JPH05236461A - Mehtod and device for encoding image - Google Patents

Abstract

PURPOSE:To decrease flicker caused by lowering of average luminance level to the level lower than that of preceding and following pictures due to the blurring of a current picture in the case of executing the interpolation predictive signaling of an image signal. CONSTITUTION:Motion-compensated signals 48 and 49 are obtained by detecting motion vectors from preceding and following pictures stored in two picture memories 30 and 32, and an interpolation predictive signal 46 is obtained. When adding difference from a signal 40 of the current picture at an adder 20 while defining this signal as a predictive signal 45, a correcting signal 50 to correct the luminance of the current picture on a display is added to the signal 40, and the average luminance of the current picture is increased. Therefore, since the average luminance of the current picture is increased at the same degree of that of preceding and following pictures, flicker is decreased and picture quality is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a moving picture signal coding method and apparatus. Specifically, it is an object of the present invention to provide an image coding method and apparatus for removing flicker that tends to occur when a moving image signal is interpolatively predictively coded and transmitted.

[0002]

2. Description of the Related Art A moving image signal has a huge amount of information. Therefore, conventionally, various methods have been used to efficiently encode a moving image signal by encoding the moving image signal. By using the correlation between the pictures (frames or fields) of the image signal, the one used for this high efficiency encoding,
There is inter-picture predictive coding that predicts the current picture from the previous picture. Image signal of current picture and 1
The difference value for each pixel from the image signal of the previous picture is obtained as prediction error data, and only the obtained prediction error data is encoded and transmitted. This reduces the amount of information of the image to be encoded and transmitted.

However, although the inter-picture predictive coding is effective in the still area, it has a drawback that large prediction error data is generated in the moving area and the image quality is deteriorated. Motion-compensated inter-picture predictive coding is used as a means for correcting this. In motion-compensated inter-picture predictive coding, a motion vector, which is the amount of movement between the current picture and the immediately preceding decoded picture, is detected before determining inter-picture prediction error data. To do. When the motion vector is obtained, the prediction error data with respect to the part to be coded of the current picture at the position shifted according to the motion vector in the immediately preceding decoded picture is obtained. The obtained prediction error data is encoded and transmitted to the receiving side together with the motion vector.

If the prediction error data between pictures corrected using the motion vector is coded and transmitted, the transmission code amount is reduced as compared with the interframe coding without motion compensation.

However, even when the processing block of the current screen is completely changed and there is no correlation between the pictures, the prediction error data becomes larger than the data of the current picture only when the inter-picture prediction coding is performed. It may end up. In that case, intra-picture coding is desirable.

Since a type is provided on the screen in moving image encoding for storage media, it will be described with reference to FIG. Here, processing is performed in block units. I picture (Intra pict
ure), intra-picture coding is performed on all blocks included in the picture. P picture (Predictive
picture), inter-picture predictive coding and intra-picture coding are selected for each block. Hereinafter, the terms I and P pictures mean I picture or P picture.

In B pictures (Bidirectional pictures), not only forward (past) I and P pictures are used for prediction (including motion compensation) but also backward (future) I and P pictures.
Pictures can also be used for prediction. This is because the processing order of the input images is changed, so that the decoded images of future I and P pictures are predicted (the processing of B pictures is backward I,
Used later than P-pictures). In a B picture, four types of prediction, that is, intra-picture coding (prediction signal is 0), forward predictive coding, backward predictive coding, and interpolative predictive coding are selected for each block.

When the interpolative prediction coding for detecting the prediction error data using the front and rear I and P pictures is selected, the front and rear two I and P for one block in the B picture are selected. Two pieces of motion vector information are used with reference to a P picture. The values of these two motion vector information are
It is obtained in the I and P pictures that are the forward and backward reference pictures. The interpolated prediction signal is formed by averaging the forward prediction signal and the backward prediction signal.

In general, in the above-described moving picture coding apparatus for storage media, B pictures are compared to I and P pictures.
The image is blurred and reproduced.

This is because the B picture is predicted by referring to both the forward and backward I and P pictures, so that the prediction error is small, and the B picture is not used for the next prediction. This is because, since it is a type, only a code amount smaller than the code amounts assigned to I and P pictures is generally assigned to B pictures. Further, in the B picture, since an image is obtained by the prediction signal obtained by averaging the prediction signals from both the front and rear I and P pictures, a fine portion (texture) of the image is lost and is easily blurred.

