Method of printing full-color frame image reproduced from full-color field image by interpolation

A color video thermal printing method for printing a full-color frame image on the basis of a color video signal of a full-color field image by interpolation. Interpolation for at least one of three primary colors is made as follows: sums A1, A2 and A3 of image data D1 and D4; D2 and D5; and D3 and D6 of respectively two pixels P1 and P4; P2 and P5; and P3 and P6 are calculated, assuming the pixels P1, P2 and P3 are aligned in this order in a line of the field image, and the pixels P4, P5 and P6 are aligned in this order in an adjacent line of the field image, wherein the pixels P2 and P5 are aligned in the vertical direction with a pixel Px to be interpolated, and are disposed on opposite sides of the pixel Px. If A1<A2<A3 or A1>A2>A3, differences S1=.vertline.D1-D6.vertline., S2=.vertline.D2-D5.vertline., and S3=.vertline.D3-D4.vertline. are calculated. If S1<S2<S3, an average value (D1+D6)/2 is used as interpolation data. If S1>S2>S3, an average value (D3+D4)/2 is used as interpolation data. In other cases, an average value (D2+D5)/2 is used as interpolation data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a color video printing method for printing 
a full-color image by using a frame signal reproduced from a field signal. 
2. Related Art 
A video image, that is, a pictorial image picked up by a video camera, is 
constituted of a plurality of lines. For example, video images picked up 
by a TV camera may be classified into frame image each constituted of 525 
lines and field images each constituted of 262.5 lines, half the number of 
the frame image lines. The frame image is superior in quality to the field 
image. However, many more memory locations are necessary for recording a 
frame signal, that is, a video signal for one frame. This is inconvenient 
particularly in an electronic still camera, because the number of 
recordable images decreases. 
Therefore, it is desirable to record an image in the form of a field 
signal, that is, a video signal for one field, and print the image in the 
form of a frame image. Because the field image is constituted of lines 
half the number of those of the frame image, it is necessary to double the 
lines by interpolation. Conventionally, an average of image data of a pair 
of pixels which are adjacent in the vertical direction in the field image 
is utilized as image data of a pixel to be interpolated between the pair 
of pixels, this conventional interpolation method will be hereinafter 
referred to as a single-directional interpolation. However, the 
conventional interpolation method has a problem when it is applied to such 
a thermal video printer as shown in FIG. 13, whose thermal head 2 has an 
array of heating elements 3a, 3b . . . aligned in a main scan direction 
(corresponding to the horizontal scanning direction of the video signal) 
and records images on a recording paper 5 in an area gradation method, 
such as disclosed in U.S. Pat. No. 5,232,294, while the recording paper 5 
is moved in a sub scanning direction perpendicular to the main scanning 
direction relative to the thermal head 2. 
In such a case, when the density of the image gradually changes, e.g., 
increases in the main scanning direction, jagged patterns as shown in FIG. 
13 would be provided because the width of an interpolated line 6 would 
increase stepwise in a similar way as adjacent lines 7 and 8. 
To avoid the above problem, an improved method has been known, for example, 
from JPA 63-187785, wherein three pairs of pixels are detected, which are 
disposed on opposite sides of a pixel to be interpolated and are aligned 
therewith in vertical and diagonal directions, respectively. Then, a 
difference of image data between the two pixels of each pair is 
determined, and is compared with one another. An average value of image 
data of one of the three pairs which has the smallest difference 
therebetween, is selected as the image data to be interpolated. This 
conventional interpolation method will be referred to as a conventional 
triple-directional interpolation. 
However, according to the above-described conventional triple-directional 
interpolation, if all the pixels of both diagonal pairs which are disposed 
on diagonal opposite sides of a pixel P to be interpolated, have image 
data "0", image data of value "0" would be interpolated as image data of 
the pixel P, through the pixels of the vertically disposed pair having 
image data of remarkably larger values, e.g., "100" and "95", as is shown 
in FIG. 14. Obviously, interpolation of image data "0" in the location of 
the pixel P is unsuitable in the case shown in FIG. 14. Rather, it is 
desired interpolate the average value "97.5" of the data "100" and "95" 
for the pixel P. Therefore, the conventional triple-directional 
interpolation still has a problem in this respect. 
