JPH0548907A - Encoder - Google Patents

Abstract

PURPOSE:To count the predicted value only by a combining circuit and to miniaturize a device by weighting and adding to the peripheral plural picture elements of the encoding object picture element and obtaining the predictive value of the encoding object picture element based on it. CONSTITUTION:Image data to be compressed are stored in an image data memory 32. The weight is respectively added to plural peripheral picture elements of the picture element to be encoded which is read from the device 32 by a picture element predictive value generator 33, the result is compared with a threshold, the predictive value is generated, and outputted to a predictive error counter 34. The device 34 outputs the predictive value error between the picture element value to be encoded which is received from the device 32 and the predictive value output from the device 33 to an entropy encoder 35, encodes it here, and successively stores it in an encoding memory 36. Here, the predictive error is predicted, the value is deviated to '0', and since as for the entropy encoding, the encoding deviated to the value to be encoded has the small number of the prepared codes, the image data can efficiently be more compressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an encoding device used for image data compression processing for compressing image data at a pixel level, and more particularly to an encoding device capable of performing data compression with a small circuit scale. Regarding the device.

[0002]

2. Description of the Related Art A process for reducing the data amount of an original image signal by removing redundant signal components contained in the image signal is performed by a high efficiency coding (hihg-efficiency co-coding) of the image.
ding), or simply picture coding. In addition, this technique is based on data compression coding, redundancy suppression coding, and bit rate reduction coding.
on coding) and so on.

An image that does not include a halftone signal component such as a document or a drawing and can be regarded as being made up of only black and white binary data is called a binary image. Binary images generally have large redundancy, and data compression is possible by the method of information source coding. Since the number of quantization levels in the amplitude direction is two, the image quality of a binary image may be determined by the sampling density.

The most important data compression encoding is
Predictive coding and transcoding
form coding and vector quantiza
tion). Often, these two and three parties are combined (hybrid coding), and entropy coding (variable-length coding, Huffman coding) is a powerful means of increasing the data compression effect by combining the above methods. Often used.

Among the entropy codings for removing redundancy, one of the most typical entropy codings is unequal length coding, which is a short code for a high output level and an appearance frequency. A long code is assigned to each low output level. As a result, the average code length of code output can be shortened. By performing entropy coding, the code generation rate is not constant. Since a general transmission line is designed to transmit information at a constant speed, it is not possible to send out information (codes) generated unevenly to the transmission line as it is. Therefore, the information is temporarily smoothed by the buffer storage and then transmitted as a signal. Further, in order to prevent the overflow and underflow of the buffer storage, the buffer storage occupancy rate is monitored, and the redundancy removal encoding parameter is controlled so that it is always within a certain range.

In the predictive coding, the past pixel value which has already been coded is stored in the memory in the encoder, and the predicted value (X) of the pixel X newly input from this is calculated. The difference (referred to as prediction error) X- (X) is quantized, the output level thereof is converted into an appropriate binary code, and the binary code is transmitted. Since the prediction value (X) can be reproduced by the same calculation on the receiving side, the prediction error is reproduced from the transmitted code, and this is added to the previously decoded image signal, and a new image signal is added. Can be restored. Since a highly accurate prediction is possible for a signal having a large correlation between sample values, the predicted value (X) becomes close to the actual signal X, and the prediction error becomes small. Therefore, it is possible to transmit with a smaller amount of information than the signal X, and high efficiency coding can be realized.

FIG. 8 is a block diagram of an image data compression apparatus 11 for compressing image data at a pixel level by encoding a prediction error. In FIG. 8, the image data compression apparatus 11 determines an address of a prediction value by an image data storage apparatus 12 that stores image data to be compressed and a value of several pixels around a pixel to be encoded (encoding target pixel). Address generator 13 to be designated and address generator 1
The pixel prediction value storage device 14 that outputs the prediction value (X) from the prediction value table 22 (FIG. 10) based on the value corresponding to the address designated by 3 and the encoding output from the image data storage device 12 And a prediction error calculation device 15 that calculates a prediction error based on the predicted value (X) output from the pixel prediction value storage device 14, and an entropy code that compresses the image data by encoding the calculated prediction error. And a code storage device 1 for sequentially storing generated codes.
7 and a control device 18 for controlling the operation of each of the above devices.

