Method and apparatus for coding an image and reducing bit generation by HVS (human visual sensitivity)

A method and apparatus for coding an image for reducing the amount of bit generation. Those are realized by eliminating a perceptual error from a predict error, which is generated in performing a differential pulse coded modulation (DPCM) procedure with respect to a movement between moving pictures, by using a look up table (LUT) defined for human perceptual color tolerance in the YUV chromatic domain.

BACKGROUND OF THE INVENTION 
The present invention relates to method and apparatus for coding an image, 
and more particularly, to method and apparatus for coding an image for 
reducing the amount of bit generation, by eliminating a perceptual error 
from a predict error, which is generated in performing a differential 
pulse coded modulation (DPCM) procedure with respect to a movement between 
moving pictures, by using a look up table (LUT) defined for human 
perceptual color tolerance in the YUV chromatic domain. 
As various video information services make advances in video media 
technology, it is inevitable that these video information services will 
increase the amount of video information generated. However, in the 
conventional analog video transmission technique, there is a problem in 
that huge amounts of video information can not be processed during a real 
time scale in accordance with various requests by users. Therefore, a 
digital video signal compression-coding technique (it is called shortly a 
coding technique) has been proposed to reduce only the amount of video 
information while keeping the picture quality, by converting analog video 
signals into digital video signals and then compressing them by using 
their redundancies. 
An MPEG2 coding technique (for a moving picture, not a still picture), for 
coding I, P, B pictures composed of a YUV chromatic coordinate system, is 
comprised of four steps, including: a DPCM procedure for defining a 
predict error in a macroblock unit; a discrete cosine transform (DCT) 
procedure for defining the magnitude of a predict error in a block unit 
for each frequency component; a rate control procedure for deciding a 
quantization step-size by using a cumulative buffer amount specifying the 
difference between a real and a target amount of bit generation; and, a 
quantization procedure for carrying out a quantization of a discrete 
cosine transformed predict error by using a quantization matrix and the 
quantization step-size defined in the rate control procedure. 
In this kind of MPEG2 coding system, the correlation between a forward and 
a current original frame is defined by a motion vector to reduce the 
redundancies of video signals. Hence, when a real current original frame 
is coded, only a predict error between a real original frame to be coded 
and a predict frame (defined in a DPCM procedure for estimating and 
compensating a motion vector) is coded and then transmitted to a decoder 
through a limited transmission line to be restored to an original frame. 
Here, a DPCM procedure to reduce the redundancies of video signals is as 
follows. In a DPCM procedure, as shown in FIG. 3, when macroblock C of a 
current original frame is coded, a macroblock having the smallest error 
within a search window is defined for a macroblock f or b located at the 
same position (x,y) of a forward or backward original frame, respectively, 
in other words, a macroblock having the most similar video signal 
characteristics to macroblock C, is defined by deciding a motion vector 
(fMVx, fMVy) or (bMVx, bMVy) having the minimum absolute error sum, dmc, 
by using the full or the half pel search methods adopting the absolute 
error sum, dmc, defined in FIG. 4. Then, using an estimated motion vector, 
a predict frame is defined, as shown in FIG. 4, by comparing square error, 
var1, and square error sum, vmc, of a forward reference frame, and average 
variance, var0, of a current original frame, to each other. The difference 
between a predict frame, defined in this method, and a current original 
frame, is called a predict error. However, there are problems in this 
method as follows: 
First, if a predict error (defined as "e" in macroblocks m1 and m2 (as 
shown in FIG. 3)) by a DPCM procedure is so small that the chromatic 
difference cannot be noticeable by a human sense, it can be negligible. 
However, in a MPEG2 coding technique, there is a problem of generating 
needless (redundant) bits because a predict error is processed by 
considering only the difference mathematically defined without considering 
the perceptual difference between a predict frame and an original frame. 
Second, a problem occurs in case of inequalities, 0&lt;D&lt;CT, 0&lt;CT&lt;2QM, and 
D&gt;QM/2, when a predict error D, with negligible perceptual difference 
within a perceptual color tolerance (CT), is quantized with a quantization 
step-size QM in a quantization procedure for compressing video signals. It 
is described in detail by using the following equation (1), that is, an 
MPEG2 quantization relationship: 
EQU D'=Round(D/QM+0.499999)QE=D-D'*QM (1) 
In the case of carrying out a quantization procedure for a predict error D 
satisfying inequalities, 0&lt;D&lt;CT, 0&lt;CT&lt;2QM, and D&lt;QM/2, by using equation 
(1), the deterioration of picture quality for a reference frame by a 
quantization error does not occur because a quantization error satisfies 
the relationship, (QE=D)&lt;CT. However, in case of carrying out a 
quantization procedure for a predict error D satisfying inequalities, 
0&lt;D&lt;CT, 0&lt;CT&lt;2QM, and D&gt;QM/2, the deterioration of picture quality for a 
reference frame by a quantization error does occur because of the 
relationship of a quantization error, QE=D-QM. Therefore, there is a 
problem of deteriorating the picture quality of a reference frame for a 
predict error even without having a real perceptual chromatic difference. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method and apparatus for 
coding an image for reducing the amount of predict error generated in a 
DPCM procedure by using a perceptual color tolerance, to solve the problem 
described above. 