SUMMARY OF THE INVENTION 
In view of the foregoing, a primary object of the present invention is to 
provide a color video printing method wherein a field signal is converted 
into a frame signal while preventing occurrence of the jagged patterns and 
solving the above described problem. 
Another object of the present invention is to provide a color video 
printing method which does not require a complicated construction for 
interpolation, nor require a long time. 
To achieve the above objects, the present invention makes a 
triple-directional interpolation for at least one of the three primary 
colors of a color video signal, wherein a direction of arrangement of the 
pair of pixels whose image data is used for interpolation and which are 
disposed on opposite sides of a pixel to be interpolated, is selected 
among from the three, that is, vertical and diagonal directions, according 
to the following steps: 
Sums A1, A2 and A3 of image data D1 and D4; D2 and D5; and D3 and D6 of 
respectively two pixels P1 and P4; P2 and P5; and P3 and P6 are 
calculated, assuming the pixels P1, P2 and P3 are aligned in this order in 
a main scanning line, and the pixels P4, P5 and P6 are aligned in this 
order in an adjacent main scanning line, and the pixels P2 and P5 are 
aligned in the vertical or sub scanning direction and are disposed on 
opposite sides of a pixel Px to be interpolated. If A1&lt;A2&lt;A3 or A1&gt;A2&gt;A3, 
differences S1, S2 and S3 of image data between two pixels of each of 
three pixel pairs which are aligned in the above-described three 
directions with the pixel Px, are calculated: 
S1=.vertline.D1-D6.vertline., S2=.vertline.D2-D5.vertline., 
S3=.vertline.D3-D4.vertline.. If S1&lt;S2&lt;S3, an average value of the image 
data D1 and D6 is used as image data of the pixel Px. If S1&gt;S2&gt;S3, an 
average value of the image data D3 and D4 is used as image data of the 
pixel Px. In other cases, an average value of the image data D2 and D5 is 
used as image data of the pixel Px. 
According to the present invention, concerning at least a remaining one of 
the three primary colors, the single-directional interpolation is 
executed, wherein image data to be interpolated is always calculated as an 
average value of image data of those pixels which are vertically aligned 
with the pixel to be interpolated and are disposed on opposite sides of 
the pixel to be interpolated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 showing the overall construction of an image data 
processing circuit of a thermal color video printer embodying a method of 
the present invention. A color video signal representing a field, which 
has been picked up, for example, by a video still camera and stored in a 
video floppy disc, and is read by a still video player, is inputted in a 
Y/C separation circuit 11 to be separated into a luminance signal Y and a 
chrominance signal C. Thereafter, the signals Y and C are converted into 
three primary color signals R, G and B through a decoder 12. If the video 
signal to be inputted into the video printer is already separated into the 
luminance signal Y and the chrominance signal C, the signals Y and C are 
inputted through a S-terminal connected to input terminals of the decoder 
12. 
The color signals R, G and B are respectively quantized through an A/D 
converter 13, to be converted into digital color data representing 64 
density grades of each pixel of the three color separated fields. The 
digital color data R, G and B is color-corrected in a color masking 
circuit 14. Thereafter, the digital color data R, G and B is sent to an 
interpolating section 15, wherein the three color data of the field are 
processed into three color data for a frame by interpolation. 
FIG. 2 illustrates 3.times.3 pixels of a frame, wherein pixels P1, P2, P3; 
and P4, P5, P6 are components of two adjacent lines or main scanning lines 
L1 and L2 of a field, and Px, Px-1 and Px+1 represent pixels of a line Lx 
interpolated between the two lines L1 and L2. According to one embodiment 
the present invention, image data Dx of the pixel Px to be interpolated, 
hereinafter referred to as interpolation data Dx, is determined on the 
basis of image data D1, D2, D3, D4, D5 and D6, e.g., luminance data or 
color data, of the pixels P1, P2, P3, P4, P5 and P6, concerning at least 
one of three primary colors of the video signal, according to a 
triple-directional interpolation sequence as shown in FIG. 3. 
