JPH09269875A - Data compression/expansion method for page printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of access times for a memory bus and to efficiently execute the expansion processing of compressed data by rasterizing document data in the block unit of a prescribed size and checking whether data of respective blocks are all white or not. SOLUTION: When CPU resterizes a PS file with a function as a PS interpreter 21, it checks whether data of one unit is all white dots or not. When they are all white, the white map table WMT of a compression memory 23 is directly set to NULL without using a compression part 22. If even only one black dot exists, the compression part 22 executes a compression processing for respective block words BW constituting the unit. An expansion part 25 reads the unit record of WMT. When data is all '0', it shows that the unit has all white dots. When data is '0', the reading and expansion processing of compressed data is not executed on BW.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data compression / expansion processing method for a page printer (including a printer section of a digital copying machine) such as a monochrome or color laser printer or LED printer.

[0002]

2. Description of the Related Art In recent years, the resolution of page printers such as laser printers has become increasingly higher, and recently 600 DPI has become mainstream. In the future, 800 DPI and 1200 DPI
Printers will emerge, and more multi-resolution printers, i.e., printers whose engine resolution changes in response to emulation, will also increase.

However, the memory capacity of a frame buffer for developing bitmap data for one page in a page printer is about 4 MB at A4,600 DPI, and A3,60.
At 0 DPI it is about 8 MB. Such an increase in the memory capacity of the frame buffer greatly affects the product price. On the other hand, since the price of the page printer has been changing to a low price, it is necessary to suppress the price increase accompanying the high resolution by some means.

For this purpose, lossless compression techniques for binary image data include MH (Huffman coding: a standard G3 facsimile coding method using a one-dimensional compression method) and MR (G3 using a two-dimensional compression method) used in facsimile and the like. Optional coding method for facsimile), MMR (standard coding method for G4 facsimile, which is a modification of MR coding method)
For example, there is LZ compression used for text files and binary files handled by a computer.
Recently, JB using the international standard arithmetic coding
There is also an IG method.

[0005]

However, in these binary image data compression techniques, since an encoding process is performed sequentially from the head of an image or a file, only a desired arbitrary block cannot be restored in real time. .

In a page printer, document information to be printed comes in the order in which it is actually created on a CRT of a computer, so that a compression process must be performed in block units of a certain size. Therefore, the above compression technique cannot be used as it is. There is no fixed-length lossless compression technology for binary image data in the world. Therefore, depending on the target image data, it may not be possible to compress it to a desired capacity.

Therefore, the inventors of the present invention first rasterize document data to be printed in a page printer to a predetermined resolution, and perform variable length lossless compression processing on the rasterized data in block units of a predetermined size. And store each compressed code in the compressed memory,
When all the units formed by a plurality of blocks continuous in the main scanning direction or the sub scanning direction are white data, the specific code is stored in the compression memory, and one page to the compression memory or a plurality of the divided data are divided. The present invention invented a method for compressing / decompressing image data in which the compressed code of data is stored and then the compressed code is expanded to output a video signal to the printer engine.

Further, when decompressing the compressed code, the presence / absence of the specific code is discriminated for each unit, and when it is determined that the specific code is present, the compressed code of the unit is not read or decompressed. The inventor also invented a method for compressing / decompressing image data in which all white data is directly transferred to a printer engine.

With this arrangement, the page printer can greatly reduce the memory capacity of the frame buffer for storing the rasterized print data, and a high-resolution page printer can be provided at a low cost. Moreover, when all the data in one unit is white data, the data of that unit is not read out, so that the memory access for each block in one unit can be reduced, so that the decompression process of the compressed code can be efficiently performed.

However, in the case of an image having a lot of ruled lines such as printing by a spreadsheet application, black data exists in a very small part even though there are many white parts. Since all the compressed codes of each block in one unit are read out and decompressed, there is a problem that the memory bus is frequently accessed and the processing efficiency cannot be sufficiently improved. The present invention solves such a problem, and more efficiently performs the process of decompressing (also referred to as decompressing) the print data compressed after rasterization and stored in the memory, and outputting the video signal to the printer engine. The purpose is to do so.

[0011]

In order to achieve the above object, a data compression / decompression processing method according to the present invention rasterizes document data to be printed in block units of a predetermined size in a page printer, and Variable length lossless compression is applied to the rasterized data in block units, the compression code is stored in the compression memory, and each unit is composed of a plurality of blocks continuous in the main scanning direction or the sub scanning direction. Then, it is checked whether the data of each block constituting the unit is all white, and the result is stored in the compression memory as status information of each block.

