JP2552350B2 - Image signal compression coding device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image signal compression encoding apparatus, and more particularly to an image signal compression encoding apparatus which makes a data amount of a compression encoded image constant.

BACKGROUND ART When storing digital image data such as image data captured by an electronic still camera in a memory,
In order to reduce the amount of data and the storage capacity of the memory, various types of compression encoding are performed. In particular, two-dimensional orthogonal transform coding is widely used because it can be coded at a large compression rate and image distortion can be suppressed along with the coding.

In such a two-dimensional orthogonal transform coding, the image data is divided into a predetermined number of blocks, and the image data in each block is two-dimensionally orthogonally transformed. The orthogonally transformed image data, that is, the transform coefficient, is compared with a predetermined threshold value, and the portion below the threshold value is truncated (coefficient truncation). As a result, the conversion factor below the threshold is
It is processed as 0 data. Next, the transform coefficient subjected to coefficient truncation is divided by a predetermined quantization step value, that is, a normalization coefficient, and quantization by a step width, that is, normalization is performed. This gives the value of the conversion factor,
That is, the dynamic range of the amplitude can be suppressed.

The comparison with the threshold value and the normalization process are often performed simultaneously. That is, when the conversion coefficient is normalized with a predetermined normalization coefficient and the result is converted into an integer, the conversion coefficient having a value lower than the normalization coefficient becomes zero.

In such a two-dimensional orthogonal transform coding, when the above normalization coefficient is normalized and coded, the amount of coded data differs depending on the image data, and the data is recorded in the memory. It was inconvenient.

That is, when the data is normalized by using a certain normalization coefficient, the amount of encoded data in the image data including many high frequency components is large, and the image data including many low frequency components is large. Reduces the amount of encoded data. Such a difference in encoded data amount may reach 5 to 10 times, which is inconvenient when recording in a memory having a fixed capacity.

Therefore, for example, so-called activity is calculated to the extent that a high frequency component is included in each divided block, and a normalization coefficient is set for each block based on this. However, when the compression coefficient is changed by changing the normalization coefficient for each block in this way, the image quality is different for each block, so that the image quality becomes unstable. In addition, there is a drawback that it is necessary to calculate the transform coefficient for each block.

Aim The present invention solves the above-mentioned drawbacks of the prior art, and in the normalization and encoding after the two-dimensional orthogonal transformation, the normalization is performed according to the image to be compressed, and the encoded image data amount is constant. It is an object of the present invention to provide an image signal compression encoding device that becomes

DISCLOSURE OF THE INVENTION According to the present invention, an image signal compression encoding apparatus that divides digital image data forming one screen into a plurality of blocks and performs two-dimensional orthogonal transform encoding on the image data of each block is provided. Orthogonal transforming means for two-dimensionally orthogonally exchanging the digital image data divided into blocks, normalizing means for normalizing the data orthogonally exchanged by the orthogonal transforming means, and encoding the data normalized by the normalizing means. Encoding means, block activity calculation means for calculating the activity of image data for each divided block, activity addition means for adding the activity for each block calculated by the block activity calculation means, and calculation by the block activity calculation means Activity and activity for each block Coded data amount distribution means for calculating the coded data amount to be distributed for each block from the total value of the activities calculated by the virtue addition means, and the coding means according to the output from the coded data amount distribution means. From the encoding means, the encoding output control means limiting the amount of output data from the encoding means, and the encoding output control means from the encoding means. The output data amount from the encoding unit is limited so that the output data amount from the encoding unit falls within the range of the encoded data amount distributed for each block.

Description of Embodiments Next, an embodiment of an image signal compression encoding apparatus according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows an embodiment of an image signal compression encoding apparatus according to the present invention.

This device has a blocking unit 12. The blocking unit 12 is composed of a frame buffer, and the still image data for one frame imaged by the electronic still camera is input and stored through the input terminal 10. Blocking unit
The image data for one frame stored in 12 is divided into a plurality of blocks, read for each block, and sent to the two-dimensional orthogonal transformation unit 14. The two-dimensional orthogonal transformation unit 14 performs two-dimensional orthogonal transformation on the image data of each block. As the two-dimensional orthogonal transform, a known orthogonal transform such as a discrete cosine transform or a Hadamard transform is used.

The image data is arranged vertically and horizontally for each block that has been two-dimensionally orthogonally transformed by the two-dimensional orthogonal transformation unit 14, low-order data is arranged in the upper left portion, and becomes higher-order data in the lower right direction. DC component data is arranged at the upper left. The output of the two-dimensional orthogonal transformation unit 14 is sent to the normalization unit 16.

