difference values vary 
smoothly across the range of uniform luminance values in FIG. 21a. This 
range of smoothly changing values is where color bleeding is found. 
FIG. 21c shows the results of the horizontal color difference correction 
process removing this color bleeding area. The area in the center is the 
area that could not be processed for color bleeding because the area of 
continuously varying color difference values is larger than the area of 
smooth luminance values. 
FIG. 21d shows the results of the vertical color difference correction 
process applied to the image after horizontal color difference correction 
(FIG. 21c). Note that the area of color bleeding that remained after just 
horizontal color difference correction (FIG. 21c) is removed. This is 
because the luminance values in the vertical direction are evenly 
distributed through these pixels and the color difference signals are 
assumed to be the same, and color bleeding that could not be removed by 
averaging the color difference values is distributed through the 
surrounding pixels and smoothed. As a result, color bleeding is removed, 
vertical variation of the color difference values is improved, and image 
quality is improved. 
Eighth Embodiment 
The eighth embodiment of a video signal correction apparatus according to 
the present invention is described below with reference to the 
accompanying figures. 
FIG. 22 is a block diagram of the video signal correction apparatus 
according to the eighth embodiment of the invention. Like parts in FIGS. 1 
and 22 are identified by like reference numerals. This eighth embodiment 
differs from the first in the addition of a low-pass filter 12 for 
low-pass filtering the luminance values read from the image memory 1. 
The video signal processing operation of this embodiment is described 
below. 
The image luminance values are read in the horizontal direction from the 
image memory 1, and are filtered by the low-pass filter 12 to remove noise 
and obtain luminance values with a reduced noise component. The leading 
edge detector 2 detects the leading edge, defined as pixel n1 at which a 
big change in the luminance value starts, and the trailing edge detector 3 
detects the trailing edge, defined as pixel n2 at which the change in the 
luminance value ends. 
If the color difference value before the leading edge or after the trailing 
edge also varies continuously, the leading edge color difference corrector 
4 and the trailing edge color difference corrector 5 read the color 
difference values of the pixels in the horizontal direction from the image 
memory 1, and change the color difference values of the pixels in each 
area. 
The vertical color difference corrector 6 then corrects the color 
difference values, which have already been corrected for reduced color 
bleeding in the horizontal direction, in the vertical direction to obtain 
a processed image in which the color bleeding is further reduced. 
In other words, while color bleeding can be reduced as described in the 
first embodiment above, edge detection and detection of smooth luminance 
value regions are made easier by adding a low-pass filter 12 to reduce the 
noise component of the luminance values before color difference 
correction. 
Note that while the low-pass filter 12 is used as a media filter in the 
above embodiment, the same effect can be obtained using a common smoothing 
filter, including an averaging filter that uses the average of plural 
pixels. 
Because the low-pass filter 12 must reference the unprocessed color 
difference values from the image memory 1 to process the pixels, the 
low-pass filter 12 also comprises a function for temporarily storing the 
processed pixel luminance values so that the processed values are not 
written back to the image memory 1. This can be achieved by providing a 
line memory with capacity to temporarily store processing results in the 
horizontal direction, or plural buffers with sufficient capacity to store 
processing results in the horizontal direction for as long as the 
corresponding pixel position is within the processing range. 
The configuration of an image data processing apparatus according to the 
present embodiment achieved in a microcomputer is as shown in FIG. 23, 
wherein like components in FIGS. 2 and 23 are identified with like 
reference numerals. In this embodiment, however, an image line memory 10c 
used as auxiliary memory for temporarily storing one horizontal line of 
the values output by the low-pass filter 12 is added. 
Other than accessing the luminance values from the image line memory 10c, 
operation of the leading edge detector 2, trailing edge detector 3, 
leading edge color difference corrector 4, trailing edge color difference 
corrector 5, and vertical color difference corrector 6 in the video signal 
processing apparatus described above is as described in the first 
embodiment. 
Table 4 shows the numeric data obtained as the result of processing the 
luminance signal and color difference signal of the source image to 
correct the color difference values in the horizontal direction of the 
luminance signal after filtering the luminance signal through the low-pass 
filter. These results were obtained from tests conducted using the values 
Th1=.alpha.=.beta.=5. Each row in the table shows the pixel position, 
column A shows the number of the pixel position, column B the luminance 
value of the source image, column C the color difference value of the 
source image, column D the color difference value corrected using the 
luminance value of the source image according to the first embodiment, 
column E the luminance value after low-pass filtering the luminance value 
of the source image according to the eight embodiment, and column F the 
color difference value of the image processed according to the first 
embodiment using the luminance value after low-pass filter processing. 
Note that while there are two color difference signals (R-Y and B-Y), only 
the values for color difference signal R-Y are shown because the same 
process is used for both color difference signals. 
TABLE 4 
______________________________________ 
A B C D E F 
Pixel Bright- Color 1st 8th Color 
position 
ness diff. embod. embod. diff. 
______________________________________ 
1 112 -26 -26 112 -26 
2 113 -24 -24 113 -24 
3 118 -22 -22 118 -22 
4 136 -20 -20 136 -20 
5 146 -15 -15 146 0 
6 146 -12 -12 146 0 
7 149 -10 -10 148 0 
8 148 -8 -8 148 0 
9 140 -5 -5 147 0 
10 147 -2 -2 147 0 
11 148 0 0 148 0 
12 150 0 0 148 0 
13 147 0 0 147 0 
14 145 0 0 145 0 
15 144 0 0 144 0 
16 140 0 0 140 0 
17 134 0 0 134 0 
______________________________________ 
FIGS. 24a-24d are graphs of the process results shown in Table 4. The 
horizontal axis shows the pixel position, and the vertical axis shows the 
luminance value or color difference value of each pixel. FIG. 24a shows 
the luminance value of the source image, FIG. 24b shows the color 
difference values in the source image, FIG. 24c shows the color difference 
values obtained by the first embodiment based on the luminance values of 
the source image, and FIG. 24d shows the luminance values obtained after 
low-pass filter processing the luminance values of the source image. In 
each of these figures, n1 is the leading edge of the luminance value, n2 
is the trailing edge, and n2+b is the end point of the uniform luminance 
area. FIG. 24e shows the corrected color difference values obtained by the 
eighth embodiment where k2 is the end of the color bleeding area for the 
color difference value. These results were obtained using the same process 
executed by the first embodiment based on the results shown in FIG. 24d. 