After the compression code of one page or the data obtained by dividing the compression code into a plurality of pieces has been stored in the compression memory, the compression code for each block of each unit stored in the compression memory is read out and decompressed. When the data is transferred to the printer engine, the status information of each block stored in the unit is read, and the compression code of the block in which the status information indicates all white is not read and expanded, The feature is that all white data is directly sent to the printer engine.

Further, the following data compression / decompression method for a page printer is also provided. In order to rasterize the document data to be printed page by page using two basic units, a block composed of t dots in the main scanning direction and a unit composed of u blocks in the main scanning direction or the sub scanning direction. The necessary page buffer is divided into units. It is checked whether the data of each divided unit are all white, and the result is stored in the m-bit white map table (WMT).

If the data of one unit is not all white, it is checked for each block that constitutes the unit whether the data is all white, and the result is stored in the white map table (WMT). In addition to saving as block status information, an address is stored in the white map table (WMT), a compressed data table (CDT) is prepared in an area designated by the address, and the data is stored for each block. Variable length lossless compression processing is performed, and the compression code is stored as a compressed data record (CDR).

When the compression processing for each block is impossible, a t-bit uncompressed data table (UDT) is prepared in the address area designated by the compressed data table, and the data of the block is stored as it is. To do. In this way, after storing one page of data, the compression code for each block of each unit stored in the compressed data table (CDT) is read and decompressed, and the video data is transferred to the printer engine. , The status information of each block of the white map table (WMT) is read in units of the above units, and the compression code of the block whose status information indicates all white is not read and decompressed, and all t dots are directly read. Send white data to the printer engine.

By doing so, the compression code is read out and decompressed for the block whose data is all white,
Further, since the decompressed data is not transferred to the video buffer, the number of accesses to the memory bus can be reduced and the decompression (decompression) process of the compressed data can be efficiently performed.

[0017]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 2 is a diagram showing a configuration example of an image forming system using a page printer according to the present invention, in which 100 is a personal computer and 200
Is a page printer. Usually, the user operates the CRT screen, keyboard and DTP of the personal computer 100.
(Desktop Publishing) A document is created using an application, transferred to the page printer 200 through the printer driver 101, and printed.

The printer driver 101 converts the content of a document to be printed normally into a printer language supported by the page printer 200, such as PCL of Hewlett-Packard (HP) or Postscript language of Adobe. The printer connected to the former is generally connected to a PC.
The L emulation printer and the printer connected to the L emulation printer are called PostScript printers. There is also a printer called a dumb printer which simply prints a bit image which is entirely rasterized on the personal computer 100 side.

The page printer 200 may be a laser printer, an ink jet printer, a thermal printer, or the like, but the laser printer is superior in terms of high speed printing. Recently, a color laser printer has begun to appear on the market, and it is also possible to use it. Further, while the resolution is increasing year by year, 600 DPI is currently the standard. In the following embodiments, the page printer 200 will be described as a 600 DPI black and white PostScript laser printer (hereinafter simply referred to as “page printer”), but the present invention is not limited to this.

FIG. 3 is an external view of the page printer, and FIG. 4 is a vertical sectional view showing the outline of the internal mechanism.
The page printer 200 has a paper feed tray 2 detachably provided, a first paper output stacker 3 provided at an upper portion, and a second paper output stacker 3 provided at a rear portion.
A discharge stacker 4 is provided. 2 paper output stackers 3,
4 can be switched by the switching claw 5. Normally, the first discharge stacker 3 is selected as the discharge stacker. However, the second discharge stacker 4 is selected in a special case such as when using easily curlable paper such as an envelope or a postcard.

Further, inside thereof, a photosensitive drum 10, a charging unit 11, an optical writing unit 12, a developing unit 13, a transfer unit 14, a fixing unit 15, which constitute an image forming unit of a printer engine, a paper feed roller 16 and A paper feed section using a pair of registration rollers 17, etc.,
Paper discharge transport unit 1 composed of transport rollers and paper guide plates
8 and a controller board 19 constituting a printer controller for controlling the entire page printer, an engine driver board 20 constituting a sequence controller of the printer engine, and the like.