In the above configuration, the address generator 1
3 specifies the address of the prediction value table 22 (FIG. 10) by the values of several pixels around the pixel to be encoded, the pixel prediction value storage device 14 determines the value corresponding to the specified address based on the prediction value table. And outputs it to the prediction error calculation device 15 as a prediction value. The prediction error calculation device 15 calculates a prediction error between the pixel to be coded and the predicted value output, and the entropy coding device 16 compresses the image data by coding the calculated prediction error. The generated codes are sequentially stored in the code storage device 17.

In the image data compression device 11, 2
The operation until the prediction error is calculated for the value image (for example, facsimile) is as follows. FIG. 9 is a diagram showing the image data 21 of a document or the like and a predicted value target obtained by enlarging a part thereof. As shown in this figure, at the time of encoding, black and white (0 or 1) binary dot data is obtained by scanning the image data 21 from the upper left to the right. In such a case, the original data is not directly encoded, but as shown in the enlarged part of FIG. Predict the value and find the error from the prediction. That is, as shown in FIG. 9, a prediction value table (prediction code) in which the prediction values are statistically stored in the pixel prediction value storage device 14 based on the values of seven pixels around a to g in the upper left direction of the pixel X. The table) 22 is created, and the address of the prediction value table 22 is designated by the values of the seven pixels around the above a to g to obtain the corresponding prediction value (X). The predicted value table 22 is for statistically previously calculating and storing predicted values (X) of 0 to 1 corresponding to all combinations of pixel values 0000000 to 1111111 of a to g. When the image data 21 is scanned and there are many 0s (for example, white) around the pixel X, it is assumed that the dot data to be encoded also has many whites, so the prediction value (X) is set to 0 (white). .. On the contrary, in the case of solid painting, the probability of being 1 (for example, black) is assumed to be high, so the prediction value (X) is set to 1 (black). Thus, the optimum predicted value (X) corresponding to the pixel values a to g is statistically obtained in advance and stored in the ROM or the like.
(X) and the pixel X are input to the EX-OR (exclusive OR) circuit 23 shown in FIG. 11 to take the EX-OR logic to calculate the prediction error. As a result of EX-OR, "0" is set if the prediction is correct, and "1" is set if the prediction is incorrect. As long as the prediction is correct, there will be many "0" s. If this data with many "0s" is passed through the encoding device 16 to be encoded, this is data with a large amount of bias, so considerable compression can be expected. In this way, if the predicted value (X) based on the pixel values of a to g matches X, the compression rate becomes high.

[0010]

However, in such a conventional image data compression apparatus, the predicted value based on the predicted value table is stored in the predicted value storage device. In order to obtain the storage capacity, a storage device (eg, ROM, RA
M) is required, which causes a problem that the circuit scale of the entire device becomes large. Therefore, it is an object of the present invention to provide a coding device which can eliminate the need for a prediction value storage device and a device for designating its address, and can perform data compression with high accuracy on a small circuit scale. ..

[0011]

In order to achieve the above object, an encoding apparatus according to the present invention provides a data supplying means for supplying data to be encoded and a plurality of pixels around a pixel to be encoded with predetermined weights. Then, the prediction value generating means for generating the prediction value of the encoding target pixel based on the addition result, the encoding target pixel output from the data storage means, and the prediction value generated by the prediction value generating means Prediction error calculating means for calculating a prediction error based on the above, and encoding means for encoding the prediction error calculated by the prediction error calculating means. The predictive value generating means includes a weighting means for weighting a plurality of surrounding pixels of the pixel to be coded with a predetermined weight, an adding means for adding the values of the surrounding plurality of pixels weighted by the weighting means, and an adding means It may be configured by a comparison unit that compares the output with a predetermined value and generates a prediction value of the pixel to be encoded.

[0012]

The operation of the means of the present invention is as follows. A plurality of pixels around the pixel to be coded are respectively weighted by the prediction value generating means and then added, and the prediction value is output using the addition result and a predetermined threshold value. Then, the prediction error is calculated by the prediction error calculation unit based on the encoding target pixel supplied from the data supply unit and the prediction value output from the prediction value generation unit. The encoding means encodes the prediction error calculated by the prediction error calculating means. Therefore, the predicted value of a pixel can be obtained by a combinational circuit without using a sequential circuit such as a storage device, so that the size of the entire device can be reduced.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. First Embodiment FIGS. 1 to 6 are views showing a first embodiment of an encoding apparatus according to the present invention, which is an example applied to an image data compression apparatus.
In this figure, an image data compression device 31 adds an image data storage device 32 for storing data to be compressed and values of several pixels around a pixel to be encoded (encoding target pixel) by weighting them and adding them. , A prediction error due to the pixel prediction value generation device 33 that generates a prediction value from the result, the value of the pixel to be encoded output from the image data storage device 32, and the prediction value output from the pixel prediction value generation device 33. A prediction error calculation device 34, an entropy coding device 35 that compresses the image data by coding the calculated prediction error, a code storage device 36 that sequentially stores the generated codes, and The control device 37 controls the operation.