Concerning the remaining colors of the video signal, interpolation data Dx 
is determined as an average value of the image data D2 and D5 of the 
pixels P2 and P5 which are disposed vertically above and below the pixel 
P, that is, the pixels which are aligned with the pixel Px in the vertical 
or sub scanning direction. 
Referring to FIG. 3, first, the image data D1, D2 and D3 of the pixels P1 
to P3 of the first line L1 is respectively added to the image data D4, D5 
and D6 of the pixels P4 to P6 of the second line L2, in pairs of two 
pixels P1 and P4; P2 and P5; and P3 and P6 which are aligned in the sub 
scanning direction: 
A1=D1+D4 
A2=D2+D5 
A3=D3+D6 
Then, the order of magnitude of the sums A1 to A3 is detected. In case 
A1&lt;A2&lt;A3 or A1&gt;A2&gt;A3, jagged patterns as illustrated in FIG. 13 might be 
produced if the interpolation data Dx is calculated as the average value 
of the image data D2 and D5 of the pair of pixels P2 and P5 which are 
disposed vertically above and below the pixel Px. Therefore, in such a 
case, differences of the image data between two of the pixels P1 to P6 are 
calculated: 
S1=.vertline.D1-D6.vertline. 
S2=.vertline.D2-D5.vertline. 
S3=.vertline.D3-D4.vertline. 
wherein, the pixels P1 and P6 of one pair are disposed diagonally above and 
below the pixel Px in a first diagonal direction, the pixels P2 and P5 of 
another pair are aligned with the pixel Px in the vertical or sub scanning 
direction, and the pixels P3 and P4 of a third pair are aligned with the 
pixel Px in a second diagonal direction. 
Thereafter, the order of magnitude of the differences S1, S2 and S3 is 
detected. In case S1&lt;S2&lt;S3, interpolation data Dx is calculated as an 
average value of the image data D1 and D6. In case S1&gt;S2&gt;S3, interpolation 
data Dx is calculated as an average value of the image data D3 and D4. 
Namely, interpolation data Dx is determined as an average value of one 
pair of image data whose difference is the smallest. 
In other cases, interpolation data Dx is calculated as the average value of 
the image data D2 and D5. 
FIG. 4 shows an embodiment of the interpolating section 15 of FIG. 1, 
wherein the digital color data R, G and B of the field is first stored in 
field memories 16a, 16c and 16e for each color. Then, the green field data 
G is read from the field memory 16c into an interpolating direction 
determination circuit 17 which determines according to the sequence shown 
in FIG. 3 one of the three interpolating directions, that is, the first 
and second diagonal directions and the vertical direction, in which a pair 
of pixels whose image data, that is, green color density data in this 
instance, is to be utilized for interpolation are aligned with the pixel 
Px to be interpolated. 
If, for instance, the first diagonal direction is selected by the 
interpolating direction determination circuit 17, a triple-directional 
interpolation circuit 18a calculates interpolation data Dx according to 
the first equation Dx=(D1+D6)/2. If the second diagonal direction is 
selected by the interpolating direction determination circuit 17, the 
triple-direction interpolation circuit 18a calculates interpolation data 
Dx according to the second equation Dx=(D3+D4)/2. If the vertical 
direction is selected by the interpolating direction determination circuit 
17, the triple-directional interpolation circuit 18a calculates 
interpolation data Dx according to the third equation Dx=(D2+D5)/2. 
Interpolation data for green thus calculated is written in a field memory 
16d. Thereafter, the green field data and the interpolation data for green 
are simultaneously read from the field memories 16c and 16d and are 
composed into a green frame signal G' for a green frame. The green frame 
signal G' is inputted in an image enhancer 21. 
In the image enhancer 21, the green frame signal G' is contoured through a 
spatial filter which may be represented, for example, by the following 
formula: 
##EQU1## 
wherein, (i, j) represent coordinates of 3.times.3 pixels, and values in 
matrix represent densities of the spatial filter called Laplacian. Through 
this spatial filter, the density of a pixel to be contoured is multiplied 
by "4", and the densities of four pixels adjacent to the pixel to be 
contoured are multiplied by "-1", and these five multiplied values are 
added to determine image enhancement components, which are added to the 
original densities. After being thus contoured, the green frame data G' is 
inputted in a gamma correction circuit 23. 
