JP2006174331A - Image compression apparatus, image compression method, and program therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image compression apparatus for controlling a code quantity of an input image. <P>SOLUTION: In the case of prioritizing the processing speed, a compression program 5 uses a page target compression rate or a line target compression rate which is tighter (higher in the target compression rate) for a line reference rate, carries out the encoding processing while monitoring that the actually measured compression rate of each line is the line reference rate or over, in the case of prioritizing the image quality, the compression program 5 sets in advance a low lower limit of a compression rate by each line and carries out the encoding processing while suppressing deterioration in the image quality by the compression processing and monitoring the compression rate of the whole pages. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image compression apparatus that compresses image data.

For example, Patent Document 1 performs compression processing in a predetermined quantization step and stores the data in the first memory and the second memory. When the amount of data stored in the first memory reaches a predetermined amount, An image processing apparatus that sets a large quantization step so as to achieve a higher compression rate, re-encodes code data stored in the second memory in this quantization step, and overwrites the code data in the first memory Is disclosed.
JP 2003-008903 A

  The present invention has been made from the above-described background, and an object thereof is to provide an image compression apparatus that controls the code amount of an input image.

[Image compression device]
In order to achieve the above object, an image compression apparatus according to the present invention compresses an input image input as a compression target for each partial image that is a part of the input image, and compression by the compression unit. Partial data amount detecting means for detecting the data amount of the partial image thus obtained, partial data for comparing the data amount of the partial image detected by the partial data amount detecting means and a reference partial data amount set for the partial image An amount comparison unit; and a parameter setting unit that sets a compression parameter that defines compression processing by the compression unit in accordance with a comparison result by the partial data amount comparison unit.

  Preferably, the reference partial data amount is a value corresponding to a higher target compression ratio among the target compression ratio of the partial image and the target compression ratio of the entire input image, and the parameter setting means Compression that can achieve a higher compression ratio than the set compression parameter when the data amount comparison means determines that the data amount of the partial image exceeds the reference partial data amount set for this partial image. Change to parameter.

  Preferably, the compression unit applies the compression parameter changed by the parameter setting unit when the partial data amount comparison unit determines that the data amount of the partial image exceeds the reference partial data amount. Then, the compression process for the partial image is performed again.

  Preferably, the entire data amount detecting means for detecting the data amount of the entire input image compressed by the compressing means, the data amount of the entire input image detected by the entire data amount detecting means, and the entire input image are set. And an overall data amount comparison means for comparing the reference overall data amount, wherein the parameter setting means is responsive to the comparison result by the overall data amount comparison means and the comparison result by the partial data amount comparison means. To set the compression parameter.

  Preferably, the sum of the initial values of the reference partial data amount is larger than the target data amount of the input image, and the reference portion set for each partial image according to the comparison result by the overall data amount comparison unit Further comprising reference value changing means for changing the data amount, the partial data amount comparing means compares the data amount of the detected partial image with the reference partial data amount changed by the reference value changing means, The parameter setting means changes the compression parameter according to the comparison result by the partial data amount comparison means.

  Preferably, when the partial data amount comparison unit determines that the data amount of the partial image exceeds the reference partial data amount set for the partial image, the parameter setting unit sets the compression that has been set. The reference value changing means changes the compression parameter so that the compression ratio is higher than the parameter, and the reference value changing means sets the whole reference amount data in which the entire input image data amount is set by the whole data amount comparison means. When it is determined that the amount exceeds the reference amount, the reference partial data amount set for each partial image is decreased.

  Preferably, the apparatus further includes parameter storage means including at least a limit compression parameter capable of realizing a compression ratio corresponding to the reference overall data amount for any partial image, and the parameter setting means includes the parameter storage means. A compression parameter to be set is selected from the stored compression parameters.

  Preferably, the parameter storage means stores a plurality of compression parameters having different realizable compression ratios, and the parameter setting means stores a realizable compression ratio from among the plurality of compression parameters for each partial image. Are selected in the order of compression parameters having higher realizable compression ratios.

  Preferably, the partial image corresponds to one line of the input image, the compression means compresses the input image in units of one line, and the partial data amount detection means is compressed by the compression means. The data amount for one line is detected, and the partial data amount comparison unit compares the data amount for one line detected by the partial data amount detection unit with the reference partial data amount for one line.