The normalization unit 16 normalizes the image data, which has been subjected to the two-dimensional orthogonal transformation in the two-dimensional orthogonal transformation unit 14, after rounding down the coefficients. The coefficient truncation is to compare the orthogonally transformed transform coefficient with a predetermined threshold value and to cut off a portion below the threshold value. The normalization is performed by dividing the conversion coefficient, which has undergone the coefficient truncation, by a predetermined quantization step value, that is, the normalization coefficient α to obtain the normalization coefficient α.
The quantization is performed by. The normalization coefficient α is obtained from the look-up table based on the value obtained by summing the activities of each block, as described later.

The image data for each block output from the blocking unit 12 is also sent to the block activity calculation unit 20. The block activity calculation unit 20 calculates the activity for each block, that is, the degree to which the block includes image data of high frequency components.

To calculate the activity ACT (i, j) for each block,
If the divided block is composed of 8 × 8 pixels as shown in FIG. 2 or 6, for example, the pixel data is Xij (where i and j indicate the block address).
formula Is calculated by here, Is.

That is, according to this formula, the average value DC (i, j) of the data of the 8x8 pixels forming the block is calculated, and the absolute value of the difference between each pixel data and the average value DC (i, j) is added. Ask for activity ACT.

If you use the above formula to find the activity,
Since (i, j) is obtained by adding each pixel data and dividing the added data by 64, it can be configured only by an adder and data shift. Further, ACT (i, j) is obtained by the absolute value conversion circuit and the adder using the obtained DC (i, j). Therefore, no multipliers and dividers are needed in the activity calculation.

For example, the calculation of the activity for each block is the third
As shown in the figure, the block may be divided into four sub-blocks, and the activity of each sub-block may be summed up. In this case, the activity ACT (i, j) of the block is calculated by the following formula.

In this equation, the first term to the fourth term respectively represent the degree to which the high frequency components are included in the image data forming the sub blocks 1 to 4. For example, the first term in the above equation is the sum of the absolute values of the differences between the respective image data forming the sub-block 1 and the average value of the image data in the sub-block 1.
This represents the degree of high frequency components included in the sub-block 1.

In this way, the activity of each block can be accurately calculated by dividing the sub-blocks, obtaining the degree to which the high frequency components are included in each sub-block, and adding them. That is, since the degree of the high frequency component is calculated not for the entire block but for each sub-block, the activity of the block can be calculated more accurately.

The calculation of such an activity does not require a multiplier and a divider as in the calculation of the activity of the whole block.

The activity calculation for each block is shown in Figures 4A to 4A.
Alternatively, a filter as shown in Fig. 4C may be used.

As shown in FIG. 5, these filters 80 are sequentially moved from the upper left of the block in the direction of the arrow, and the pixel data values output from the filter 80 are summed to obtain the activity of the block. For example, the filter in Figure 4A
If 80 is located at the upper left corner of the block, the value obtained by multiplying the pixel Xi + 1, j + 1 in FIG. 6 by 8 is Xi, j, Xi + 1, j,
X i + 2, j, X i, j + 1, X i + 2, j + 1, X i, j + 2, X i
The values obtained by multiplying +1, j + 2, Xi + 2, j + 2 by -1 are output, and these values are summed. If the values of these 9 pixel data are the same, that is, there is no change in the pixel,
If it is a DC component, it is output from the filter 80.
The total value of the pixel data is 0. Therefore, moving such a filter 80 to scan the block and sum the output values gives the activity of the block. For example, the filters shown in FIGS. 4A to 4C may be selected according to the frequency component to be emphasized when obtaining the activity.

The calculation of this activity also does not require a multiplier and a divider.

In calculating the activity, a non-linear conversion coefficient T () may be used for conversion as shown in the following equation.

Here, the conversion coefficient T () is, for example, as shown in FIG. 7, for obtaining an output obtained by subjecting the input [X (i, j) -DC] to nonlinear conversion. By adding such a conversion, the activity corresponding to the low threshold frequency component is increased, and the bit allocation to the low activity block is relatively increased. As a result, the image quality of the part where the gradation is gentle can be improved, and the visual image quality can be improved.

As described above, the block activity calculation unit 20
Calculates the activity for each block and outputs it to the total activity calculating unit 26 and the bit allocation calculating unit 40.