When the print sequence is started by the sequence controller of the printer engine,
Paper is fed from the paper feed tray 2 by the paper feed roller 16,
The sheet is temporarily stopped in a state where the leading end of the sheet abuts against the pair of registration rollers 17. On the other hand, the photosensitive drum 10 rotates in the direction of arrow A in FIG. 4, and the surface charged by the charging unit 11 is
A laser beam modulated by the optical writing unit 12 according to the image data from the printer controller is irradiated while performing main scanning in the drum axis direction to be exposed, and the photosensitive drum 10 is exposed.
To form an electrostatic latent image on the surface of.

The developing unit 13 develops it with toner, and the registration roller pair 17 transfers it to a sheet fed at a predetermined timing by the transfer unit 14, and the fixing unit 1
The print paper heated and fixed in step 5 is sent to the second discharge stacker 4 or is conveyed to the first discharge stacker 3 above through the discharge conveyance section 18.

FIG. 5 is an internal block diagram of the controller board 19. This controller board 19 has a CPU 20.
1, NVRAM 203, program ROM 204, font ROM 205, RAM 206, and four interfaces (hereinafter abbreviated as “I / F”) 207, 209,
211 and 213 and a bus line 215 connecting them
It is constituted by.

The CPU 201 is a program ROM 204.
The controller is controlled by a program stored in the controller, a mode instruction from the operation panel 210, a command from a personal computer (personal computer) 100 as a host device, and the like. In addition, the inserted IC card 2
From 02, font data and programs can be imported. The data compression / decompression processing according to the present invention is also performed under the control of the CPU 201. NVRA
M203 is a non-volatile memory that stores the contents of the mode instruction from the operation panel 210.

The program ROM 204 is a read-only memory that stores a control program for this controller. The font ROM 205 stores character font pattern data and the like. RAM206 is CPU2
No. 01 is a random access memory used as a work memory, an input buffer for input data, a page memory (frame buffer) for print data, a video buffer, a memory for downloaded fonts, and the like.

The engine I / F 207 is an interface that is connected to a printer engine 208 that actually performs printing and that communicates commands, status, and print data. A panel I / F 209 is an interface that is connected to the operation panel 210 and communicates commands and status. The operation panel 210 displays and informs the user of the current status of the printer, and allows the user to specify the mode. Panel device.

The host I / F 211 is an interface for communicating with the personal computer 100 which is a host device, and is usually a Centronics I / F or RS23.
Use 2C. The disk I / F 213 is a disk interface for communicating with the disk device 214. The disk device 214 is an external storage device for storing various data such as font data, programs, and print data, and is a floppy disk device, a hard disk device, or the like.

Next, the page printing operation of the page printer 200 will be described. FIG. 1 is a block diagram showing a functional configuration of a part related to page printing processing by the controller board 19 and the printer engine 208 of 600 DPI shown in FIG. The PS interpreter 21, the compression unit 22 and the decompression unit 25 are the CPU of FIG.
Functions such as 201, compressed memory 23 and uncompressed memory 2
Reference numeral 4 indicates the memory area of the RAM 206, and the video output unit 27 indicates the function of the engine I / F 207.

FIG. 6 is an operation flow chart when printing one page by the data compression / decompression method according to the present invention. Therefore, along with the flow of FIG. 6, the page printing operation will be described with reference to FIG. Printer driver 1 in personal computer 100 shown in FIG.
01 converts the document data to be printed page by page into a Postscript (hereinafter referred to as PS) file and sends it to the page printer 200.

When the page printer 200 receives the PS file, the PS interpreter 21 shown in FIG.
However, it has a resolution of 600 DPI, which is the same as the printing resolution due to the performance of the printer engine 208, and rasterizes it in block units of a certain size, and the compression unit 22 has a variable length in block units for the rasterized data. Attempt lossless compression processing. This block unit will be described later in detail.

If compression is possible, the resulting compressed code is stored in the compression memory 23. This compression memory 23
Is a page memory having a capacity equivalent to one page at 300 DPI, and uses the memory area of the RAM 206 shown in FIG. If the data cannot be compressed, the rasterized data of 600 DPI is stored in the non-compressed memory 24. In practice, the uncompressed memory 24 is
Therefore, it is not necessary to increase the memory capacity of the RAM 206. The compression memory 23
In the page memory including the uncompressed memory 24 and the uncompressed memory 24, the compressed code and uncompressed data may be sequentially stored in each area obtained by dividing the area for one page into N areas.