To obtain the object described above, there is provided an image coding 
method according to the present invention, comprising of the steps: 
(a) generating a predict frame from current original and forward reference 
frames by a differential pulse coded modulation (DPCM); 
(b) deciding a perceptual color tolerance of a chromatic component for each 
pixel of the current t0 original frame; 
(c) generating a predict error between the predict frame and the current 
original frame; 
(d) comparing the predict error output from the step (c) and the perceptual 
color tolerance output from the step (b); 
(e) generating a perceptual predict error recomposed by the predict error 
having a perceptual chromatic difference according to a result compared in 
step (d); and, 
(f) coding the current original frame by using the predict frame and the 
perceptual predict error output from step (e) and then composing a 
restored frame. 
Further, to obtain the object described above, there is provided an image 
coding apparatus according to the present invention, comprising: 
an image input portion having a current original, a forward original, and a 
forward reference frame, as inputs, and for generating a predict frame 
from the current original and forward reference frames by a differential 
pulse coded modulation (DPCM); 
a perceptual color tolerance decision portion for deciding a perceptual 
color tolerance of a chromatic component for each pixel of the current 
original frame input from the image input portion; 
a predict error generator for generating a predict error between the 
predict frame input from the image input portion and the current original 
frame; 
a perceptual predict error comparator for comparing the predict error 
output from the predict error generator and the perceptual color tolerance 
output from the perceptual color tolerance decision portion; 
a perceptual predict error generator for generating a perceptual predict 
error recomposed by the predict error having a perceptual chromatic 
difference according to a result compared in the perceptual predict error 
comparator; and, 
a decoder for coding the current original frame by using the predict frame 
and the perceptual predict error output from the perceptual predict error 
generator and then composing a restored frame.

DETAILED DESCRIPTION OF THE INVENTION 
A block diagram of an image coding apparatus according to the present 
invention is illustrated in FIG. 1 which comprises an image input portion 
11 having a current original, forward original, and forward reference 
frame as inputs, and for generating a predict frame from the current and 
forward reference frames by differential pulse coded modulation (DPCM). 
The image coding apparatus also includes a perceptual color tolerance 
decision portion 12 for deciding a perceptual color tolerance of each 
pixel of the current original frame input from image input portion 11. A 
predict error generator 13 also is disclosed for generating a predict 
error between the current original frame and the predict frame input from 
image input portion 11. Perceptual predict error comparator 14 compares 
the predict error output from predict error generator 13 with the 
perceptual color tolerance output of each pixel output from perceptual 
color tolerance decision portion 12. Perceptual predict error generator 15 
generates a perceptual predict error which is recomposed by the predict 
error having a perceptual chromatic difference according to the result 
compared in perceptual predict error comparator 14. A decoder 16 is also 
disclosed for coding the current original frame by using the predict frame 
and the perceptual color tolerance output from perceptual predict error 
generator 15 and then composing a restored frame. 
The present invention relates to an MPEG coding system for improving the 
picture quality of a restored frame, as well as for reducing the 
compressed bit generation amount, based on the fact that if a predict 
error, generated in the procedure of DPCM, is negligible to human sensory 
perception, it does not affect the picture quality of a restored frame. 
Image input portion 11, as shown in FIG. 2, is composed of motion vector 
estimation portion 21 and motion vector compensation portion 22. Image 
input portion 11 estimates, as shown in FIGS. 3 and 4, a motion vector to 
be used for extracting the correlation between frames, composed on a YUV 
chromatic coordinate system in a time frequency region, by using the 
forward and current original frame. The image input portion 11 also 
defines, as shown in FIG. 4, a predict frame by applying an estimated 
motion vector to the current original and forward reference frame by DPCM. 
Therefore, image input portion 11 outputs the current original frame, the 
forward original frame and predict frame with respect to the current 
original frame and the forward original frame, as shown in FIG. 2. 