[Image compression method]
The image compression method according to the present invention compresses an input image input as a compression target for each partial image that is a part of the input image, and detects the data amount of the compressed partial image. Compare the partial image data amount with the reference partial data amount set for the partial image, and change the compression parameter that defines the compression process according to the comparison result between the partial image data amount and the reference partial data amount To do.

[program]
The program according to the present invention includes a step of compressing an input image input as a compression target for each partial image that is a part of the input image in an image compression apparatus including a computer, The step of detecting the amount of data, the step of comparing the data amount of the detected partial image with the reference partial data amount set for the partial image, and the comparison result between the data amount of the partial image and the reference partial data amount In response, the computer of the image compression apparatus executes a step of changing a compression parameter that defines the compression process.

  According to the image processing apparatus of the present invention, the code amount of an input image can be flexibly controlled.

[background]
First, in order to help understanding of the present invention, the background that led to the present invention will be described.
In order to supply raster data while maintaining the printing process speed in a printer or the like, it is necessary to control the amount of image data. As a method of controlling the data amount, there is a method of compressing pixel data by various compression processes. For example, in a printer that prints an image for each line, it is necessary to perform code amount control for each page and each line of the image. That is, the target compression rate for each page and each line is set independently, and it is necessary to satisfy both.
However, there are various requests for each printer, such as processing speed priority, image quality priority, or a compromise between them, and all these requests are not answered at present.

Further, for example, in Patent Document 1, in order to perform code amount control in units of pages in one pass, when the image is compressed and the memory is consumed up to a predetermined threshold, decoding of the previously stored image compressed data and A method has been proposed in which requantization is performed using new hardware and the subsequent image compression is resumed. In this method, when the memory is again consumed exceeding the threshold, the requantization process is repeated, and a code having a data amount determined in one pass can be generated.
However, with this method, depending on the amount of installed memory, the image pattern, or the threshold value, the number of re-quantizations increases as the head of the image increases, resulting in an increase in quantization error and causing deterioration in image quality. .
Also, assuming the case where the re-quantization process is performed exceeding the threshold at the end of image encoding, a large amount of new entropy decoding and re-quantization processes are required to reduce the processing time. Hardware resources are required.

  Therefore, the present invention provides a compression method that can flexibly respond to the demands of the system and the user without sacrificing image quality and hardware scale.

[Embodiment]
Next, an embodiment of the present invention will be described.
FIG. 1 is a diagram illustrating a hardware configuration of a printer apparatus 2 (image compression apparatus) according to the present invention, centering on a control apparatus 20.
As shown in FIG. 1, the printer device 2 includes a control device 20 and a printer main body 26. The control device 20 includes a control device main body 210 including a CPU 212 and a memory 214, a communication device 220, a user interface device (UI device) 230 including an LCD display device or a CRT display device and a keyboard / touch panel, and an HDD / CD device. And the like.

[Compression program]
FIG. 2 is a diagram illustrating a functional configuration of the compression processing program 5.
As shown in FIG. 2, the compression program 5 includes a compression unit 500, a parameter setting unit 540, a line compression rate calculation unit 550, a line compression rate comparison unit 560, a page compression rate calculation unit 570, a page compression rate comparison unit 580, and A reference value changing unit 590 is included.
The compression program 5 is supplied to the control device 20 via, for example, the recording medium 242 (FIG. 1), loaded into the memory 214, and executed. A part of the compression program 5 may be installed in the client terminal as driver software, and may realize the functions described in the present embodiment in cooperation with each other.

In the compression program 5, the compression unit 500 encodes an input image input as an encoding target for each partial image. In this example, a case where the input image is a page image corresponding to one page of printed matter and the partial image is an image (line image) of one scan line of the page image will be described as a specific example. The partial image is desirably an image obtained by equally dividing the input image.
In addition, the compression unit 500 encodes the partial image by an irreversible encoding method in accordance with the compression parameter set by the parameter setting unit 540 (described later).

The parameter setting unit 540 sets a compression parameter that defines the compression process performed by the compression unit 500. This compression parameter defines the compression rate by the compression process. The compression parameter of this example is information (for example, a quantization step, a scaling factor, etc.) that defines irreversibility in the compression process.
Further, the parameter setting unit 540 sets a compression parameter according to the comparison result from the line compression rate comparison unit 560. For example, the parameter setting unit 540 is a compression that can realize a higher compression rate when the compression rate (actual value) for each line is lower than a reference value (hereinafter referred to as a line reference value) set for one line. Change to parameter. In other words, the parameter setting unit 540 further reduces the code amount when the code amount (actual value) for each line exceeds the line reference value (the upper limit code amount) set for one line. Change to a compression parameter that can.