When the processing for one page is completed, the compressed image is expanded and the video signal is output. That is, the compression code stored in the compression memory 23 is decompressed (also referred to as decompression processing) by the decompression unit 25 in order from the upper left of the page, and 600
The image data is restored to the DPI image data, and is developed into a bitmap in a video buffer (frame memory) in the video output unit 27. The uncompressed image data of 600 DPI stored in the uncompressed memory 24 is directly expanded to an address specified by a compressed data table (CDT) of the video buffer, which will be described later.

Then, the video output unit 27 sends the image data expanded in the video buffer as a serial video signal to the printer engine 208 for printing. At this time, the video buffer in the video output unit 27 uses the frame memory area for one page secured in the RAM 206 of FIG. 5 or another memory. A specific example will be described later. Alternatively, the memory area of 1 / N of the frame memory for one page is used to expand the image data for one page and output the video signal by N.
It can also be done in batches.

Next, details of the variable length lossless compression processing in block units described with reference to FIGS. 1 and 6 will be described with reference to FIGS. 7 to 14. A block unit of data for rasterizing a PS file is a block word BW of t dots as shown in FIG. And FIG.
As shown in (a) or (b) of this t dot (dot)
The block words BW of are continuously formed by u in the main scanning direction (a) or the sub scanning direction (b) to form one unit. That is, 1 unit = u * BW.

Then, the CPU 201 shown in FIG. 5 has the function of the PS interpreter 21 of FIG.
When the file is rasterized (drawn), it is checked whether or not the data of one unit is all white dots. If all the data is white, the compression unit 22 is not used and the white map table WMT of the compression memory 23 is directly set to NULL ( 2xffffffff). If there is even one black dot, the compression unit 22 is caused to perform compression processing for each block word BW that constitutes that unit.

FIG. 8 shows a concrete example of the block word BW and the unit. For the rasterized data of the PS file, 8 (line) of the block word BW consisting of 64 dots (dots) of white dots and / or black dots is shown. ) Minutes
Unit. That is, 1 unit = 8 *
BW = 8 * 64 (dot).

Furthermore, as shown in FIG. 9, for each unit, a 32-bit white map table (White Ma
p Table: WMT) is prepared. If all the data for one unit is white dots, WMT is set to NULL (0xffff
ffff), and if there is more than one black dot,
The 32-bit compressed data table (Commpress shown in FIG.
The actual address (CDT address) of the ion data table (CDT) is stored.

The length of the record indicated by the CDT address is 8 * CDT = 8 * 32 bits (bit) = 32 bytes as shown in FIG. The CDT corresponds to one block word BW, and if the compression method described later is tried and if compression is possible, the compression code is stored in bits 29 to 0 of the compression data table CDT shown in FIG. Also, if compression is not possible, bit 31
= 1 and the data storage memory address (Uucompression Data Table Address: UD) at the time of non-compression in bit 30 to bit 0
(T address) is stored (FIG. 12).

The compression method used here consists of the following two steps. First, it is checked whether the target block word BW is all white dots or all black dots. If so, set the CDT address to 0x0000
Set to 0000 or 0x7fffffff. If not, depending on whether the first dot is a white dot or a black dot (designated by "0" or "1" in bit 30 of CDT shown in Fig. 10), the run length of the white dot or the black dot Figure 13
The coding is performed by the so-called run length compression coding method in which the Huffman code shown in FIG.

For example, the 64-dot block word BW shown in FIG. 14A is converted into the 32-bit compressed code shown in FIG. 14B. Note that the last white dot (run length = 10) is not included in the compressed code, but can be restored because all the dots after the last black dot (run length = 19) are white dots. Also, 1 is written in the surplus area of the CDT.

Further, according to the present invention, when the data of one unit is not all white, it is checked whether or not the data is white for each block constituting the unit, and the result is white-mapped. Table (WMT)
The status information of each block is saved in.

Therefore, in the white map table (WMT) in the compression memory 23, the CDR as shown in FIG. 15 is added to the unit record prepared for each unit.
A u bit (upper 8 bits in this example) is reserved as a status, and it is used by each BW in the unit to store a flag of status information indicating whether or not all white is present, and compression of this unit in the compressed data table is performed. The remaining 24 bits are used for the CDR address pointer indicating the code storage area.

Since the compressed data table CDT and the uncompressed data table UDT have a size of 64 bits (8 bytes) or more, the lower 3 bits of the address pointer to the CDT & UDT area are always "0". Therefore, the CDR address pointer in the unit record of the white map table WMT can be information omitting the lower 3 bits.
That is, there is no problem if 27 bits can be used as the address pointer information.