Perceptual color tolerance decision portion 12 is used in the image coding 
apparatus of the present invention for deciding a perceptual color 
tolerance for an arbitrary pixel of the current original frame input from 
image input portion 11 with respect to Y, U, V color components, 
respectively. As illustrated in FIG. 6; since it is possible to keep a 
more precise linearity by defining a perceptual color tolerance in a 
chromatic coordinate system in which human perceptual chromatic 
differences are uniformly distributed, the YUV nonuniform chromatic space 
for defining chromatic information of a color video is transformed into 
the CIE L*a*b* uniform chromatic space in first transformer 61. In second 
transformer 62, a perceptual color tolerance is decided for L, a, b, 
chromatic components by using a perceptual chromatic difference equation, 
recommended by the Color Measurement Committee (CMC). In this transformed 
CIE L*a*b* chromatic coordinate system, the decided CIE L*a*b*perceptual 
color tolerance is transformed into a perceptual color tolerance for Y, U, 
and V chromatic components, and then the YUV perceptual color tolerance 
for a pixel is decided. A perceptual color tolerance decided by this kind 
of method for the YUV chromatic coordinate system is processed in a real 
time scale by using the perceptual chromatic difference look-up table 
(LUT) shown in FIG. 7. 
The perceptual chromatic difference LUT shown in FIG. 7 is composed and 
indexed by the following procedures. As the first step, each chromatic 
axis, Y, U and V of the YUV chromatic coordinate system, is divided into 
eight uniform spaces to have a total of 512 representative color 
divisions. As the second step, perceptual color tolerances, pY, mY, pU, 
mU, pV, mV, of each chromatic component, Y, U, and V for the 512 
representative colors, are defined. As the third step, perceptual color 
tolerances for Y, U and V chromatic components are decided by indexing YUV 
chromatic coordinates of a current original frame input from image input 
portion 11, using equation (2): 
EQU index=(Y/32)*64+(U/32)*8+(V/32) (2) 
Predict error generator 13 generates a predict error from the difference 
between the current original frame and the predict frame output from image 
input portion 11, using subtractor 51 shown in FIG. 5. At this stage, the 
larger the average dispersion and the movement of a macroblock, the larger 
a predict error will be. Also, the smaller a real perceptual color 
tolerance is, the larger the relative dimension of a predict error will 
be. Thus, the reason for using a predict error instead of a current 
original frame in a coding procedure is to reduce the bit generation 
amount by eliminating the redundancy of frame signals which have a 
characteristic that identical video signals are redundantly distributed. 
Perceptual predict error comparator 14 is for recognizing predict errors 
which are not perceptual by using the method described in connection with 
FIG. 8 and by comparing the perceptual color tolerance output (for Y, U 
and V chromatic components of the current original frame output) from 
perceptual color tolerance decision portion 12, with the predict error 
output from predict error generator 13. 
As illustrated in FIG. 8, the procedure for recognizing predict errors 
comprises: in a first step, inputting perceptual color tolerances (mY,pY), 
(mU,pU) and (mV,pV) for a current original frame, defined by perceptual 
chromatic difference LUT defining; in the second step, defining; predict 
errors DY, DU and DV for each Y, U and V chromatic component comparing; 
and in the third step comparing; predict errors DY, DU and DV with 
perceptual color tolerances (mY,pY), (mU,pU) and (mV,pV) (by steps 
81.about.83, 84.about.86 and 87.about.89, respectively, as illustrated in 
FIG. 8) and defining a predict error within the perceptual color tolerance 
as a non-perceptual predict error to be set to 0 as the predict error 
value. 
In the perceptual predict error generator 15, a new perceptual predict 
error is generated by defining a predict error value recognized as a 
non-perceptual predict error as 0, as a result of a comparison of a 
perceptual color tolerance of the current original frame in perceptual 
predict error comparator 14. 
In decoder 16, as shown in FIG. 9, the quantization portion 91 quantizes 
perceptual predict error outputs from the perceptual predict error 
generator 15 to compress them. The inverse quantization portion 92 carries 
out inverse quantization of quantized perceptual predict errors, and the 
adder 93 combines restored perceptual predict errors and predict frames to 
compose a restored frame to be supplied to a user through video media. 
As described above, the method and apparatus for coding an image, according 
to the present invention, for reducing the redundancy of digital video 
signals by the procedures of DPCM and DCT and for compressing a real frame 
in a quantization procedure, can reduce the deterioration of the picture 
quality of a restored frame (resulted from a quantization error) and the 
amount of bit generation due to the reduction of the amount of predict 
error generation. Also, a real time processing is possible by reducing the 
arithmetic operation time being used for defining perceptual color 
tolerance for each chromatic component by using a perceptual chromatic 
difference LUT. 
Accordingly, the present invention is compatibly applicable to several 
digital video media coding techniques because it has the effect of 
reducing the amount of bit generation and of improving the picture quality 
of a restored frame.