  The line compression rate calculation unit 550 calculates a value indicating the code amount for one line encoded by the compression unit 500. The value indicating the code amount is, for example, the code amount itself or the compression rate. The line compression rate calculation unit of this example calculates the compression rate of code data for one line encoded by the compression unit 500.

  The line compression rate comparison unit 560 compares the compression rate (actually measured value) for one line calculated by the line compression rate calculation unit 550 with a line reference value set for one line, and compares the comparison result with a parameter. Output to the setting unit 540.

  The page compression rate calculation unit 570 calculates a value indicating the code amount of one page encoded by the compression unit 500 (that is, the entire input image). The page compression rate calculation unit 570 of this example calculates the compression rate of the code data for one page encoded by the compression unit 500.

  The page compression rate comparison unit 580 compares the compression rate (actual value) for one page calculated by the page compression rate calculation unit 570 with a reference value (hereinafter referred to as a page reference value) set for one page. The comparison result is output to the reference value changing unit 590. This page reference value is a value corresponding to a target code amount or compression rate for one page.

  The reference value changing unit 590 changes the line reference value according to the comparison result by the page compression rate comparison unit 580. For example, the reference value changing unit 590 increases the line reference value (lower limit of the compression rate for each line) when the compression rate (actual value) for one page is lower than the page reference value (target compression rate). . In other words, when the code amount (actual value) for one page exceeds the page reference value (target code amount), the reference value changing unit 590 has a line reference value (upper limit of code amount for one line). Pull down.

FIG. 3 is a diagram illustrating a functional configuration of the compression unit 500.
As illustrated in FIG. 3, the compression unit 500 includes a filter processing unit 510 and a lossless compression unit 520.
The filter processing unit 510 is a block that performs pixel value conversion processing according to the compression parameter, and includes a first prediction unit 512a, a second prediction unit 512b, a pixel value change processing unit 514, and an error distribution processing unit 516. In addition, although this embodiment demonstrates the form which has the two prediction parts 512 (1st prediction part 512a and 2nd prediction part 512b), the prediction part 512 should just be one or more, for example, five prediction parts. 512 may be configured.

The first prediction unit 512a and the second prediction unit 512b each predict the pixel value of the target pixel based on the image data by a predetermined method, and output the predicted pixel value to the pixel value change processing unit 514 as a predicted value. For example, the first prediction unit 512a and the second prediction unit 512b use pixel values of pixels at different positions as prediction values of the target pixel.
The pixel value change processing unit 514 compares the pixel value of the target pixel with the predicted value, and if the difference is smaller than a preset change allowable value (compression parameter in this example), the pixel value change processing unit 514 In addition, the difference between the pixel value of the target pixel and the predicted value (hereinafter referred to as an error value) is output to the error distribution processing unit 516. On the other hand, when the difference between the pixel value of the target pixel and the predicted value is equal to or greater than the change allowable value, the pixel value change processing unit 514 outputs the pixel value of the target pixel to the lossless compression unit 520 as it is, and performs error distribution. 0 is output to the processing unit 516. That is, when the error value is greater than or equal to the change allowable value, the filter processing unit 510 does not convert the pixel value of the target pixel and does not distribute the error value that is greater than or equal to the change allowable value. In other words, as the change allowable value (compression parameter) is larger, the pixel value of the input image is changed, the prediction hit rate by the lossless compression unit 520 in the subsequent stage is improved, and the compression rate is increased.
The error distribution processing unit 516 generates an error distribution value based on the error value input from the pixel value change processing unit 514, and adds this to the pixel value of a predetermined pixel included in the image data. The error distribution value is calculated by multiplying the error value by the value of the weight matrix, for example, according to an error diffusion method using a weight matrix or an average error minimum method.

  In this way, the filter processing unit 510 converts pixel values included in the image data so that the subsequent lossless compression unit 520 can easily compress the image data. At that time, the filter processing unit 510 distributes the difference from the true pixel value generated by the change of the pixel value to the peripheral pixels, thereby making the change of the pixel value macroscopically inconspicuous.