The total activity calculation unit 26 adds the activities for each block sent from the block activity calculation unit 20 to calculate the total activity. The total activity calculation unit 26 outputs the total value of activities to the normalization coefficient setting unit 22 and the bit allocation calculation unit 40.

The normalization coefficient setting unit 22 sets the normalization coefficient according to the total value of the activities input from the total activity calculation unit 26. The normalization coefficient is set, for example, by using a look-up table stored in a storage unit (not shown) and performing conversion as shown in FIGS. 9A and 9B.
According to the conversion shown in FIG. 9A, the normalization coefficient changes in proportion to the total value of activities. The conversion shown in FIG. 9B has a small increase in the normalization coefficient with respect to the increase in the total value of activities, and thus high-precision encoding can be performed. The normalization coefficient setting unit 22 outputs the normalization coefficient thus set to the normalization unit 16.

The normalization unit 16 performs normalization using the normalization coefficient sent from the normalization coefficient setting unit 22. That is, the image data for each block is divided by the normalization coefficient. The normalization coefficient used for normalization is set based on the total value of activities for each block as described above,
It is common to the whole image, that is, all blocks.

Note that this normalization is changed to dividing the conversion coefficient that has undergone coefficient truncation by the value α of one selected normalization coefficient, and replaces the data stored in the weight table T as shown in FIG. And the normalization coefficient α may be used together. The low-frequency component of the conversion coefficient is important as data,
Since the high-frequency components are not important, the weight table T as shown in FIG. 10 assigns small values to the low-threshold components and large values to the high-frequency components. By the value α · T obtained by multiplying the normalization coefficient α of
The normalization may be performed by gradually dividing the conversion coefficient that has been subjected to the coefficient truncation.

Further, the conversion coefficient is gradually divided by the normalization coefficient α or α · T to obtain 1 / α or 1 / (α · T) in advance, and the conversion coefficient is multiplied by this value. By doing so, you can reduce the number of dividers,
The scale of the device can be reduced.

The normalized transform coefficients are arranged in blocks like the pixel data shown in FIG. 2, and as shown in FIG. 11, they are sequentially scanned in a zigzag form from the low-frequency component to the two-dimensional Huffman encoder 28. Is output.

The output of the normalization unit 16 is output to a two-dimensional Huffman encoding unit 28. The two-dimensional Huffman encoding unit 28 often has consecutive zeros in the normalized transform coefficient input by scanning in a zigzag manner as described above, so that the continuous amount of zero-valued data, that is, zero. The run length is detected, the run length of zero and the amplitude of non-zero are obtained, and this is two-dimensionally Huffman coded. The output from the two-dimensional Huffman encoding unit 28 is output to the output terminal 32 and the code amount counting unit 44.

The bit allocation calculation unit 40 calculates the coded bits allocated to each block using the total value of the activity for each block sent from the block activity calculation unit 20 and the activity sent from the total activity calculation unit 26. The coded bits allocated to each block are
It is a number of bits that defines how far the data output from the low-frequency component is two-dimensionally Huffman-coded for each block, that is, how many bits of the encoded data are output.

The coded bit bit (i, j) allocated to each block is
It is given by the following formula.

bit (i, j) = {total number of bits of the entire image xACT (i, j) / total activity} + [remaining bits to previous block] ...
(1) In the above equation, "total number of bits of the entire image" is the number of bits assigned to the entire image formed by a plurality of blocks in encoding, that is, two-dimensional Huffman encoding is performed for each block. This is the number of bits that represents the number of bits to be assigned to the entire image when the output of data is cut off by a predetermined number of bits when the data is output in order from the low-frequency component. The amount of output data is determined.

ACT (i, j) is an activity for each block. The total activity is the total value of the activity of the block.

Therefore, the total number of bits xACT (i,
j) / total activity} is for allocating the number of bits allocated to the entire image by the ratio of the activity of each block to the total value of the block activity.

[Remainder bit to previous block] is 1 of the block for which the distributed encoded bit bit (i, j) is to be obtained.
This is the difference between the number of surplus bits up to the preceding block, that is, the number of bits distributed up to the preceding block, and the number of bits actually encoded and output. For example, in the previous block, if the allocated number of bits is 50 bits and the actual number of encoded output bits is 45 bits,
The remaining bits are 50−45 = 5 bits. By adding such surplus bits, it is possible to prevent the difference between the number of bits allocated to each block and the number of bits actually output from being accumulated.