FIG. 2 is a circuit diagram of the pixel prediction value device 33. In FIG. 2, the pixel prediction value generation device 33 includes several pixels around the pixel to be encoded (pixels a _{1 to} a _{n -1} )
Of the weighting coefficient Wk (where 1 ≦ K ≦
n−1, or n = 2 ¹ ) with predetermined bits (m bits) to output the weighting factor generation circuits 41 to 44, the peripheral pixels a _{1 to} a _{n−1 of} the pixel to be encoded, and the peripheral pixel a. ₁ ~
a _n-1 weighting coefficient corresponding to W ₁ ~W _n-1 are respectively input,
A gate circuit 4 for controlling whether or not to transmit the value (0 or 1) m-bit weighting coefficient Wk of the peripheral pixels a _{1 to} a _n-1.
5 to 48 and m bits (n- of gate circuits 45 to 48).
1) Adder 49 that adds inputs and outputs as (m + 1) bit 1 output, and threshold generation circuit 50 that generates (m + 1) bit threshold value for determining a predicted value of (m + 1) bit output And the comparator 51 which compares the result added by the adder 49 with the threshold value from the threshold value generation circuit 50 and outputs the predicted value (X) = 1 when the addition result is equal to or more than the threshold value. It is configured.

The weighting factors Wk generated by the weighting factor generators 41 to 44 are for weighting the values of the peripheral pixels a _{1 to} a _n-1 of the pixel X to be encoded, respectively. If a _{1 to} a _n−1 are, for example, n = 8, then a ₁ to
next a _7, corresponding to around 7 pixels a~g of FIG 9.
In this case, the weighting factor W _{1 is applied to} each of the seven pixels surrounding a to g.
Set ~ W ₇ (see Figure 3). Then, the weighting factor W ₁ ~
In the case of a black-and-white binary image as a weighting method by W ₇ , it is considered that the closer the surrounding pixels are to the pixel X to be encoded, as shown in FIG. 3, the stronger the correlation. According to this concept, the c and g pixels are closest to the pixel X among the seven pixels around a to g, the b, d and f pixels are next, and the a and e pixels are the farthest. As shown in FIG. 4, the weighting factors W ₃ and W ₇ of the c and g pixels are set to “4”, and the weighting factors W ₂ , W ₄ and W ₆ of the b, d and f pixels are set to “2” as the correlations are strong in order. , The weighting factors W ₁ and W ₅ of the a and e pixels are set to “1”, respectively.

The gate circuits 45 to 48 are provided for the peripheral pixels a.
_{It is} composed of (n-1) AND gate circuits provided corresponding to ₁ to an _-1 , and in the above example, the value (0 or 1) of 7 pixels around a to g is input by 1 bit. To be done. Therefore, depending on the value of each pixel a to g, an m-bit weighting factor W ₁
It is determined whether ~ W ₇ is output to the adder 49.

The adder 49 is a weighted m
It is an adder that adds all bit inputs and outputs as (m + 1) bit output. The threshold value generating circuit 50
Is a circuit for generating a predetermined threshold value of (m + l) bits, and when the above weighting coefficient is used, the threshold value is set to “7”, for example. The comparator 51 is the adder 4
It is a comparator or the like for comparing the output of 9 and the output of the threshold generation circuit 50.

Next, the operation of this embodiment will be described. Overall Operation First, the image data storage device 32 stores image data to be compressed. The pixel prediction value generation device 33 adds the weights W _{1 to} W ₇ to the values of the seven pixels a to g around the pixel X to be encoded read from the image data storage device 32 and adds them, and the result is thresholded. The predicted value (X) is generated by comparison with the value and output to the prediction error calculation device 34.

The prediction error calculation device 34 calculates the prediction error between the value of the pixel X to be encoded received from the image data storage device 32 and the prediction value (X) output from the pixel prediction value generation device 33 as an entropy coding device. To 35. The entropy coding device 35 codes the received prediction error, and the coding storage device 36 codes the entropy coding device 35.
The codes generated by are sequentially stored. Here, the prediction error is biased toward the value of 0 because the prediction is correct, and
In entropy coding, when the values to be coded are biased, the number of generated codes is small, that is, the efficiency is high, and the image data is compressed more.