The lossless compression unit 520 includes a first prediction unit 522a, a second prediction unit 522b, a prediction error calculation unit 524, a selection unit 526, a run counting unit 528, and a code generation unit 529.
The first prediction unit 522a and the second prediction unit 522b are configured to calculate the pixel value of the pixel of interest based on the image data by a predetermined method, respectively, in substantially the same manner as the first prediction unit 512a and the second prediction unit 512b in the filter processing unit 510. Predict and output to the selection unit 526 as a predicted value.
The prediction error calculation unit 524 predicts the pixel value of the target pixel based on the image data by a predetermined method, subtracts the prediction value from the actual pixel value of the target pixel, and outputs the prediction error value to the selection unit 526. Output.
The selection unit 526 detects a prediction match / mismatch in the target pixel from the image data and the prediction value. As a result, the selection unit 526 gives the identification number to the run counting unit 528 and the code generation unit 529 if there is a prediction unit 522 that has made the prediction, and the prediction error value if none has been made. Output.

The run counter 528 increments the internal counter by 1 when the identification number indicates the first predictor 522a. When the identification number indicates other than the first prediction unit 522a and the internal counter is not 0, the value of the internal counter is output as run data to the code generation unit 529.
When the run data and the prediction error value are given simultaneously, the code generation unit 529 first generates a code of the run data and then generates a code of the prediction error value. On the other hand, when only the identification number or the prediction error value is given, the code generation unit 529 generates a code for the identification number or the prediction error value.

  The code generation processing by the run counting unit 528 and the code generation unit 529 is assumed to have a high probability of hitting the first prediction unit 522a, but other compression methods may be used. For example, when a fixed-length code is given for the purpose of high-speed decoding or the like, the code generation unit 529 indicates that the first prediction unit 522a hits the binary number “01” and the second prediction unit 522b hits the hit. The binary number “10” and the fact that any prediction unit has been removed are respectively compressed with a binary number “00”, and the prediction error is compressed with a binary number of sign + 8 bits. In addition, when it is desired to improve the compression rate, the code generation unit 529 may generate a code using a variable length code such as Huffman compression or an arithmetic code. For example, in the case of a Huffman code in which a 1-bit code is given to the first predictor 522a that seems to have a high probability of occurrence, the binary number “0” indicates that the first predictor 522a has hit, and the second predictor 522b The fact that the prediction is out of prediction is encoded with the binary number “11”, and any prediction unit 522 encodes that the prediction is lost. Further, the code generation unit 529 may encode using an arithmetic code. In this way, a number of compression methods can be considered.

  Next, the operation of the compression program 5 when processing speed priority, image quality priority, or a compromise between these is required will be described.

[Process speed priority]
First, encoding processing when processing speed is prioritized will be described.
FIG. 4 is a flowchart for explaining the operation (S10) of the compression program 5 when the processing speed is prioritized.
In this case, the line reference value (lower limit of the compression rate for each line) is set in advance to a value equal to or higher than the higher target compression rate of the page or the target compression rate of the line.
In this case, since the page compression rate calculation unit 570, the page compression rate comparison unit 580, and the reference value change unit 590 do not need to operate, in the printer device 2 that performs only the operation with priority on the processing speed, the page compression The rate calculation unit 570, the page compression rate comparison unit 580, and the reference value change unit 590 are not required.

  As shown in FIG. 4, in step 100 (S100), the parameter setting unit 540 initializes the compression parameters set in the compression unit 500. More specifically, the parameter setting unit 540 sets the initial value of the change allowable value in the pixel value change processing unit 514 (FIG. 3).

  In step 110 (S110), the compression unit 500 encodes one line (hereinafter, a target line) of the input image according to the set compression parameter.

  In step 140 (S140), the line compression rate calculation unit 550 calculates the compression rate of the target line encoded by the compression unit 500.

In step 145 (S145), the line compression rate comparison unit 560 calculates the compression rate (actual value) of the target line calculated by the line compression rate calculation unit 550 and a preset line reference value (the compression rate of each line). (Lower limit).
The compression program 5 proceeds to the process of S155 when the compression rate (actually measured value) of the target line is equal to or greater than the line reference value, and performs the process of S150 when the compression rate of the target line is smaller than the line reference value. Transition.

In step 150 (S150), the parameter setting unit 540 sets, in the compression unit 500, a compression parameter that can realize a higher compression ratio than the currently set compression parameter. More specifically, the parameter setting unit 540 increases the value of the change allowable value set in the pixel value change processing unit 514 (FIG. 3).
The compression program 5 returns to the process of S110, applies the changed compression parameter (change allowable value), and performs the encoding process of the attention line again. That is, the compression program 5 repeats the encoding process of the target line while changing the compression parameter (change allowable value) until the compression rate of the target line becomes equal to or greater than the line reference value.