The coded bit bit (i, j) distributed in each block, which is shared by the bit allocation calculation unit 40, may be expressed by the following formula.

In the above formula, Is a recurrence formula showing the sum of the number of bits distributed up to the previous block. Therefore, [total number of bits in the entire image Represents the total number of bits assigned to the remaining blocks. Also, Indicates the total value of the activity of the blocks up to the previous block. Therefore, select Total Activity Indicates the total activity of the remaining blocks.

According to the above equation, the total number of bits allocated to the remaining block is distributed by the ratio of the activity of the block to the total of the remaining block activity. According to this formula, the coded bit bit (i, j) to be allocated to the block is obtained in consideration of the number of bits allocated to the previous block and the activity of the block to the previous block. From (1) above
The difference between the total number of bits allocated to each block and the total number of bits of the entire image is smaller than that of the expression.

The coded bit bit (i, j) allocated to each block may also be obtained by the following formula.

bit (i, j) = [total number of bits of the entire image−Σ actual sending bits up to the previous block] xACT (i, j) / [total activity−ΣΣACT (i, j)] (3) above In the equation, the actual sending bit up to the block before Σ represents the sum of the number of bits actually encoded and output up to the previous block. Therefore, [total bits of the entire image−actual sending bits up to the preceding block] is the total number of bits assigned to the remaining blocks in consideration of the number of bits actually coded and output up to the preceding block. Represents a number.

Therefore, according to the above equation, the total number of bits allocated to the remaining blocks, which takes into account the number of bits actually encoded and output up to the preceding block, is calculated by the ratio of the block activity to the total activity of the remaining blocks. It is to distribute. According to this formula, 1
The coded bit bit allocated to the block considering the number of bits actually coded and output up to the block before and the activity of the block up to the previous block.
Since (i, j) is obtained, it is not necessary to consider the surplus bits up to the previous block as in the above equations (1) and (2).

The coded bit bit (i, j), which is output from the bit allocation calculation unit 40 and is distributed to each block, is a coding stop determination unit.
Sent to 42. On the other hand, the encoded output from the two-dimensional Huffman encoding unit 28 is output to the code amount counting unit 44. The code amount counting unit 44 counts, for each block, the code amount of the output from the two-dimensional Huffman encoding unit 28, that is, the cumulative number of bits. The code amount counting unit 44 outputs the data obtained by adding the number of bits of the coded data output next from the two-dimensional Huffman coding unit 28 to the coding stop determination unit 42.

The coding stop determination unit 42 compares the number of bits input from the code amount counting unit 44 and the coded bit bit (i, j) distributed to each block, which is input from the bit distribution calculation unit 40. When the number of bits input from the code amount counting unit 44 is the encoded bit bit (i, i,
When it is detected that j) is exceeded, a signal indicating that the encoded output should be stopped is output to the two-dimensional Huffman encoding unit 28. As a result, the two-dimensional Huffman encoding unit 28 stops the output of the encoded data corresponding to the number of bits added last in the code amount counting unit 44, that is, the encoded data to be output next, and outputs the encoded output. The number of bits of data to be encoded is the encoded bit bit (i,
Do not exceed j). That is, the Huffman-coded output of the block is terminated up to the preceding coded data that exceeds the coded bits to which the coded output data is distributed.

The coding stop determination unit 42 further obtains a difference between the distributed coded bits and the number of bits of the coded output data, that is, a residual bit, and outputs it to the bit allocation calculation unit 40. The bit allocation calculation unit 40 uses the remainder bits when obtaining the coded bit bit (i, j) allocated by the above equations (1) and (2).

The compression-coded image data output from the two-dimensional Huffman coding unit 28 is sent to the output terminal 32. Output terminal
The data sent to 32 is transmitted to a transmission line (not shown),
Alternatively, it is recorded on a recording medium such as a magnetic disk.

The operation relating to the bit allocation of this device will be described using the twelfth flowchart.

The block activity calculation unit 20 calculates the activity for each block (102). In the total activity calculation unit 26, the block activity calculation unit 20
The activity for each block is added from the input to obtain the total value of the activities of the entire image, that is, the total activity (104).

The normalization coefficient setting unit 22 sets the normalization coefficient α using the total activity from the total activity calculation unit 26,
The data is output to the normalization unit 16 (112). The normalization unit 16 uses the normalization coefficient α to perform normalization. The data normalized by the normalizer 16 is sent to the two-dimensional Huffman encoder 28, and the two-dimensional Huffman-encoded compressed data is scanned and output (114). The coded output from the two-dimensional Huffman coding unit 28 is sent to the code amount counting unit 44, and the code amount is counted (116).

