Image signal coding system

Abstract: An adaptive blocking coding system selects an effective blocking of an input image signal to be encoded in accordance with the correlation between fields, even if motion is detected between the fields. The blocking patterns include an individual field blocking, a non-interlace blocking, a split blocking and an inverted split blocking. Further, the coding system searches for motion from both odd and even fields of a frame for producing a motion compensated prediction signal in order to provide high-efficient coding.

Description: The present invention relates to an adaptive blocking image coding system according to the pre-characterizing part of claim 1. As is known in the art, means for eliminating redundant components included in an image signal is used for coding an image signal. A typical approach to image coding is the transform coding method wherein an image is divided into blocks, an orthogonal transform is carried out for each of the blocks, and the transform coefficients are encoded. In the case of television signals such as an NTSC signal, interlaced scanning is used whereby an image signal of one frame is scanned twice, once in the odd field and once in the even field. The two fields scan different but complementary spaces of an image. The fields have image information at different times but there is a relatively strong correlation therebetween because the scanned lines of the two fields are alternate and adjacent. There is a technique in which coding is carried out after combining the fields and dividing them into blocks when coding an image signal produced by the interlaced scanning. Fig. 1 is a block diagram showing the structure of an embodiment of "High Efficiency Image Coding System" described in the Japanese Patent Public Disclosure No. 1688/1991. In Fig. 1, the coding system includes a non-interlacing section 1, a motion detecting section 2, a non-interlace blocking section 3, an individual field blocking section 4, an orthogonal transform section 5, a quantizing section 6 for quantizing a conversion coefficient at the output of the orthogonal transform section 5, and coding section 7. In operation, a series of input image signals 100, which are produced by the interlaced scanning method and applied to each field, are converted to a non-interlaced signal 101 in the non-interlacing section 1 as indicated in Fig. 2(C). As shown, the pixels belonging to the odd field and the pixels belonging to the even field appear alternately in every other line. When an object is stationary and the correlation between adjacent lines is high, it is effective to use a non-interlaced signal and to code the image signal in a block including components from both fields. Fig. 3(A) shows an example of such a condition. On the other hand, when an object is moving, the correlation between adjacent lines is lowered and it is considered to be effective to execute the coding in units of individual fields. This is because a non-interlaced signal is used for the moving object results in discontinuation as shown in Fig. 3(B), causing a power to be generated in high frequency coefficients during the transform coding. In this case, the blocking as indicated in Fig. 3(C) is adequate. Thus, the motion detector 2 detects the motion of an object and changes the operation when the object is detected as being stationary by a signal 103 indicating motion, to conduct the blocking shown in Fig. 3(A) (hereinafter, this arrangement of Fig. 3(A) is called the non-interlace blocking) in the non-interlace blocking circuit 3. If the object is detected to be moving, the motion detector 2 changes the operation to conduct the blocking shown in Fig. 3(C) (hereinafter, this arrangement of Fig. 3(C) is called the individual field blocking) in the individual field blocking circuit 4. The blocks obtained by changing the blocking as explained above are subjected to the discrete cosine transformation (DCT) in the orthogonal transform section 5. The transform coefficients obtained as described above are quantized in the quantizing section 6, and a variable length code is assigned in the coding section 7 in accordance with the occurrence probability of respective events. Since a conventional image coding system has been structured as described above, it has been difficult to realize the blocking utilizing the correlation between fields when an object is moving. Moreover, such a system has not utilized the property of different intensities in power distribution of the coefficients after conversion caused by the difference in arrangement of pixels within the block. In addition, there is the difference in power between the stationary blocks and moving blocks, the moving blocks having a high signal power which has not been utilized. US-A-4 849 812 already discloses a television system in which digitalized picture signals subjected to a transform coding are transmitted from an encoding station to a decoding station. In order to detect motion effects between the two fields of a picture, these fields are examined in a motion detector. If no motion ist detected, intraframe transform is employed, and if motion is detected, intrafield transform is employed. Small motion effects can be eliminated by subjecting the picture signal to a median filtering before subjecting it to an intraframe transform. The present invention has been proposed to overcome the problems in the prior art. Therefore it is an object of the present invention not only to adaptively discriminate between a block which is effective for non-interlace blocking and a block which is effective for individual field blocking, but also to enhance coding efficiency by adding a class of blocking so that field correlation is used even for a moving image, the quantization accuracy is controlled and the scanning sequence of transform coeffecients is changed in accordance with switching of the blocking. In order to solve this object, an adaptive blocking image coding system comprises the features of claim 1. Specific