The image blurring phenomenon means that the AC amplitude of the image signal becomes small. It is felt that the average level of luminance is lowered because the input signal has a non-linear characteristic called a gamma characteristic that takes into account the non-linear characteristic of a display device typified by a cathode ray tube due to the reduction of the AC amplitude.

The relationship between the input signal level Y which is the image signal and the brightness I on the display will be described with reference to FIG. Here, for convenience of display, the curvature of the curve indicated by the thick line representing the inverse gamma characteristic is represented large.

For example, when the AC amplitude of a clear vertical stripe image signal is large and the minimum and maximum values of the input signal level are represented by Y _A1 and Y _B1 , the points A 1 and 1 on the curve are indicated.
B1 corresponds and the average visually perceived luminance in that case is the luminance I _C1 of _C1 at the midpoint between points A1 and B1. However, when the image with vertical stripes is blurred, the AC amplitude of the image signal becomes small, and the minimum and maximum values of the input signal level are represented by Y _A2 and Y _B2 , and the points A2 and B2 on the curve are indicated. Corresponds. Then, the average brightness at this time becomes the brightness I _C2 of C2, which is the middle point between the points A2 and B2, and the average brightness level is perceived to be reduced by the difference between I _C1 and I _C2 . That is, the blurred B picture is perceived as having a lower brightness than the clear I and P pictures.

[0014]

Since the preceding and following I and P pictures are clear and the B picture is perceived as having low brightness due to blurring, the B pictures and the I and P pictures are alternately arranged in time. Then, there remains a problem to be solved that it is perceived as a flicker having a period of I and P pictures.

[0015]

A means for obtaining a prediction error signal by taking a difference between an input signal which is an image signal and a prediction signal and adding a correction signal for correcting the brightness on a display (CRT) is provided. ..

[0016]

By such means, the input signal levels Y _A2 and Y _B2 of the B picture which is the blurred current image of FIG. 5 are shifted to the right, and the corresponding points A2 and B2 on the curve and the intermediate point C thereof.
2 is shifted to the right by the correction signal so that the brightness level I _C2 at the intermediate point C2 matches the brightness level I _C1 . As a result, flicker can be reduced and a more natural image can be reproduced.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described with reference to FIG.

The B picture which is the current image signal is input terminal 4
It is input from 0 and applied to the adder 20 and the correction circuit 21. The correction circuit 21 receives the image signal of the B picture applied to the input terminal 40 and the prediction signal 45, and outputs a correction signal 50 for brightness correction described in detail later.

The adder 20 takes the sum of the signal level of each pixel included in the block to be processed in the B picture and the correction signal applied via the terminal a of the changeover switch 36, and then calculates the sum. The prediction signal 45 is subtracted to obtain the prediction error signal 44.

The prediction error signal 44 is orthogonally transformed by the transformer 22 that performs orthogonal transformation, quantized by the quantizer 23, and encoded data 41 is output. Encoded data 4
At the same time as 1 is sent to the receiving side, it is inversely quantized by the inverse quantizer 24, which is inversely orthogonally transformed by the inverse transformer 25 to obtain the quantized error signal 47.

The quantizer error signal 47 and the prediction signal 45 are applied to the adder 26, and the decoded signal 51 is obtained by adding them to the picture memory 30 or 32 via the terminal d or e of the changeover switch 27. Applied and stored. An image signal of a B picture is applied to the input terminal 40, the changeover switch 27 is connected to the terminal c when processing a B picture, and the input terminal 40 is connected to the front or rear I,
When the picture signal of the P picture is applied and the forward or backward I and P pictures are processed, the changeover switch 27 is connected to the terminal d or e. Therefore, the decoded signal 51 at the time of processing the forward or backward I and P pictures is stored in the picture memories 30 and 32. Motion compensation circuit 3
Forward or backward I, P stored in the picture memory 30 or 32 using the motion vector at 1, 33
The image signal of the motion-compensated block is read out from the picture to obtain the motion-compensated signal 48 or 49, which is the terminals f and h of the interpolation circuit 34 and the changeover switch 35.
Applied to.