C in the unit record shown in FIG.
The DR statuses are all "1" when the white map table WMT is initialized. Then, each bit of the 8 bits is made to correspond to each BW constituting the unit, and when it is checked whether the data of the target BW is all white dots or all black dots as described above, it is determined that all white dots are white dots. For example, the bit corresponding to that BW is cleared to "0".

In rewriting to each unit,
If there is a block word BW newly judged to be all white in the above judgment before the compression operation, the corresponding bit in the CDR status is cleared to "0". In the above operation, when all the flags in the CDR status are "0", all the bit contents of the unit record are "0".
Then, the memory used for the CDR is released.

Next, the process of "decompression of compressed image / output of video signal" in FIG. 6 will be described with reference to FIGS. FIG. 16 shows the compression memory 2 in FIG.
3 is a block diagram schematically showing the relationship among the RAM 206 having the uncompressed memory 24 and the memory area of the video buffer, the decompression unit 25, the video output unit 27, and the printer engine 208. A FIFO, not shown in FIG.
A memory 28 is provided. 29 is a decompression (decompression) unit 25
And a memory bus for accessing the RAM 206 from the video output unit 27.

FIG. 17 is a flow chart showing the expansion processing of the compressed image by the expansion unit 25 (actually controlled by the CPU 201 of FIG. 5). The processing by the decompression (decompression) unit 25 shown in FIG. 16 will be described with reference to this flowchart.

The decompression unit 25 first reads the above-mentioned unit record of the white map table WMT in the first storage unit (area storing WMT, CDR, UCDR) by the RAM 206. This data is all "0"
If, then the unit (eg: 64 dots x 8 lines)
Indicates that all are white dots. In this embodiment, a NULL table is provided as a third storage unit in the decompression unit 25, and the CDR status information is copied and stored in the corresponding area. This NULL table has 8-bit information for one WMT unit record, that is, information for 8 scan lines (BW).

Read 1 of WMT unit record
After scanning lines, the line count value is set to u (8 in this example, the number of block words), and it is checked whether or not the corresponding bit in the NULL table is "0". If it is "0", the compressed data is not read from the first storage unit and decompressed for the BW. When the corresponding bit is "1", the compressed data is read from the first storage unit and decompressed, and the restored data is restored to the video buffer V which is the second storage unit.
Transfer to B.

When the processing for one scan line (BW) is completed, the line count value is decremented by 1 until the line count value becomes 0 (8 scan lines in this example), sequentially for each scan line (BW). Then, the above processing is performed. FIG. 18 shows the relationship between the unit record in the WMT, the state of each BW in the CDR, and the NULL table.

(A) is an example of CDR. BW in which some black dot data is present is shown by shading. In the illustrated example, some black dot data exists in the second, fourth, fifth and seventh BWs, and the first, third, sixth and eighth BWs are all white. (B) shows the contents of the unit record corresponding to the CDR of (A). In each BW, the bit in the CDR status corresponding to the all white BW is "0".
It has become. 1st BW to 8th BW correspond to bits 31 to 24 in the CDR status, respectively.

(C) is an example of the NULL table, and bit 7 to bit 0 of Xn dots in the scan line direction are (B).
CDR status bits 31 to b in the unit record of
It corresponds to it24. Therefore, since the compressed data is neither read from the first storage unit nor decompressed for the BW whose bit is "0" in the NULL table 30, the number of accesses to the memory bus 29 can be reduced. The processing efficiency can be improved.

The data that has been decompressed (decompressed) is temporarily stored in the video buffer BV which is the second storage unit.
Since the data transfer is not performed and the data transfer from the video buffer BV to the video output unit 27 is not performed,
The number of accesses to the memory bus 29 can be further reduced.

In FIG. 16, the data stored in the video buffer VB which is the second storage unit is the video output unit 2
7 is sent to the printer engine 208 via the video queue 31 and the FIFO 28. However, N of the extension part 25
The data for BW whose flag in the ULL table is "0" is not read from the video buffer VB, but is directly output from the video queue 31 of the video output unit 27 as t dot (64 dots in this example) all-white data. To the printer engine 208.

[0056]

As described above, according to the present invention, it is possible to reduce the memory capacity of the page printer to achieve high resolution and low cost, and to decompress (decompress) and compress compressed data. When sending a video signal, it is possible to significantly reduce the number of accesses to the memory bus, improve processing efficiency, and improve performance.