On the other hand, the red and blue field data R and B stored in the field 
memories 16a and 16e are respectively processed in single-directional 
interpolation circuits 19a and 19b, which always calculate interpolation 
data Dx according to the third equation Dx=(D2+D5)/2. Interpolation data 
for red and blue is written in field memories 16b and 16f, respectively. 
Then, the red field data R and the interpolation data for red are 
simultaneously read to be composed into a red frame signal R', which is 
contoured in the image enhancer 21 in the same way as for the green frame 
signal, and is sent to the gamma correction circuit 23. Also the blue 
field data B and the interpolation data for blue are simultaneously read 
to be composed into a blue frame signal B', which is contoured in the 
image enhancer 21 in the same way as for the green frame signal G', and is 
sent to the gamma correction circuit 23. 
The gamma correction circuit 23 accomplishes gamma correction for each of 
the three color frame signals R', G' and B' in accordance with color 
developing properties of a thermosensitive color recording paper 24, so 
that the three color frame signals R', G' and B' are converted into 
magenta, yellow and cyan frame signals MA, YE and CY, respectively. A 
thermal head 25 is sequentially driven in accordance with the magenta, 
yellow and cyan frame signals MA, YE and CY. As conventionally, the 
thermal head 25 has a plurality of heating elements aligned in the main 
scanning direction. 
The thermosensitive color recording paper 24 has a construction as shown in 
FIG. 5, wherein a cyan recording layer 32, a magenta recording layer 33 
and a yellow recording layer 34 are formed on a base material 31 made of 
an opaque coating paper or plastic film in this order from the inside, and 
a protection layer 35 is formed on the outermost surface of the color 
recording layers 32 to 34. The thermal head 25 records three color frames 
on the corresponding color recording layers 32 to 34 in the order from the 
outside, that is, from yellow to cyan in this instance. 
The cyan recording layer 32 contains an electron donating dye precursor and 
an electron accepting compound as main components, and is colored cyan 
when the cyan recording layer 32 is heated. The magenta recording layer 33 
contains a diazonium salt compound having a maximum absorption factor at a 
wave length of about 360 nm and a coupler which acts upon the diazonium 
salt compound and is developed in magenta when the magenta recording layer 
33 is heated. The magenta recording layer 33 loses its capacity of 
color-developing when exposed to electromagnetic or ultraviolet rays of 
about 360 nm, because the diazonium salt compound is photochemically 
decomposed by this range of rays. The yellow recording layer 34 contains a 
second diazonium salt compound having a maximum absorption factor at a 
wave length of about 420 nm and a coupler which acts upon the second 
diazonium salt compound and is colored in yellow when the yellow recording 
layer 34 is heated. The yellow recording layer 34 is also optically fixed, 
that is, loses its capacity of color-developing when exposed to near 
ultraviolet rays of about 420 nm. 
Accordingly, the thermal head 25 is first driven by the yellow frame signal 
YE to record the yellow frame on the yellow recording layer 34 of the 
thermosensitive recording paper 24, and then the recording paper 24 is 
exposed to the near ultraviolet rays of about 420 nm to optically fix the 
yellow recording layer 34. Next, the thermal head 25 is driven by the 
magenta frame signal MA to record the magenta frame on the magenta 
recording layer 33. After the magenta recording layer 33 is optically 
fixed, the cyan frame is recorded on the cyan recording layer 32 in 
correspondence with the cyan frame signal. 
FIGS. 6 and 7 illustrate other embodiments of the interpolating section 15, 
wherein the triple-directional interpolation is applied to two of the 
three primary colors of the video signal. In the embodiment shown in FIG. 