Prediction Value Calculation Operation in Pixel Prediction Value Generation Device 33 First, the weighting coefficient generation devices 41 to 44 output weighting coefficients W _{1 to} W ₇ to the gate circuits 45 to 48 in m bits. The gate circuits 45 to 48 take AND logic as to whether or not to transmit the values of the weighting factors W _{1 to} W ₇ to the adder 49 according to the values of a to g (0 or 1). As described above, a _{1 to} a ₇ when n = 8 in FIG. 2 correspond to a to g in FIG. 9, and the weights in FIGS. 3 and 4 correspond to the pixels in a to g. shall coefficients W ₁ to _W-7 is set.

Pixels a weighted by weighting factors W _{1 to} W _7.
The values of ~ g are input to the adder 49, and the adder 49 adds all these inputs and outputs the addition result to the comparator 51. Therefore, for example, when the value of the seven pixels around a to g is "0", the value of the adder 49 becomes "0", and when only the c pixel has "1" and the other pixels have "0". “4” according to the weighting factor W ₃ of the c pixel is output from the adder 49.

The comparator 51 compares the output of the adder 49 with the output of the threshold value generating circuit 50. When the output of the adder 49 is equal to or more than the output of the threshold value generating circuit 50, the predicted value (X) = 1. Otherwise, outputs (X) = 0. That is, similarly to the conventional method of FIG. 9, the image data 21 is scanned from the upper left to the right, and addition is performed by the adder 49 from the values of a _{1 to} a _n−1 , and the result is as shown in Expression 1.

[0023]

[Equation 1]

Then, the comparator 51 compares the addition result expressed by the equation 1 with the threshold value output of the threshold value generating circuit 50 and outputs the predicted value (X). A prediction error for the value X of the pixel to be coded is obtained from this prediction value (X), and this is input to the entropy coding device 36 for coding.

In FIG. 5, the pixels around the pixel X to be encoded are 7 pixels around a to g, and the weights corresponding to the pixels a to g are Wa = 1, Wb = 2, Wc = 4. Wd =
2, We = 1, Wf = 2, Wg = 4 (see FIG. 4),
Further, it is an output example of the prediction value table when the threshold value is set to "7", and FIG. 6 is an output example of the prediction value table when the threshold value is set to "8".

As a result of data compression using the output examples of the prediction value tables shown in FIGS. 5 and 6, a compression effect equal to or higher than that obtained when using the prediction value table stored in the conventional ROM or the like is obtained. I was able to.

As described above, the image data compression device 31 of the present embodiment has the image data storage device 32 for storing the data to be compressed and several pixels around the pixel to be encoded (encoding target pixel). Of the pixel values and the pixel prediction value generating device 33 that generates a prediction value from the result, and the value of the pixel to be encoded output from the image data storage device 32 and the pixel prediction value generating device. A prediction error calculation device 34 that calculates a prediction error based on the prediction value output from 33, and an entropy coding device 35 that compresses the image data by coding the calculated prediction error.
Since the prediction value is generated by the combination circuit such as the adder 49 and the comparator 51, the storage device (image prediction value storage device 14 of FIG. ) And a device (address generating device 13 in FIG. 8) for designating an address of the storage device can be eliminated, and the scale of the entire device can be significantly reduced. Further, since the image data compression apparatus having a small circuit scale can be realized in this way, it is possible to reduce the size of a device incorporating the apparatus.

Second Embodiment FIG. 7 is a diagram showing a second embodiment of the coding apparatus, which is an example applied to the image data compression apparatus as in the first embodiment. In this embodiment, the circuit configuration of the pixel prediction value generation circuit 33 is simplified. The same components as those of the first embodiment shown in FIG. 2 are designated by the same reference numerals, and the description of the overlapping portions will be omitted.

First, the structure will be described. FIG. 7 is a circuit configuration diagram of the pixel prediction value generator 61. In the first embodiment, the threshold value can be any value of (m + 1) bits, but in the pixel prediction value generation device 61, the threshold value is a power of 2 (2 ^m ). By limiting the above, the threshold value generating circuit 50 and the comparator 51 shown in FIG. 2 can be replaced with the logical sum of the upper bits of the output of the adder 45 by using the OR gate circuit 62. The pixel prediction value generator 61 of FIG. 7 has a threshold value of 2 ^m ,
An example is shown in which the predicted value (X) is replaced with the logical sum of the upper (l + 1) bits of the output of the adder 49.