  In step 155 (S155), the compression program 5 determines whether or not the encoding process for all lines included in the input image has been completed. If the encoding process for all lines has been completed, the encoding process is performed. In other cases, the process returns to S100, and the next line is set as the target line, and the encoding process is performed.

  Thus, when giving priority to the processing speed, the compression program 5 uses the stricter one (the higher target compression ratio) of the page and line target compression ratios as the line reference value, and the actual compression ratio of each line. The encoding process is performed while monitoring so that is equal to or greater than the line reference value. Thereby, efficient encoding processing by one pass is realized.

[Image quality priority]
Next, an encoding process when image quality is given priority will be described.
FIG. 5 is a flowchart for explaining the operation (S12) of the compression program 5 when image quality is prioritized. It should be noted that among the processes shown in this figure, the same reference numerals are assigned to the processes that are substantially the same as those shown in FIG.
In this example, there is an upper limit for the code amount of the entire page, but a case where there is no upper limit for the code amount of each line will be described as a specific example.

  As shown in FIG. 5, in step 160 (S160), the line compression rate comparison unit 560 initializes the line reference value. The initial value of the line reference value (the lower limit of the compression rate for each line) is set to a value lower than at least one of the target compression rate of the page and the target compression rate of the line (that is, a loose condition). More preferably, the initial value of the line reference value (lower limit of the compression rate for each line) is set to a value lower than both the target compression rate of the page and the target compression rate of the line.

In S100, the parameter setting unit 540 initializes the compression parameters set in the compression unit 500.
In S110, the compression unit 500 encodes one line (target line) of the input image according to the set compression parameter.
In S140, the line compression rate calculation unit 550 calculates the compression rate of the target line encoded by the compression unit 500.

In S145, the line compression rate comparison unit 560 calculates the compression rate (actual value) of the target line calculated by the line compression rate calculation unit 550 and a preset line reference value (lower limit of the compression rate for each line). In comparison, when the compression rate (actual value) of the target line is equal to or greater than the line reference value, the compression program 5 proceeds to the process of S155, and when the compression rate of the target line is smaller than the line reference value, S150 Move on to processing.
In S150, the parameter setting unit 540 sets, in the compression unit 500, a compression parameter that can realize a higher compression rate than the currently set compression parameter, and the compression program 5 returns to the processing of S110.
In S155, the compression program 5 determines whether or not the encoding process for all lines included in the input image has been completed. If the encoding process for all lines has been completed, the process proceeds to S165. In other cases, the process returns to the process of S100, and the encoding process is performed using the next line as the target line.

  In step 165 (S165), the page compression rate calculation unit 570 calculates the compression rate (actual value) for one page compressed by the compression unit 500. For example, the page compression rate calculation unit 570 calculates the compression rate for one page based on the compression rate for one line calculated by the line compression rate calculation unit 550.

In step 170 (S170), the page compression rate comparison unit 580 calculates the compression rate (actual value) for one page calculated by the page compression rate calculation unit 570 and a preset page reference value (for one page). Compared with the lower limit of the set compression rate). For example, the page reference value is the target compression rate of this page.
The compression program 5 terminates the encoding process when the compression ratio (actual value) of the entire page is equal to or greater than the page reference value, and when the compression ratio of the entire page is smaller than the page reference value, the process of S175 Transition to processing.

In step 175 (S175), the reference value changing unit 590 raises the line reference value (lower limit of the compression rate for each line) set in the line compression rate comparison unit 560.
The compression program 5 returns to the process of S100, applies the changed line reference value (lower limit of the compression rate for each line), and performs the encoding process of the target line again. That is, the compression program 5 repeats the encoding process for the entire page while gradually increasing the lower limit of the compression ratio for each line until the compression ratio for the entire page becomes equal to or greater than the page reference value.

As described above, when giving priority to image quality, the compression program 5 sets the lower limit (line reference value) of the compression rate for each line low in advance, and suppresses image quality deterioration due to compression processing, while reducing the compression rate of the entire page. The encoding process is performed while monitoring. As a result, image quality deterioration due to lossy encoding can be suppressed as much as possible while paying attention to the limitation of the code amount for a page.
When there is an upper limit for the code amount of each line, the compression program 5 uses the initial value of the line reference value as the target compression rate of each line (the compression rate corresponding to the upper limit of the code amount of each line. ), It is possible to perform an encoding process while suppressing image quality deterioration while paying attention to the upper limit of the code amount of each line.