embodiments of the system according to the invention are defined in the claims. According to the present invention, an adaptive blocking image coding system encodes an input image signal obtained by interlaced scanning in a unit of the block of M pixels x N lines. More specifically, the adaptive blocking image coding system comprises blocking means for selectively forming a first type block including only the pixels of M pixels x N lines belonging to the odd field of the input image signal or the pixels of M pixels x N lines belonging to the even field of the input image signal, a second type block wherein the pixels of the M pixels x N/2 lines belonging to the odd field and the M pixels x N/2 lines belonging to the even field are arranged alternately in every other line corresponding to scanning positions on a display screen, a third type block wherein the pixels of M pixels x N/2 lines belonging to the odd field are arranged in the upper or lower half of the block and the pixels of M pixels x N/2 lines belonging to the even field are arranged in the remaining half of the block, and a fourth type block wherein the pixels of M pixels x N/2 lines belonging to the odd field are arranged in the upper of lower half of the block, the pixels of M pixels x N/2 lines belonging to the even field are arranged in the remaining half of the block and the pixels of either field are inverted upside down in the vertical direction with respect to the display screen. The system further includes blocking determining means for determining the selected type of block formed by the blocking means, a transform means for orthogonally transforming the selected block formed by the blocking means, quantizing means for quantizing the transform coefficient obtained by the transform means, and coding means for encoding the quantized index obtained by the quantizing means. With the structure described above, the block of the M pixels x N lines if obtained by one blocking selected from the non-interlace blocking where the pixels of M pixels x N/2 lines belonging to the odd field and the pixels of M pixels x N/2 lines belonging to the even field are arranged in every other line corresponding to the scanning positions on the display screen, an arrangement (hereinafter, called split blocking) where the pixels of M pixels x N/2 lines belonging to the odd number field are arranged in the upper half or lower half block and the pixels of M pixels x N/2 lines belonging to the even field are arranged in the remaining half block, and an arrangement (hereinafter, called inverted split blocking) where the pixels of M pixels x N/2 lines belonging to the odd field are arranged in the upper or lower half block, the pixels of M pixels x N/2 lines belonging to the even field are arranged in the remaining half block and the pixels of either field are inverted in the vertical direction with respect to the display screen. The obtained block is orthogonally transformed the transform coefficients are quantized, and then the quantization index is encoded. According to an embodiment of the present invention, the quantizing means for quantizing the transform coefficient in the adaptive blocking image coding system variably controls the quantization accuracy in accordance with the type of arrangement provided by the blocking means. More specifically, one of the arrangements including individual field blocking, non-interlace blocking, split blocking, or inverted split blocking is orthogonally transformed, the transform coefficient is quantized and the quantizing index is encoded with the quantizing accuracy in accordance with the information, indicating the selected blocking. According to another embodiment of the present invention, the coding means for encoding the quantized index obtained by quantizing the transform coefficient determines the scanning sequence (path) of quantizing the transform coefficient in accordance with the type of arrangement to be blocked by the blocking means. More specifically, one of the arrangements to be blocking-processed by the individual field blocking, non-interlace blocking, split blocking, or inverted split blocking is orthogonally transformed, the transform coefficient is quantized, and the quantizing index is encoded with the quantization accuracy and the scanning sequence in accordance with the information indicating the selected blocking. According to a further embodiment of the present invention, the blocking determining means determines the type of arrangement to be blocked in the blocking means in accordance with a value obtained by multiplying a predetermined weighting coefficient with the pixels of each line included in the block and then totaling the multiplied values. More particularly, one of the arrangements to be blocking-processed by individual field blocking, non-interlace blocking, split blocking, or inverted split blocking is selected by the value obtained by multiplying the predetermined weighting coefficient with the pixels of each line included in the block and then totaling such multiplied values. The selected block is orthogonally transformed, the transform coefficient is quantized and the quantizing index is encoded. According of a still further embodiment of the present invention, the blocking determining means determines the type of arrangement having the minimum coefficient power of a predetermined high frequency component among the transform coefficients obtained by discrete cosine transform of the block. In other words, one of the arrangements to be blocking-processed by individual field blocking, non-interlace blocking, split blocking, or inverted split blocking is selected in such a manner that the coefficient power of the predetermined high frequency element component is the minimum among the transform coefficients obtained by discrete cosine transform of the block. The determined block is orthogonally transformed, the transform coefficient is quantized, and the quantizing index is encoded. The invention will be more fully understood from the following