That is, in the first embodiment, all the bits of the adder 49 had to be output and set as the threshold value from the threshold value generating circuit 50, but the threshold value should be 2 (2
^If it is limited to ^m ), the bits lower than the threshold value of the output of the adder 49 will not be taken into consideration at all, and only the upper bits can be compared. For example, when the threshold value limited to 2 ^m is m = 3 (that is, the threshold value is 2 ³ = 8), if the value input to the adder 49 is 0 to 7, this is Since it is 0000 to 0111, all the bits higher than 2 ^m bits are 0 and no value is output. That is, when the output is 0 to 7, no value is output from the adder 49 up to 2 ^m (= 2 ³ ) bits. 100 when the input becomes 8
It becomes 00 and a value appears in the 2 ^m bit of the adder 49. Further, when the number of inputs is 8 or more, at least 1 exists in any of the upper bits, so that if the OR gate circuit 51 takes the logical sum of the outputs of the adder 46, the predicted value (X) = 1 is output. ..

Since the threshold value when m = 3 is "8", the prediction value table is the same as the prediction value table shown in FIG. As described above, in this embodiment, the threshold value generation circuit and the comparator can be omitted by limiting the threshold value to a power of 2, and the circuit scale can be further simplified.

In each of the above embodiments, the encoding device is applied to the image data compression device, but of course the invention is not limited to this, and it can be applied to any device as long as it uses a predicted value. Needless to say.

Further, the extraction method of the surrounding pixels of the pixel to be encoded, the number thereof, and the setting method of the weighting coefficient are not limited to those in the above-mentioned embodiment, and any method can be used. Needless to say, the numbers and types of circuits and members forming the image data compression devices 31 and 61 and the pixel prediction value generation devices 33 and 61 are not limited to those in the above-described embodiments.

[0034]

According to the present invention, a plurality of pixels around the pixel to be coded are weighted with predetermined weights and added, and the predicted value of the pixel to be coded is obtained based on the addition result. The predicted value of the pixel can be calculated only by the combinational circuit without using the storage device, and the size of the entire device can be reduced.

[Brief description of drawings]

FIG. 1 is a block configuration diagram of an encoding device.

FIG. 2 is a circuit configuration diagram of a pixel prediction value generation device of the encoding device.

FIG. 3 is a diagram for explaining a method of setting a weight coefficient of a weight coefficient generation circuit of the encoding device.

FIG. 4 is a diagram for explaining a method of setting a weight coefficient of a weight coefficient generation circuit of the encoding device.

FIG. 5 is an output example of a table of prediction values of the encoding device.

FIG. 6 is an output example of a table of prediction values of the encoding device.

FIG. 7 is a circuit configuration diagram of a pixel prediction value generation device of an encoding device according to another embodiment.

FIG. 8 is a block diagram of a conventional image data compression device.

FIG. 9 is a diagram showing conventional image data and pixels surrounding a pixel to be encoded.

FIG. 10 is a diagram showing a conventional prediction value table.

FIG. 11 is a circuit configuration diagram of a conventional prediction error calculation device.

[Explanation of symbols]

 31 image data compression device 32 image data storage device 33, 61 pixel prediction value generation device 34 prediction error calculation device 35 entropy coding device 36 coding storage device 37 control device 41-44 weighting coefficient generation circuit 45-48 gate circuit 49 addition 50 threshold generator 51 comparator 62 OR gate circuit

Claims (2)

[Claims] 1. A data supply unit for supplying data to be encoded, a plurality of pixels around a pixel to be encoded are weighted with predetermined weights, respectively, and added, and the pixel to be encoded is added based on the addition result. A prediction value generating means for generating a prediction value; a prediction error calculating means for calculating a prediction error based on the encoding target pixel supplied from the data supplying means and the prediction value generated by the prediction value generating means; An encoding device comprising: an encoding unit that encodes the prediction error calculated by the error calculating unit. 2. The prediction value generating means adds weighting means for weighting a plurality of surrounding pixels of a pixel to be coded to a predetermined weight respectively, and adding means for adding the values of the plurality of surrounding pixels weighted by the weighting means. The encoding device according to claim 1, further comprising: a comparison unit configured to compare an output of the addition unit with a predetermined value to generate a prediction value of a pixel to be encoded.