[Eclectic]
Next, an encoding process when the processing speed priority and the image quality priority are compromised will be described.
In the image quality priority encoding process, there is no upper limit to the number of times the encoding process is re-executed (fallback), and the processing time until the encoding process for the entire page is completed cannot be estimated.
Therefore, the compression program 5 in the case where the processing speed priority and the image quality priority are compromised prepares a predetermined number of compression parameters in advance and sets an upper limit on the number of selectable compression parameters, so that the entire page can be encoded. An upper limit can be set for the processing time until completion.

FIG. 6 illustrates a compression parameter table 542 stored in the parameter setting unit 540.
As illustrated in FIG. 6, the parameter setting unit 540 stores a compression parameter table 542 in which a plurality of compression parameters are registered. Each compression parameter registered in the compression parameter table 542 is a parameter that realizes a different compression rate for the same image, and is associated with a compression rate level. In this example, “level 1” having the lowest realizable compression rate is associated with “parameter 1”. In other words, “parameter 1” is a compression parameter (for example, the minimum change allowable value) that can be encoded with the highest image quality.
Further, “parameter 5” associated with “level 5” is a compression parameter (for example, the maximum change allowable value) capable of realizing the highest compression rate among the registered compression parameters. “Parameter 5” is a compression parameter (limit compression parameter) that can always realize a compression ratio equal to or higher than the target compression ratio of the line and the target compression ratio of the page.
The compression program 5 of this example applies the compression parameters in order from “level 1” in the compression parameter table 542, so that the target compression rate of the line and the target compression rate of the page are always set within the predetermined number of times. Can be realized.

  FIG. 7 is a flowchart for explaining the operation (S14) of the compression program 5 when the compression parameter table 542 is used. Note that, among the processes shown in this figure, the same reference numerals are given to the processes that are substantially the same as those shown in FIG.

  As shown in FIG. 7, in S160, the line compression rate comparison unit 560 initializes the line reference value. The initial value of the line reference value (lower limit of the compression rate for each line) is set to a value that is equal to or higher than the target compression rate of the line and lower than the target compression rate of the page.

  In step 180 (S180), the parameter setting unit 540 initializes the compression parameters set in the compression unit 500. More specifically, the parameter setting unit 540 selects the compression parameter with the lowest compression rate (that is, “parameter 1” corresponding to “level 1”) from the compression parameter table 542, and sends it to the compression unit 500. Set.

In S110, the compression unit 500 encodes one line (line of interest) of the input image by applying the set compression parameter.
In S140, the line compression rate calculation unit 550 calculates the compression rate of the target line encoded by the compression unit 500.

  In S145, the line compression rate comparison unit 560 calculates the compression rate (actual value) of the target line calculated by the line compression rate calculation unit 550 and a preset line reference value (lower limit of the compression rate for each line). In comparison, when the compression rate (actual value) of the target line is equal to or greater than the line reference value, the compression program 5 proceeds to the process of S155, and when the compression rate of the target line is smaller than the line reference value, S185 Move on to processing.

  In step 185 (S185), the parameter setting unit 540 selects the compression parameter at the next level of the currently set compression parameter from the compression parameter table 542 and sets it in the compression unit 500. The compression program 5 The process returns to S110.

In S155, the compression program 5 determines whether or not the encoding process for all lines included in the input image has been completed, and when the encoding process for all lines has been completed, the process proceeds to S165. In other cases, the process returns to the process of S180, and the encoding process is performed using the next line as the target line.
In step S165, the page compression rate calculation unit 570 calculates the compression rate (actual value) for one page compressed by the compression unit 500.
In S170, the page compression rate comparison unit 580 displays the compression rate (actual value) for one page calculated by the page compression rate calculation unit 570 and a preset page reference value (compression set for one page). The compression program 5 ends the encoding process when the compression ratio (actual value) of the entire page is equal to or greater than the page reference value, and the compression ratio of the entire page is the page reference. When the value is smaller than the value, the process proceeds to S190.

In step 190 (S190), the reference value changing unit 590 raises the level of the compression parameter selected as the initial value by one. That is, the reference value changing unit 590 prohibits the parameter setting unit 542 from selecting the lowest level compression parameter among the compression parameters (parameter groups registered in the compression parameter table 542) that are currently selectable. In response to this, the parameter setting unit 542 selects the compression parameter of the next level (the next level of the compression parameter whose selection is prohibited) and sets it in the compression unit 500.
The compression program 5 returns to the process of S110, applies the set compression parameter as an initial value, and redoes the encoding process of this attention line.