[Brief description of drawings]

FIG. 1 is a block diagram showing a functional configuration of a portion related to a process for printing document data of a PS file in page printer 200 shown in FIGS. 2 to 4.

FIG. 2 is a diagram showing a configuration example of an image forming system using a page printer to which the present invention is applied.

FIG. 3 is an external view showing an example of the page printer in FIG.

FIG. 4 is a longitudinal sectional view showing the outline of the internal mechanism of the same.

5 is a block diagram showing a configuration of a controller board 19 in FIG.

6 is an operation flow chart when printing one page by each unit shown in FIG. 1. FIG.

FIG. 7 is a diagram showing a configuration of one unit of data obtained by rasterizing a PS file.

FIG. 8 is a diagram similarly showing a specific example of the number of dots of the block word BW and the number of lines of one unit.

FIG. 9 is a diagram showing the size of a white map table (WMT) for each unit.

10 is a diagram showing the configuration of a compressed data table (CDT) stored in the WMT of FIG.

11 is a real address of the CDT shown in FIG. 10 (CDT
3 is a diagram showing the length of a record of (address). FIG.

FIG. 12 is a data storage memory address (UD when uncompressed)
It is explanatory drawing in the case of storing T address) in BW.

FIG. 13 is an explanatory diagram of a Huffman code that describes a run length of white dots or black dots.

FIG. 14 is a diagram showing a conversion example of a compressed code.

FIG. 15 is a diagram showing a bit configuration of a unit record in a white map table (WMT).

16 is a block diagram for explaining a part related to the process of expanding the compressed image and outputting the video signal in FIG. 6 according to the present invention.

17 is a flowchart showing a decompression process of a compressed image by the decompression unit 25 shown in FIG.

FIG. 18: Unit record in WMT and each B in CDR
It is a figure which shows the relationship between the state of W and a NULL table.

[Explanation of symbols]

 10: Photoconductor drum 11: Charging part 12: Optical writing part 13: Developing part 14: Transfer part 15: Fixing part 19: Controller board 20: Engine driver board 21: Postscript (PS) interpreter 22: Compressing part 23: Compressed memory 24: Non-compressed memory 25: Decompression unit 27: Video output unit 28: FIFO memory 29: Memory bus 30: NULL table 31: Video queue 100: Personal computer 101: Printer driver 200: Page printer 201: CPU 206: RAM 208: Printer engine VB: Video buffer

Claims (2)

[Claims] 1. A page printer rasterizes document data to be printed in block units of a predetermined size, performs variable length lossless compression processing on the rasterized data in block units, and compresses the compressed code. Is stored in the compression memory, and it is checked whether or not the data of each block constituting the unit is all white, for each unit constituted by a plurality of blocks continuous in the main scanning direction or the sub scanning direction. , The result is stored in the compression memory as status information of each block, and is stored in the compression memory after the compression code of one page or the data obtained by dividing the data into a plurality of pieces is stored in the compression memory. Reads the compressed code for each block of each unit, decompresses it, and transfers the video data to the printer engine. At the time of transmission, the status information of each block stored in the unit unit is read, and the compression code of the block in which the status information shows all white is not read and decompressed, and all white data is directly read. A data compression / decompression method characterized by sending to a printer engine. 2. Document data to be printed in a page printer using two basic units, a block composed of t dots in the main scanning direction and a unit composed of u blocks in the main scanning direction or the sub scanning direction. The page buffer required to rasterize each page by one page is divided into the above-mentioned units, and it is checked whether or not the data of each divided unit is all white, and the result is an m-bit white map table (WMT ), And if the data of one unit is not all white, it is checked whether the data is all white for each block that constitutes the unit, and the result is stored in the white map table (WMT). Save as block status information and store address in the white map table (WMT). , A compressed data table (CDT) is prepared in the area designated by the address, variable length lossless compression processing is applied to the data for each block, and the compression code is stored as a compressed data record (CDR). If the compression processing for each block is impossible, a t-bit uncompressed data table (UDT) is prepared in the address area specified by the compressed data table (CDT), and the data of the block is directly used. By storing the data for one page by storing, the compression code for each block of each unit stored in the compressed data table (CDT) is read and decompressed, and the video data is transferred to the printer engine. Read the status information of each block of the white map table (WMT) in units of A data compression / decompression processing method characterized in that t-dot all-white data is directly sent to a printer engine without performing reading and decompression processing on a compression code of a block whose status information indicates all white.