When the luminance values of the source image shown in Table 4 are used, 
the leading edge is detected at the second pixel and the trailing edge is 
detected at the fifth pixel. Because the luminance value of the trailing 
edge is 146, the condition for the uniform luminance value area is a range 
of consecutive pixels with a luminance value is greater than or equal to 
141 and less than or equal to 151 (146.+-.Th1). 
The range of smooth values satisfying the conditions from trailing edge n2 
is through the eighth pixel because the luminance value of the ninth pixel 
is 140. 
As shown in FIG. 24a, however, only one pixel (pixel 9) does not satisfy 
the conditions for inclusion in the uniform luminance value area. The 
luminance value of this pixel is therefore likely affected by noise. 
This conclusion is possible because the surrounding pixels are visually 
within the same region, and the luminance value of the pixels offset one 
pixel vertically from pixel 9 is different from the luminance value of 
pixel 9. The luminance value of pixel 9 can therefore be concluded to be 
due to noise. 
To correct the color difference value, the color difference values are in a 
continuously increasing state through the range of smooth pixel luminance 
values from the trailing edge to pixel 8, and pixels with minimal change 
in the color difference value cannot be detected. 
Therefore, as shown in FIG. 24c, the color difference values shown in FIG. 
24b are output without correcting the color difference values. There is, 
of course, no improvement in image quality. In other words, because noise 
is contained in the luminance value, the range of smooth luminance values 
cannot be correctly detected, and correction of the color difference 
values is therefore not possible. 
This is compensated for in the present embodiment by passing the luminance 
values through a low-pass filter 12 to reduce the noise component in the 
luminance values used for color difference value correction. 
The luminance values after passing the luminance values through the 
low-pass filter 12 are shown in Table 4 column E, and are graphed in FIG. 
24d. Noise has been removed from the luminance value of pixel 9, which is 
now within the range of uniform luminance values. Note that the pixel 
positions of the leading edge and trailing edge are the same as those 
obtained with the luminance values of the source image when evaluated 
using the luminance values after low-pass filter processing at this time. 
If the luminance value of the trailing edge is 146, the condition obtaining 
the uniform luminance value area is a range of consecutive pixels with a 
luminance value from 141 to 151 (146.+-.Th1). As a result, the smooth 
range of luminance values is from the trailing edge to pixel 15. In other 
words, the color difference values of the source image from the trailing 
edge to pixel 15 continue to increase from the trailing edge, but the 
pixel position k2 at which the simple increase in color difference value 
ends can be detected at pixel 11. The color difference values from the 
trailing edge to pixel k2 are then corrected using the average of the 
color difference values from pixel position k2 to pixel 15. The result of 
this operation is shown in Table 4 column F and FIG. 24e. 
If the luminance values are not filtered by the low-pass filter 12, 
however, there is no change in the color difference values and color 
bleeding remains uncorrected in the image. Reduced color bleeding and 
improved image quality can be obtained, however, by filtering the source 
image luminance values through the low-pass filter 12. By then executing 
the process of the vertical color difference corrector 6, image quality 
that is improved over that achieved by the first embodiment can be 
obtained. 
It is to be noted that while an "image memory" is used as the storage means 
in the above embodiments, the invention shall not be so limited and other 
storage devices, including magnetic disks, optical disks, and external 
RAM, can also be used. 
Furthermore, the video signal processed in the above embodiments comprises 
a luminance signal and color difference signals R-Y, B-Y, but the 
invention shall not be so limited. The video signal can be any signal 
expressing an image by a luminance signal and color difference signal. In 
the signal format, the broadcast color difference signals R-Y, B-Y are 
stored in the image data storage means, and in the NTSC signal format the 
broadcast color difference signals I and Q are converted to color 
difference signals R-Y, B-Y before storage to the image data storage 
means. In addition, if the video signal is an RGB signal, color difference 
correction by the present invention is applied after conversion to a 
luminance signal and color difference signals, and the corrected signal 
may then be re-converted to an RGB signal before final output. 
Both of the color difference signals R-Y, B-Y are described as being 
corrected in each of the above embodiments, but the invention shall not be 
so limited. For example, the invention can be comprised to correct only 
color difference signal R-Y to correct the pronounced color bleeding that 
appears in still images. Processing time can be halved in this case, and 
the processed image signal can be output faster. This method is therefore 
particularly effective when storing a continuous video signal input 
stream. 
The color difference values of all pixels within the range of variable 
color difference values are corrected in the above embodiments, but the 
invention may also be configured to correct only part of the pixel values. 
The color difference values at both the leading edge side and the trailing 
edge side are also corrected in the above embodiments, but the invention 
may also be configured to correct the color difference values on only one 
side. 
Finally, the low-pass filter is used to process all luminance values in the 
eighth embodiment above, but it is also possible to filter the luminance 
values only when the color difference values cannot be corrected in the 
uniform luminance value range, and then apply the color difference value 
correction process again after filtering noise from the luminance values. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.