On the other hand, the output from the block activity calculator 20 and the output from the total activity calculator 26 are sent to the bit allocation calculator 40, and the bit allocation calculator 40 calculates the bit allocation for each block as described above (10
6). The encoding stop determination unit 42 compares the bit allocation set by the bit allocation calculation unit 40 with the code amount counted by the code amount counting unit 44 (108). When it is detected that the number of bits of the counted code amount exceeds the allocated number of bits, the coded output from the two-dimensional Huffman coding unit 28 is stopped up to the preceding code that exceeds the allocated number of bits. (110) When the coded output from the two-dimensional Huffman coding unit 28 is stopped, it is judged whether or not the processing is completed for all blocks (118), and the processing for all blocks is completed. In this case, the compression encoding ends. If the processing of all blocks has not been completed, the remainder bits to be allocated to the encoded output of the next block are calculated as described above (120), and then the process returns to step 106 to calculate the bit allocation of the next block. .

According to the present apparatus, the total value of the activities calculated for each block as described above, that is, the total activity is obtained, and the normalization coefficient α is set based on the total value to perform normalization. Therefore, the normalization coefficient α is set so that the encoded image data has a constant amount, so that the output data can be easily processed and is convenient when stored in the memory.

The device defines the coded bits to be distributed to the coded output of each block based on the ratio of activity per block to total activity. Therefore, the number of bits assigned to the entire image consisting of multiple blocks is fixed, and the number of bits assigned to each block is distributed according to the activity of that block. Therefore, the data of each block is effective depending on the activity of the block. Encoding is performed. That is, a block containing a lot of high frequency components has a large activity, so many bits are assigned, and a block containing a lot of low frequency components has a small activity, so few bits are assigned to perform coding according to each block. To do.

Further, according to the present apparatus, since the image data is normalized by the normalization coefficient α corresponding to the frequency component of the entire image, as in the case of setting the normalization coefficient for each block and normalizing it,
The reproduced image quality does not become unstable.

Further, the calculation of the activity for each block and the calculation of the total value thereof can be realized by a linear circuit having only the data shift means of the adder, and it is not necessary to use the multiplier and the divider, so that the device has a simple configuration. can do. Further, since the activity is calculated from the image data, a buffer for accumulating the data after the orthogonal transformation is not needed unlike the case where the data after the orthogonal transformation or the activity is calculated. As the look-up table for obtaining the normalization coefficient α from the total value of the activities, various look-up tables can be selected such that the amount of compression-coded data is constant. Further, the normalization coefficient α may be obtained, for example, by a predetermined calculation from the total value of the activities without using the look-up table like the device described later.

Further, the data of the normalization coefficient α may be output together with the encoded image data. In this way, the reproducing apparatus can perform decoding using this normalization coefficient α.

FIG. 14 shows another embodiment of the image signal compression coding apparatus according to the present invention.

As shown in the figure, the output from the two-dimensional orthogonal transformation unit 14 is connected to the normalization unit 16 as well as the DC component encoding unit 28a, and the DC component is the DC component of the transform coefficients obtained by the two-dimensional orthogonal transformation. It is sent to the encoding unit 28a, and only the AC component of the transform coefficient is sent to the normalization unit 16 and is normalized. The output from the normalization unit 16, that is, the AC component of the normalized conversion coefficient,
It is sent to the AC component encoding unit 28b. DC component encoder 28a
And the AC component encoder 28b is 2 in the device of FIG.
Corresponds to the three-dimensional Huffman coding unit 28, and DC component coding 28a
Then, the DC component of the conversion coefficient is encoded, and the AC component encoding unit 28b encodes the normalized AC component of the conversion coefficient.

The outputs of the DC component encoder 28a and the AC component encoder 28b are sent to the multiplexer 50. The multiplexer 50 sequentially selects the encoded DC component data from the DC component encoding unit 28a or the encoded AC component data from the AC component encoding unit 28b in response to a control signal from a control unit (not shown) and outputs it. Output to terminal 32.

In this device, the block activity calculator
The output from 20 is limited to the maximum value by the high limiter 46. By limiting the maximum value of the activity for each block in this way, it is possible to limit the activity value of a very large block of activity and prevent an increase in the scale of the device. Block activity calculator
The output from 20 is limited to the maximum value by the high limiter 46, and then input to the total activity calculation unit 26.