6, an interpolating direction determination circuit 17 determines one of 
the three interpolating directions on the basis of the green field data G 
according to the sequence shown in FIG. 3. The difference from the first 
embodiment is in that the triple-directional interpolation is applied not 
only to green data G but also to blue data B. Therefore, interpolation 
data for blue is calculated in a triple-directional interpolation circuit 
18b according to the interpolating direction determined by the 
interpolating direction determination circuit 17. Interpolation data for 
red is calculated according to the third equation for interpolation. That 
is, interpolation data for red is calculated as an average value of two 
pixels of the red field which are disposed vertically on opposite sides of 
a pixel to be interpolated. Other constructions and operations may be 
identical to those of the embodiment shown in FIG. 4. 
In the embodiment shown in FIG. 7, interpolation data for red as well as 
green is determined according to the triple-directional interpolation, 
while interpolation data for blue is calculated as an average value of 
pixels disposed in the vertical direction on opposite sides of the pixel 
to be interpolated. Therefore, a triple-directional interpolation circuit 
18c calculates the interpolation data for red in accordance with the 
interpolating direction determined depending on the green field data by an 
interpolating direction determination circuit 17. Other constructions and 
operations may be identical to those of the embodiment shown in FIG. 4. 
The triple-directional interpolation for the embodiments of the present 
invention prevents occurrence of the jagged patterns and reproduces a 
natural frame image based on a field image even when the field image has 
such an arrangement of pixels as shown in FIG. 14. Applying the 
triple-directional interpolation to merely one or two of the three primary 
colors simplifies the construction of the interpolating section and 
shortens the time required for interpolation, as compared with a case 
where the triple-directional interpolation is applied to all the three 
primary colors. 
FIG. 8 shows a thermal color video printer according to another embodiment 
of the present invention, wherein interpolation is made with respect to a 
luminance signal Y and color difference signals R-Y and B-Y of a color 
video signal representing a field. The video signal is separated into the 
luminance signal Y and a chrominance signal C through a Y/C separation 
circuit 11, and the chrominance signal C is converted into the color 
difference signals R-Y and B-Y through a decoder 41. An A/D converter 42 
quantizes these signals Y, R-Y and B-Y to convert them into digital data 
representing, for example, 64 density grades. The digital luminance data Y 
and the digital color difference data R-Y and B-Y is color-corrected in a 
color masking circuit 43. 
Then, all the luminance data Y and the color difference data R-Y and B-Y is 
inputted into both first and second interpolating sections 44 and 45. In 
the first interpolating section 44 as shown in FIG. 9, the field data Y, 
R-Y and B-Y are respectively stored in field memories 46a, 46c and 46e. 
Then, the luminance data Y is read from the field memory 46a into an 
interpolating direction determination circuit 47 which determines 
according to the sequence shown in FIG. 3 one of the three interpolating 
directions, that is, the first and second diagonal directions and the 
vertical direction. 
Depending on the determined interpolating direction, a triple-directional 
interpolation circuit 48 calculates luminance data for lines to be 
interpolated, according to one of the above-described first to third 
equations for interpolation. The calculated luminance data is written in a 
field memory 46b as interpolation data. Thereafter, the luminance data Y 
stored in the field memory 46a and the interpolation data stored in the 
field memory 46b is simultaneously read and composed into luminance data 
Y' for a frame. The luminance data Y' is inputted in a matrix circuit 49a. 
A field memory 46d is written with the same color difference data R-Y as 
stored in the field memory 46c, as color difference data of lines to be 
interpolated. The color difference data R-Y stored in the field memory 46c 
and the field memory 46d is simultaneously read and composed into color 
difference data R'-Y' for the frame. Also the color difference data R'-Y' 
is inputted in the matrix circuit 49a. A field memory 46f is written with 
the same color difference data B-Y as stored in the field memory 46e, as 
color difference data of lines to be interpolated. The color difference 
data B-Y stored in the field memory 46e and the field memory 46f is 
simultaneously read and composed into color difference data B'-Y' for the 
frame. The color difference data B'-Y' is also inputted in the matrix 
circuit 49a. 
In the second interpolating section 45 as shown in FIG. 10, the field data 
Y, R-Y and B-Y are respectively stored in field memories 51a, 51c and 51e. 