detailed description and the accompanying drawings in which: Fig. 1 is a block diagram of an image coding system in the prior art; Fig. 2 is a diagram for explaining non-interlace blocking; Fig. 3 is a diagram for explaining an adaptive blocking of the prior art; Fig. 4 is a block diagram of an embodiment of the present invention; and Fig. 5 is a diagram for explaining adaptive blocking in the embodiment shown in Fig. 4. Referring to Fig. 4, an embodiment of the present invention is shown as an adaptive blocking image coding system. In Fig. 4, the image coding system comprises a non-interlacing section 1 for conducting non-interlace processing; a blocking determination section 8; an individual field blocking section 4; a non-interlace blocking section 3; a split blocking section 9; an inverted split blocking section 10; an orthogonal transform section 11; a quantizing section 12 and coding section 13. Such various types of blocking are shown in Fig. 5. Figs. 5A - 5D show individual field blocking non-interlace blocking, split blocking and inverted split blocking, respectively. The operation will be explained with reference to Fig. 4 and Figs. 5A - 5D. The input image signal series 100 which is scanned by the interlace scanning method and is inputted field by field is converted into a non-interlaced signal 101 in the non-interlace section 1. Fig. 2 shows a profile of non-interlace processing in the prior art similar to the non-interlace processing in the present invention. When (A) is defined as an input image signal from the odd field and (B) as an input image signal of the even field, the non-interlaced signal 101 shown in (c), alternately combining the lines from respective fields, can be obtained. The individual field blocking section 4 executes, as shown in Fig. 5(A), blocking in which the fields are processed individually. This blocking is effective when the correlation between the fields is not available because of quick motion. The non-interlace blocking section 3 executes the blocking shown in Fig. 5(B). In the case of a stationary or still image, a continuous image can be obtained by non-interlaced processing of the fields. The wavelength of the signal thereby becomes substantially longer, resulting in power being concentrated on low frequency components in the successive transform coding. The split blocking section 9 conducts the blocking as shown in Fig. 5(C). This blocking is effective in the case where the correlation between the fields exists but the fields are noncontinuous when non-interlace blocking is carried out. The inverted split blocking section 10 conducts the blocking shown in Fig. 5(D). This blocking is also effective in the case where the correlation between fields exists but the fields are noncontinuous when non-interlace blocking is carried out. This blocking prevents discontinuation at the center of the block when the split blocking method is used. The blocking determinating section 8 determines the optimum blocking from a plurality of blockings as explained above and outputs a blocking arrangement selecting signal 102 for selecting the determined blocking. Here, it is important to enhance the concentration of power, on the low frequency coefficients in the transform coding. For this purpose, it is effective to evaluate the amplitude of high frequency components in each blocking and select the blocking having the minimum amplitude. In one of the evaluation methods, a weight is multiplied with the pixels of each line and the obtained values are then totaled. For example, the weight of +1 is given to the lines 0, 2, 4, 6 using the line numbers shown in Fig. 5, and the weight of -1 is given to the lines 1, 3, 5, 7. Thereafter, the obtained values are totaled to obtain the absolute value of the sum. Moreover, the weight +1 is given to the lines 8, 10, 12, 14, and the weight -1 is given to the lines 9, 11, 13, 15. The obtained values are then totaled to also obtain the absolute values of the sum. Both absolute values are totaled. Thus, the weighting is inverted alternately for respective lines and it is equivalent to the evaluation of the maximum frequency component when non-interlace blocking has been conducted. Further, the weight +1 is given to the lines 0, 4, 8, 12 and the weight -1 to the lines 2, 6, 10, 14. The obtained values are totaled to obtain the absolute value of the sum. In addition, the weight +1 is given to the lines 1, 5, 9, 13 and the weight -1 to the lines 3, 7, 11, 15. The obtained values are then totaled to obtain the absolute value of the sum. These absolute values are also totaled to evaluate the maximum frequency component of the individual field blocking. In addition, the weight +1 is given to the lines 0, 4, 1, 5 and the weight -1 to the lines 2, 6, 3, 7. The obtained values are totaled to obtain the absolute value of the sum. The weight +1 is also given to the lines 8, 12, 9, 13 and the weight -1 to the lines 10, 14, 11, 15. The obtained values are totaled to obtain the absolute value of the sum. Both absolute values are then totaled to evaluate the maximum frequency component of the split blocking. The weight +1 is given to the lines 0, 4, 7, 3 and the weight -1 to the lines 2, 6, 5, 1. The obtained values are totaled to obtain the absolute value of the sum. Moreover, the weight +1 is given to the lines 8, 12, 15, 11 and the weight -1 to the lines 10, 14, 13, 9. The obtained values are totaled to obtain the absolute value of the sum. These absolute values are totaled to evaluate the maximum frequency component of the inverted split blocking. In another method for evaluation of each blocking, the number of orthogonal transformed coefficients having an amplitude larger than a predetermined threshold value for the respective blockings is counted, and the blocking having the minimum number is selected. The orthogonal transform section 11 carried out the orthogonal transform of the selected block to obtain the transform coefficients. The obtained transform coefficients are quantized in a fixed sequence by the