The total activity calculation unit 26 is the same as the device of FIG.
The total activity is calculated by adding the total activity for each block from the block activity calculation unit 20. The output of the total activity calculation unit 26 is limited to a minimum value by the row limiter 48. By limiting the minimum value of the total activity in this way, the maximum value of the distribution bit number described later is limited.

The high limiter 46 and the low limiter 48 may be omitted in some cases.

The output from the total activity calculation unit 26 is sent to the multiplier 22a of the normalization coefficient setting unit 22. The multiplier 22a multiplies the total activity data (total ACT) input from the total activity calculation unit 26 by a predetermined coefficient K1 to calculate K1 · total ACT. The output of the multiplier 22a is sent to the adder 22b, and the adder 22a
In b, it is added to the predetermined coefficient K2 to calculate K1 · total ACT + K2. The normalization coefficient setting 22 thus sets the normalization coefficient α from the total activity by using the coefficients K1 and K2, and outputs the normalization coefficient α to the normalization unit 16.

The output from the total activity calculating unit 26 is also sent to the divider 40a of the bit allocation calculating unit 40. The divider 40a has
"Total bits-DC bits" are input from the AC component bit number storage unit 40c. Here, “total bits” is the number of bits allocated for encoding the entire normalized transform coefficient,
“DC bit” is the number of bits allocated for encoding the DC component of the transform coefficient. Therefore, "total bits-DC bits" represents the number of bits allocated for encoding the AC component of the normalized transform coefficient. The divider 40a, the "total bits-DC bits" from the AC component bit number storage unit 40c,
It is divided by the total activity data sent from the total activity calculation unit 26. As a result, [total bit-DC bit] / total activity is calculated.

The output from the divider 40a is sent to the multiplier 40b. Activity data for each block is also input from the block activity calculation unit 20 to the multiplier 40b.
The multiplier 40b is input from the divider 40a.

The [total bit-DC bit] / total activity is multiplied by the activity for each block input from the block activity calculation unit 20 to calculate [total bit-DC bit] / block activity / total activity. As a result, [total bits-DC bits], that is, the number of bits allocated to the coding of the AC component is distributed according to the ratio of the activity of each block to the total activity, and the number of bits allocated to the coding of the AC component is obtained for each block. . The output from the multiplier 40b is output to the coding stop determination unit 42.

In this device, the output from the AC component encoding unit 28b is output to the code amount counting unit 44. The code amount counting unit 44 counts the code amount of the AC component coded by the AC component coding unit 28b and outputs it to the coding stop determination unit 42. The coding stop determination unit 42 compares the code amount counted by the code amount counting unit 44 with the number of bits allocated by the multiplier 40b, and the number of bits of the counted code amount exceeds the allocated number of bits. , The encoded output from the AC component encoding unit 28b is stopped up to the preceding code which exceeds the allocated number of bits.

As described above, according to the present embodiment, the transform coefficient obtained by the two-dimensional orthogonal transform is divided into a DC component and an AC component. The DC component data is sent to the DC component encoding unit 28a and encoded, and since the encoded data amount of the DC component is constant, it is output to the multiplexer 50 without limiting the number of bits. On the other hand, the AC component data is normalized by a normalization coefficient corresponding to the activity of each block, and then encoded by the AC component encoding unit 28b and output by the number of bits allocated according to the activity of each block. The number of bits used is limited. Bit number allocation is
AC component bits are assigned according to the activity of each block. In this way, the number of bits allocated to the AC component, which is the total number of bits minus the number of bits allocated to the DC component, is distributed according to the activity of each block. Can be set.

Also in this embodiment, the normalization coefficient α or α ·
When the conversion coefficient is renewed by T and normalization is performed, if 1 / α or 1 / (α · T) is calculated and this value is multiplied by the conversion coefficient, the number of dividers can be reduced. Therefore, the scale of the device can be reduced.

In each of the above-described embodiments, the activity for each block is calculated from the pixel data forming the image. However, the data of the transform coefficient orthogonally transformed by the two-dimensional orthogonal transforming unit 14 is input to the block activity calculating unit 20. ,
The activity may be calculated by the following formula using the data of the conversion coefficient.

However, ii = jj ≠ 0.

Further, the activity may be calculated by weighting the orthogonally exchanged transform coefficients as in the following equation.

This weight W (ii, jj) improves the image quality by increasing the bit allocation of a block including a large number of low frequency components, like the weight T () when calculating the activity from the pixel. is there.