Then, the luminance data Y is read from the field memory 51a into a 
single-directional interpolation circuit 52 which calculates luminance 
data for lines to be interpolated, according to the third equation for 
interpolation: Dx=(D2+D5)/2. The calculated luminance data is written in a 
field memory 51b. Thereafter, the luminance data Y stored in the field 
memory 51a and the calculated luminance data stored the field memory 51b 
is simultaneously read and composed into luminance data Y" for the frame. 
The luminance data Y" is inputted in a matrix circuit 49b. 
Field memories 51d and 51f are written with the same color difference data 
R-Y and B-Y as stored in the field memories 51c and 51e, respectively, as 
interpolation data. The color difference data R-Y stored in the field 
memory 51c and the field memory 51d is simultaneously read and composed 
into color difference data R'-Y' for the frame. Also the color difference 
data R'-Y' is inputted in the matrix circuit 49b. The color difference 
data B-Y stored in the field memory 51e and the field memory 51f is 
simultaneously read and composed into color difference data B'-Y' for the 
frame. Also the color difference data B'-Y' is inputted in the matrix 
circuit 49b. 
The matrix circuit 49a converts the luminance data Y' and the color 
difference data R'-Y' and B'-Y' into a green frame signal G'. The green 
frame signal G' is converted into a magenta frame signal MA through a 
gamma-correction circuit 53a which accomplishes gamma-correction in 
accordance with color developing properties of a thermosensitive color 
recording paper 24. The matrix circuit 49b converts the luminance data Y" 
and the color difference data R'-Y' and B'-Y' into blue and red frame 
signals B' and R'. The blue signal B' is converted into a yellow frame 
signal YE through a gamma-correction circuit 53b. The red signal R' is 
converted into a cyan frame signal CY through a gamma-correction circuit 
53c. Also the gamma-correction circuits 53b and 53c accomplish 
gamma-correction in accordance with color developing properties of the 
thermosensitive color recording paper 24. Depending on these yellow, 
magenta and cyan signals YE, MA and CY, a thermal head 25 is driven in the 
same manner as described above. 
The relationships between the luminance signal Y and the color difference 
signals R-Y and B-Y, on one hand, and the three primary color signals R, G 
and B, on the other hand, are represented by the following known 
equations: 
Y=0.3R+0.59G+0.11B 
R-Y=0.7R-0.59G-0.11B 
B-Y=-0.3R-0.59G+0.89B 
FIG. 11 shows a modification of the embodiment shown in FIG. 8, wherein the 
luminance data Y' and the color difference data R'-Y' and B'-Y' for one 
frame, which is generated from the first interpolating section 44, is 
converted into green and blue frame signals through a matrix circuit 49c, 
while the luminance data Y" and the color difference data R'-Y' and B'-Y' 
for one frame, which is generated from the second interpolating section 
45, is converted into a red frame signal through a matrix circuit 49d. 
FIG. 12 shows another modification of the embodiment shown in FIG. 8, 
wherein the luminance data Y' and the color difference data R'-Y' and 
B'-Y' for one frame, which is generated from the first interpolating 
section 44, is converted into green and red frame signals through a matrix 
circuit 49e, while the luminance data Y" and the color difference data 
R'-Y' and B'-Y' for one frame, which is generated from the second 
interpolating section 45, is converted into a blue frame signal through a 
matrix circuit 49f. 
The three primary color frame signals G', B' and R' are converted into 
magenta, yellow and cyan frame signals MA, YE and CY through the matrix 
circuits 53a, 53b and 53c, respectively, in the same way as in the 
embodiment shown in FIG. 10. 
Although the above described embodiments only relate to a direct color 
thermal line printer using a thermosensitive color recording paper, the 
embodiments of the present invention may be applicable to another type of 
printer, such as a serial printer, a thermal transfer printer using an ink 
film, or an ink jet printer. 
It is also possible to accomplish the interpolation with respect to the 
yellow, magenta and cyan signals. 
Thus, the present invention should not be limited by the above described 
embodiments but, on the contrary, various modifications of the present 
invention can be effected without departing from the spirit and scope of 
the appended claims.