  Thus, the compression program 5 can set an upper limit for the number of fallbacks by selecting a compression parameter to be applied in order from a predetermined number of compression parameters registered in the compression parameter table 542.

Next, the encoding process (S110) in the flowchart will be described in more detail.
FIG. 8 is a flowchart for explaining the encoding process (S110) in the flowcharts of FIGS. 4, 5, and 7 in more detail.
As shown in FIG. 8, in step 112 (S112), the first prediction unit 512a and the second prediction unit 512b of the filter processing unit 510 (FIG. 3) set the pixels on the target line as the target pixels in the scan order, The pixel value of the target pixel is predicted with reference to the pixel values of peripheral pixels around the target pixel, and is output to the pixel value change processing unit 514 as a predicted value.
In step 114 (S114), the pixel value change processing unit 514 determines whether or not the difference between the pixel value of the target pixel and the predicted value is within a change allowable value (compression parameter). When the difference between the pixel value and the predicted value is within the predetermined value, the compression program 5 proceeds to the process of S116, and in other cases, the process proceeds to the process of S122.
In step 116 (S116), the pixel value change processing unit 514 outputs the predicted value closest to the pixel value of the target pixel to the lossless compression unit 520 as the pixel value of the target pixel, and the actual pixel of the target pixel. The difference between the value and the nearest predicted value is output to the error distribution processing unit 516.
In step 118 (S118), the error distribution processing unit 516 distributes the difference between the pixel value of the target pixel and the nearest predicted value to the peripheral pixels.

In step 122 (S122), the first prediction unit 522a and the second prediction unit 522b in the lossless compression unit 520 predict the pixel value input from the filter processing unit 510. Further, the prediction error calculation unit 524 calculates a difference between the prediction value obtained by the predetermined prediction unit 522 and the actual pixel value, and outputs the difference as a prediction error value to the selection unit 526.
In the filter processing unit 510 illustrated in FIG. 3, when the pixel value change processing unit 514 changes the pixel value of the target pixel to the prediction value by the first prediction unit 512a, the prediction of the first prediction unit 522a. If the target value is changed to the predicted value by the second prediction unit 512b, the prediction by the second prediction unit 522b is correct. That is, the predictive hit rate by the lossless compression unit 520 is improved by the filter processing by the filter processing unit 510. Therefore, the compression rate of the compression process by the reversible compression unit 520 can be increased.

The selection unit 526 determines which prediction unit 522 has hit. When the first prediction unit 522a hits (S123: Yes), the selection unit 526 outputs the identification information of the first prediction unit 522a to the run counting unit 528 (S125). Further, when the second prediction unit 522b hits (S123: No and S124: Yes), the selection unit 526 outputs the identification information of the second prediction unit 522b to the run counting unit 528 and the code generation unit 529. (S125). If none of the prediction units 522 has been hit (S123: No and S124: No), the selection unit 526 uses the prediction error value calculated by the prediction error calculation unit 524 as the run counting unit 528 and the code generation unit 529. (S126).
In step 127 (S127), the code generation unit 529 generates a code of run information of the first prediction unit 522a, a code of identification information of the second prediction unit 522b, or a code of a prediction error value.
In step 128 (S128), the compression unit 500 determines whether or not the processing has been completed for all the pixels included in the target line, and is encoded when it is determined that the processing has been completed for all the pixels. In other cases, the processing returns to S112, and the processing from S112 to S127 is performed on the next pixel in the same manner as described above.

  As described above, the printer device 2 according to the present embodiment can flexibly cope with a plurality of requests of processing speed priority, image quality priority, and compromise with the simple configuration as described above.

2 is a diagram illustrating a hardware configuration of a printer device 2 (image compression device) according to the present invention, centering on a control device 20. FIG. 3 is a diagram illustrating a functional configuration of a compression processing program 5. FIG. 3 is a diagram illustrating a functional configuration of a compression unit 500. FIG. It is a flowchart explaining operation | movement (S10) of the compression program 5 when a processing speed is prioritized. It is a flowchart explaining operation | movement (S12) of the compression program 5 when priority is given to image quality. The compression parameter table 542 memorize | stored in the parameter setting part 540 is illustrated. It is a flowchart explaining operation | movement (S14) of the compression program 5 in case the compression parameter table 542 is used. FIG. 8 is a flowchart for explaining in more detail the encoding process (S110) in the flowcharts of FIGS. 4, 5, and 7. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 ... Printer apparatus 5 ... Compression program 500 ... Compression part 510 ... Filter process part 514 ... Pixel value conversion process part 520 ... Reversible compression part 540 ... Parameter setting part 542 ..Compression parameter table 550... Line compression rate calculation unit 560... Line compression rate comparison unit 570... Page compression rate calculation unit 580... Page compression rate comparison unit 590.