Further, when the block activity calculation unit 20 calculates the activity for each block, the obtained block activity ACT (i, j) may be subjected to the limiter conversion T1 as in the following equation.

ACT ′ (i, j) ＝ T1 (ACT (i, j)) By such conversion, the activity of the block
ACT (i, j) is a predetermined threshold value, for example, 20 as shown in FIG.
Limited at 48. Like this activity ACT
By limiting the upper limit of (i, j), the number of bits of total activity of the entire image can be reduced.

When the input image data is a color image signal, compression encoding is performed for the luminance signal Y and the color difference signals RY, B.
-Y for each. Allocation of the bit number of the encoded output in this case will be described with reference to the flowchart of FIG.

First, for each of the luminance signal Y and the color difference signals RY and BY, the activity for each block is obtained in the same manner as described above (202), and these are added to obtain the luminance signal Y.
Then, the total activity of the entire image data composed of the color difference signals RY and BY is obtained (204). Based on the ratio of the activities of the luminance signal Y and the color difference signals RY and BY to the total activity, the distribution of bits to the blocks of the luminance signal Y and the color difference signals RY and BY is calculated in the same manner as above. (206).

Normalization factor α for the entire image based on total activity
Is calculated (212). The luminance signal Y is first compression-encoded by the normalization coefficient α, and the encoded data is output (214). The number of bits of the encoded output is counted (216), the bit allocation assigned to each block of the luminance signal Y is compared with the bits of the encoded output (208),
When it is detected that the bits of the coded output exceed the bits allocated to each block of the luminance signal Y, the output is stopped up to the code one before the same as described above (210). It is determined whether or not the processing of all blocks has been completed (218). In this case, since the processing of the color difference signals RY and BY remains, the surplus bits in the encoding of the luminance signal Y are removed. It is calculated (220), and the process returns to step 206 and is added to the bit allocation allocated to each block of the color difference signal RY.

In this way, the color difference signal R-Y is compression-encoded by the bit allocation with the extra bits added (214).
The encoded output is counted (216), the bit allocation allocated to each block of the color difference signal RY calculated by adding the remainder bits of the luminance signal Y and the bit of the encoded output are compared (208), When it is detected that the coded output bits exceed the allotted bits, the output is stopped until the code immediately before that bit (210). Further, the color difference signal R
Calculate the extra bits in the Y encoding (220),
Add to the bit allocation assigned to each block of the color difference signal BY. (206).

Similarly, the color-difference signal BY is compression-encoded by bit allocation with the extra bits added (214). In the same manner as described above, the coded output is stopped one bit before the allocated bit is exceeded (210).

In this way, the color image signal is converted into a compressed signal.

As described above, in compression encoding of the color image signal, the remainder of the bits allocated to the encoding of the luminance signal Y is used for the encoding of the color difference signal R-Y, and is allocated to the encoding of the color difference signal R-Y. By using the bit remainder for encoding the color difference signal BY, the output of the encoded data can be made a constant amount in the compression encoding of the color image signal. Moreover, since the bit remainders allocated to the encoding of the luminance signal Y are used for encoding the color difference signal, the information amount of the color difference signal can be increased and the color data can be output with high image quality. it can.

Further, in this case, since the luminance signal Y is output first in the compression-encoded data and is recorded in the recording medium first, the luminance signal Y is not changed even if the length of the encoded data is variable. Since the address at which the signal Y is recorded is fixed, it is advantageous for searching the luminance signal Y from the recording medium.

Contrary to the operation of the flowchart of FIG. 13, the encoding process of the color difference signals RY and BY is performed first, and the remaining bits in the encoding of the color difference signals RY and BY are replaced with the luminance signal. It may be added to the bits allocated in the Y encoding. Even in this case, the output of the encoded data can be set to a constant amount in the compression encoding of the color image signal. In addition, since the surplus bits in the encoding process of the color difference signals RY and BY are added to the bits distributed in the encoding of the luminance signal Y, the luminance signal Y
The amount of information can be increased, and image data close to a monochrome image with little color information can be output with high image quality.

Effect According to the present invention, the compression encoding device performs normalization by setting a normalization coefficient from the total value of activity of blocks in the normalization after orthogonal exchange. Therefore, it is possible to perform compression encoding according to the frequency component of the image and make the encoded data amount constant.