quantizing section 12. In this case, some difference lies in the power of the coefficients depending on the type of blocking. In general, non-interlace blocking tends to be selected for a stationary region and the power is comparatively small. Meanwhile, since the correlation between fields becomes small in a quick motion area, individual field blocking is often selected and the power is large. Moreover, split blocking and inverted split blocking are considered to be intermediate to the above two blockings. Therefore, efficiency can be improved by variably controlling quantization accuracy in accordance with the type of blocking. The quantizing accuracy can also be controlled variably in accordance with not only the type of blocking but also the combination of actual signal power and quantization error power. In this case, it is also possible to execute variable length coding by combining the information indicating type of blocking and the information indicating quantization accuracy. Indexes obtained by quantizing the coefficients are encoded in the coding section 13. In this case, the coefficients are scanned from those having a larger coefficient power to those having a smaller one in order to enhance the efficiency of encoding. For the coefficients having a power lower than a certain specified value, the encoding may cease. Therefore, it is very convenient if the power distribution can be anticipated. There is a tendency with respect to the distribution of power of the coefficients that the power is increased as the frequency is lower. However, if the blocking is adaptively changing, as in the present invention, the coefficients having lower power do not always correspond to low frequency components. Then, the coding efficiency can be improved by changing the scanning sequence or path in accordance with the type of blocking. Since the present invention is structured as explained above, the following effects can be obtained. The coding efficiency of transform coding is improved by switching the blocking of an image signal scanned by the interlaced scanning method into an adapted blocking. Moreover, the efficient assignment of information quantity can be realized by variably controlling the quantizing accuracy of transform coefficients correspondingly to the switching of the blocking. In addition, the encoding efficiency can also be improved in transform coding by changing the scanning sequence of the transform coefficients within the block.

Claims: An adaptive image block coding system for encoding an input image signal (100) obtained by interlaced scanning in block units of M pixels x N lines, comprising: block forming means for selectively forming a first type of blocks (4) including only the pixels of M pixels x N lines belonging to the odd field of the input image signal (100) or the pixels of M pixels x N lines belonging to the even field of the input image signal (100), and a second type of blocks (3) wherein the pixels of the M pixels x N/2 lines belonging to the odd field and the pixels of the M pixels x N/2 lines belonging to the even field are arranged alternately in every other line corresponding to scanning positions on a display screen, block type determining means (8) for determining the selected type of block formed by said block forming means, transform means (11) for orthogonally transforming the selected block formed by said block forming means, quantizing means (12) for quantizing a transform coefficient obtained by said transform means (11), and coding means (13) for encoding a quantized index obtained by said quantizing means, characterized in that the block forming means is adapted to selectively form also a third type of blocks (9) wherein the pixels of M pixels x N/2 lines belonging to the odd field are arranged in the upper or lower half of the block and the pixels of M pixels x N/2 lines belonging to the even field are arranged in the remaining half of the block, and a fourth type of blocks (10) wherein the pixels of M pixels x N/2 lines belonging to the odd field are arranged in the upper or lower half of the block, the pixels of M pixels x N/2 lines belonging to the even field are arranged in the remaining half of the block and the pixels of either field are inverted upside down in the vertical direction with respect to the display screen. An adaptive image block coding system according to Claim 1 wherein said quantizing means (12) for quantizing the transform coefficient variably controls the quantization accuracy in accordance with the type of arrangement provided by said block forming means (3,4,9,10). An adaptive image block coding system according to Claim 2 wherein said coding means (13) for encoding the quantized index obtained by quantizing said transform coefficient determines the scanning sequence of quantizing the transform coefficient in accordance with the type of arrangement to be blocked by said block forming means (3,4,9,10). An adaptive blocking image coding system according to Claim 2 wherein said block type determining means is adapted to (8) determine the type of arrangement to be blocked in said block forming means (3,4,9,10) in accordance with a value obtained by multiplying a predetermined weighting coefficient with the pixels of each line including in the block and then totaling the multiplied values. An adaptive blocking image coding system according to Claim 2 wherein said block type determining means is adapted to (8) determine the type of arrangement having the minimum coefficient power of a predetermined highfrequency component among the transform coefficients obtained by a discrete cosine transform of the block.

Inventors: ASAI KOHTARO, MURAKAMI TOKUMICHI, NISHIKAWA HIROFUMI, YAMADA YOSHIHISA, ASAI, KOHTARO, MURAKAMI, TOKUMICHI, NISHIKAWA, HIROFUMI, YAMADA, YOSHIHISA, ASAI, KOHTARO, MITSUBISHI DENKI K.K., MURAKAMI, TOKUMICHI, MITSUBISHI DENKI K.K., NISHIKAWA, HIROFUMI, MITSUBISHI DENKI K.K., YAMADA, YOSHIHISA , MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N7
IPC codes: H04N7, H04N11, G06T9