Claims (11)

Compression means for compressing an input image input as a compression target for each partial image that is a part of the input image;
Partial data amount detection means for detecting the data amount of the partial image compressed by the compression means;
A partial data amount comparing means for comparing the data amount of the partial image detected by the partial data amount detecting means with a reference partial data amount set for the partial image;
An image compression apparatus comprising: a parameter setting unit that sets a compression parameter that defines a compression process by the compression unit according to a comparison result by the partial data amount comparison unit. The reference partial data amount is a value corresponding to a higher target compression ratio among the target compression ratio of the partial image and the target compression ratio of the entire input image,
When the partial data amount comparison unit determines that the data amount of the partial image exceeds the reference partial data amount set for the partial image, the parameter setting unit uses a compression parameter that has been set. The image compression apparatus according to claim 1, wherein the compression parameter is changed to a compression parameter capable of realizing a high compression ratio. The compression unit applies the compression parameter changed by the parameter setting unit when the partial data amount comparison unit determines that the data amount of the partial image exceeds the reference partial data amount. The image compression apparatus according to claim 2, wherein the compression processing for the partial image is performed again. An overall data amount detection means for detecting the data amount of the entire input image compressed by the compression means;
A total data amount comparing means for comparing the data amount of the entire input image detected by the total data amount detecting means with a reference overall data amount set for the entire input image;
The image compression apparatus according to claim 1, wherein the parameter setting unit sets the compression parameter according to a comparison result by the total data amount comparison unit and a comparison result by the partial data amount comparison unit. The sum of the initial values of the reference partial data amount is larger than the target data amount of the input image,
Reference value changing means for changing a reference partial data amount set for each partial image according to a comparison result by the whole data amount comparing means;
The partial data amount comparison unit compares the data amount of the detected partial image with the reference partial data amount changed by the reference value changing unit,
The image compression apparatus according to claim 4, wherein the parameter setting unit changes the compression parameter in accordance with a comparison result by the partial data amount comparison unit. The parameter setting unit compresses more than the set compression parameter when the partial data amount comparison unit determines that the data amount of the partial image exceeds the reference partial data amount set for the partial image. Change the compression parameter so that the rate is high,
The reference value changing unit is configured to perform the processing for each partial image when the total data amount comparing unit determines that the data amount of the entire input image exceeds the reference total data amount set for the entire input image. The image compression apparatus according to claim 5, wherein the set reference partial data amount is reduced. Parameter storage means including at least a limit compression parameter capable of realizing a compression ratio corresponding to the reference overall data amount for any partial image;
The image compression apparatus according to any one of claims 1 to 6, wherein the parameter setting unit selects a compression parameter to be set from compression parameters stored in the parameter storage unit. The parameter storage means stores a plurality of compression parameters having different realizable compression rates,
The parameter setting means selects, for each partial image, a plurality of compression parameters in order of a compression parameter having a lower realizable compression rate and a compression parameter having a higher realizable compression rate. The image compression apparatus described. The partial image corresponds to one line of the input image,
The compression means compresses the input image in units of one line,
The partial data amount detection means detects the data amount for one line compressed by the compression means,
The image compression apparatus according to claim 1, wherein the partial data amount comparison unit compares the data amount for one line detected by the partial data amount detection unit with a reference partial data amount for one line. The input image input as a compression target is compressed for each partial image that is a part of the input image,
Detect the amount of compressed partial image data,
Compare the data amount of the detected partial image with the reference partial data amount set for the partial image,
An image compression method for changing a compression parameter that defines a compression process in accordance with a comparison result between a partial image data amount and a reference partial data amount. In an image compression apparatus including a computer,
Compressing an input image input as a compression target for each partial image that is a part of the input image;
Detecting the amount of data of the compressed partial image;
Comparing the amount of data of the detected partial image with a reference partial data amount set for the partial image;
A program for causing the computer of the image compression apparatus to execute a step of changing a compression parameter defining compression processing according to a comparison result between the data amount of the partial image and the reference partial data amount.

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