Moreover, since the data amount of the coded output of each block is limited by the activity of each block with respect to the total value of the activity, each block can output the data amount according to the frequency component.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an image signal compression / encoding apparatus according to the present invention, FIG. 2 is a diagram showing an example of pixel data constituting a block, and FIG. 3 is a sub-block of the block. FIG. 4A to FIG. 4C are diagrams showing an example of a filter used to calculate an activity, FIG. 5 is a diagram showing a method of calculating an activity by the filter, and FIG. FIG. 7 is a diagram showing an example of pixel data forming a block, FIG. 7 is a diagram showing an example of conversion performed in calculation of a block activity, FIG. 8 is a diagram showing an example of conversion of a block activity, and FIG. 9A. And FIG. 9B is a diagram showing an example of a lookup table for converting the total value of activities into a normalization coefficient, FIG. 10 is a diagram showing an example of weight table data, and FIG. 11 is a run length and non-zero. Of the amplitude of FIG. 12 is a flowchart showing the order of encoding, FIG. 12 is a flowchart showing an operation relating to bit allocation of the apparatus of FIG. 1, FIG. 13 is a flowchart showing an operation relating to bit allocation of a color image signal, and FIG. FIG. 9 is a block diagram showing another embodiment of the image signal compression encoding apparatus according to the present invention. Description of codes of main parts 14 …… two-dimensional orthogonal transformation unit 16 …… normalization unit 20 …… block activity calculation unit 22 …… normalization coefficient setting unit 26 …… total activity calculation unit 28 …… two-dimensional Huffman code Encoding unit 28a ...... DC component encoding unit 28b …… AC component encoding unit 40 …… Bit allocation calculation unit 40c …… AC component bit number storage unit 42 …… Encoding stop determination unit 44 …… Code amount counting unit 50 ...... Multiplexer

Claims (6)

(57) [Claims] 1. An image signal compression encoding apparatus which divides digital image data constituting one screen into a plurality of blocks and performs two-dimensional orthogonal transform encoding on the image data of each block, wherein the apparatus comprises Orthogonal transformation means for two-dimensionally orthogonally transforming digital image data divided into a number of blocks, normalization means for normalizing the data orthogonally transformed by the orthogonal transformation means, and data normalized by the normalization means An encoding means for encoding, a block activity calculating means for calculating the activity of the image data for each of the divided blocks, and an activity adding means for adding the activity for each block calculated by the block activity calculating means, For each block calculated by the block activity calculation means From the ratio of the activity and the total value of the activities calculated by the activity adding means, the encoded data amount distributing means for calculating the encoded data amount distributed for each block, and the encoded data amount distributing means. And an encoding output control means for limiting the output data amount from the encoding means according to the output of the encoding output control means, the encoding output control means, for each block output from the encoding data amount distribution means. The distributed encoded data amount is compared with the output data amount from the encoding unit so that the output data amount from the encoding unit falls within the range of the encoded data amount distributed for each block. An image signal compression / encoding apparatus characterized in that the amount of output data from the encoding means is limited. 2. The apparatus according to claim 1, wherein the coded data amount distribution means multiplies the coded data amount of the entire image data by the ratio of the activity for each block and the total value of the activities. An image signal compression encoding apparatus, wherein the encoded data amount distributed to each block is calculated by adding the remaining data amount of the previously encoded block. 3. The apparatus according to claim 1, wherein the encoded data amount distribution means subtracts the encoded output data amount of the already encoded block from the encoded data amount of the entire image data. An image signal compression encoding apparatus, wherein a value is multiplied by a ratio of an activity for each block and a total value of the activities to calculate an encoded data amount to be distributed for each block. 4. The apparatus according to claim 1, wherein the coded data amount distribution unit calculates a ratio between the activity of each block and the total value of the activities,
An image signal compression coding apparatus, characterized in that a coded data amount of AC component data distributed for each block is calculated by multiplying a coded data amount of AC component data of the orthogonally transformed data. 5. The apparatus according to claim 4, wherein the encoded data amount allocating means adds the AC component of the block previously encoded to the encoded data amount of the AC component data distributed for each block. An image signal compression coding apparatus, characterized in that a coding data amount of AC component data distributed for each block is calculated by adding a residual data amount of data. 6. The apparatus according to claim 4, wherein the coded data amount distribution means is a block of AC component data that has already been coded and output from a coded data amount of AC component data of the orthogonally transformed data. To calculate the encoded data amount of the AC component data distributed to each block by multiplying the value obtained by subtracting the encoded output data amount of the above by the ratio of the activity for each block and the total value of the activities. An image signal compression / encoding device characterized by.