The interpolation circuit 34, which has received the motion-compensated signals 48 and 49 from the front and rear I and P pictures, calculates the interpolation prediction signal and outputs the interpolation prediction signal 46, which is a changeover switch. 35 is applied to the terminal g.

When the terminal f or h is selected by the change-over switch 35, a motion-compensated signal from the front or rear I or P picture is selected, and is selected as the prediction signal 4.
It will be output as 5. When the terminal g is selected, the interpolation prediction signal 46 is output as the prediction signal 45. When the terminal i is selected, the zero (0) value 43 is output as the prediction signal 45. The changeover switch 35 is switched by an optimum mode selected for each block in the image and by a control signal from the outside (not shown).

The terminal a is selected by the changeover switch 36 regardless of the selection of the changeover switch 35 at the time of processing a B picture. When processing I and P pictures, the terminal b of the changeover switch 36 is selected. The changeover switch 36 is controlled by a control signal (not shown).

At the time of processing the forward or backward I and P pictures, either the terminal e or the terminal d of the changeover switch 27 is alternately selected, which will be described with reference to FIG.

In FIG. 2, B pictures are B1, B2,
B3 ..., I, P pictures are I1, I2, I
, 3, and B pictures and I pictures (I, P pictures) are alternately arranged.

It is assumed that when the I1 picture is processed, the terminal d of the changeover switch 27 is selected and the decoded signal 51 of the I1 picture, which is the picture preceding the B1 picture, is stored in the picture memory 30. Next, the changeover switch 27 is switched to the terminal e, and the I2 picture, which is the picture behind the B1 picture, is encoded. At this time,
A signal is transmitted from the picture memory 30 through the motion compensation circuit 31, and the changeover switch 35 selects the terminal h. The decoded signal 51 of the I2 picture is stored in the picture memory 32. Since the screens used for the forward and backward prediction are prepared, the B1 picture is encoded next. During the B picture processing, the changeover switch 27 is connected to the terminal c. For the prediction of the B1 picture, the changeover switch 35 can use any of the terminals i, h, g, and f.

Next, the changeover switch 27 is changed over to the terminal d, and the I3 picture which is the picture behind the B2 picture is coded. In that case, the terminal h of the changeover switch 35 is selected.
Is selected. The decoded signal 51 is stored in the picture memory 30 and is already stored in the picture memory 32.
The I2 picture stored in the above is used as a picture ahead of the B2 picture so that the prediction signal 45 of the next encoded screen B2 picture can be obtained from the changeover switch 35.

Next, the operation of the correction circuit 21 will be described with reference to FIG. FIG. 3 shows the same curve as the luminance characteristic shown in FIG. 5, and points A1, B1, C1 and A shown there are shown.
2, B2 and C2 are the same as those already described in the section of the prior art. That is, according to the conventional interpolation predictive coding, the average brightness of the forward or backward I and P pictures which are clear images is indicated by I _C1 , and the average brightness of the B picture which is a blurred image is I _C2 . To be shown
Since the B picture is perceived with lower luminance than the I and P pictures, flicker occurs.

Therefore, in the correction circuit 21, the input signal level Y _A2 in FIG. 3 is moved in parallel to Y _A3 and Y _B2 is moved in parallel to Y _B3 , and the brightness I _{C3 at} the middle point C3 of both points A3 and B3 is the brightness I _{C3. The} input signal level is shifted by Y _A3 -Y _A2 = Y _B3 -Y _B2 in order to generate a correction signal 50 to make it equal to _C1 and realize I _C1 = I _C3 .

Therefore, in the correction circuit 21, the input terminal 4
Upon receiving the B picture image signal from 0 and the prediction signal 45, the following calculation is performed.

It is assumed that the input signal level of the image signal from the input terminal 40 is Y, the brightness on the display (for example, a cathode ray tube) is I, and the inverse gamma characteristic is I = f (Y). It is assumed that the image signal is processed for each block in the picture, the input signal level in the block is Y _ij, and i and j indicate the spatial position in the block. The number of pixels to be processed in the block is N, the signal level of the prediction signal 45 (FIG. 1) is Y _2ij , and the level of the correction signal 50 is α.
Therefore, the average of luminance corresponding to the input signal Y _ij in the block included in the current image signal applied to the input terminal 40 and the display corresponding to the signal level Y _2ij of the predicted signal 45 after luminance correction (see FIG. 1). (1 / n) Σf (Y _ij ) = (1 / N) Σf (Y _2ij + α) ( 1) should be satisfied. Here, Σ represents the cumulative sum of i and j. The same applies hereinafter.

The inverse gamma characteristic can be approximately represented by I = kY ² (2). Here, k is a constant, and the exponent 2 is a value of 2.2 to 2.8. However, when 2 is adopted, there is a feature that the operation (calculation) is easy.

From equations (1) and (2), _{ΣY ij} ² = Σ (Y _2ij + α) ² (3) is obtained. From this equation (3), α ² + 2αΣY _2ij + _{ΣY 2ij} ² −ΣY _ij ² = 0 (4) is obtained, and from this equation (4), α = −ΣY _2ij + β ^1/2 (5) is obtained. Here, β = ( _{ΣY 2ij} ) ² + ΣY _ij ² −ΣY _2ij ² .

Further, if the condition α << _{ΣY 2ij} is used, α = (ΣY _ij ² −ΣY _2ij ² ) (2ΣY _2ij ) ⁻¹ (6) is obtained.

The value of α shown in the equation (5) or (6) represents the level of the correction signal 50. The α obtained in this way is added as a correction signal 50 to the image signal of the B picture from the input terminal 40 in advance during interpolation prediction coding of the B picture, so that the flicker due to the blurred B picture and the gamma characteristic is caused. Can be reduced.

[0037]

As is apparent from the above description, according to the present invention, a display device can be used for an image signal using interpolation predictive coding in which a clear image and a blurred image are reproduced alternately. It is possible to reduce the flicker caused by the gamma characteristic in which the nonlinear characteristic is taken into consideration. Therefore, the effect of the present invention is extremely large.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention.

FIG. 2 is a picture position diagram for explaining a switching operation of two picture memories which are constituent elements of FIG.

FIG. 3 is a luminance characteristic diagram with respect to an input signal level for explaining an operation of performing luminance correction in the correction circuit which is a component of FIG.

FIG. 4 is a picture position diagram for explaining an operation of detecting a motion vector from conventional forward and backward pictures.

FIG. 5 is a luminance characteristic diagram showing luminance with respect to an input signal at the time of conventional interpolation prediction coding.

[Explanation of symbols]

 20 adder 21 correction circuit 22 converter 23 quantizer 24 inverse quantizer 25 inverse converter 26 adder 27 changeover switch 30, 32 picture memory 31, 33 motion compensation circuit 34 interpolation circuit 35, 36 changeover switch 40 Input terminal 41 Coded data 43 Zero value 44 Prediction error signal 45 Prediction signal 46 Interpolation prediction signal 47 Quantization error signal 48,49 Motion compensated signal 50 Correction signal 51 Decoded signal 100,101 Picture 102,103 Block 104 Search area m, n picture interval

Claims (2)

[Claims] 1. A correction signal (50) for correcting the luminance of the current picture by taking the difference between the image signal (40) of the current picture and the prediction signals (45) obtained from the pictures before and after the current picture. In addition (20), a prediction error signal (44) is obtained, and from the prediction signal and the image signal of the current picture, the brightness of the prediction signal on the display is displayed on the display of the image signal of the current picture. An image coding method for obtaining the correction signal for correcting the prediction signal so as to approximate the luminance. 2. A correction signal (50) for correcting the luminance of the current picture by taking the difference between the image signal (40) of the current picture and the prediction signals (45) obtained from the pictures before and after the current picture. In addition, a first accumulating means (20) for obtaining a prediction error signal (44) and a memory means (3) for temporarily storing each of the decoded signals obtained from the forward and backward pictures.
0, 32), and motion-compensated signals respectively detected from the forward and backward pictures stored in the memory means and motion-compensated from the forward and backward pictures (4
Motion compensation means (31, 3) for outputting
3), interpolation means (46) for outputting an interpolation prediction signal (46) from the motion-compensated signals obtained from the forward and backward pictures, the prediction signal and the image signal of the current picture Received
An image coding apparatus, comprising: a correction unit (21) for outputting the correction signal for correcting the prediction signal so that the brightness of the prediction signal on the display approaches the brightness of the display of the current picture.