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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for processing data representative of a visual representation, typically including a combination of text, graphics, and images, that is to be output to a visual-output device, such as a screen
display or print device. More particularly it relates to such a method and apparatus in which a data memory, referred to herein functionally as a "buffer memory", has reduced capacity requirements resulting from the selective compression of data.
2. Related Art
The preferred embodiment of, and preferred method of practicing the present invention is directed to printers that form a raster image typically connected indirectly over a network, or directly to a computer for printing documents created on the
computer. The invention is realizable for other forms of output devices as well, such as a video display generated on a CRT monitor or an LCD. Thus, the device creating the actual visual representation is referred to as a "visual-output device". The
visual area within which the visual representation exists is referred to as a "page", regardless of its actual form. The complete visual representation is referred to as a "page representation". A separately defined part of a page representation is
referred to as an "object".
One of the significant cost elements in a conventional printer is a buffer memory, also referred to as a frame buffer, for storing raster data defining the page representation. Conventional printer configurations employ buffer memories that are
capable of storing all of the raster data required to define each pixel on a page. An extensive amount of memory capacity is therefore typically required. A black-and-white representation for a 8.5 inch.times.11 inch sheet of paper at a pixel density
of 300 dpi (dots or pixels per inch) requires in excess of 1 MByte (1 million 8-bit bytes) of memory. Higher spatial and tonal resolutions, color printing and larger paper sizes require even more memory. A continuous tone, four-color representation at
a pixel density of 600 dpi for the same sized page requires about 135 MBytes of memory. Since the printer costs rise with memory size, it is desirable to provide printers with reduced memory requirements.
A memory device known by the proprietary name of "Memory Miser" produced by Advanced Micro Devices of Santa Clara, Calif., stores data in a resident memory by applying a compression algorithm to all of the data input. When required for output it
is decompressed based on the reverse of the compression algorithm and output. If used in a printer, such a device would reduce the amount of memory required. However, the memory would need to be at least large enough to store the most complex page
representation in order to be able to process any page that is input. This printer would have little flexibility in processing the variety of page representations possible with present day printers.
SUMMARY OF THE INVENTION
The present invention provides a method for using, and an apparatus permitting a reduced-size memory. Further, it provides a method and apparatus that can accommodate a variety of page representation characteristics and data processing
objectives.
The invention is directed generally to an apparatus and a method for processing data representative of a page representation for output to a visual-output device, such as the electro-mechanical printing apparatus (also referred to as the print
device), of a printer. The method begins with the step of receiving data that defines a page representation. A plurality of regions of the page are selected, which regions contain at least a portion of the page representation. In one aspect of the
invention, separate data for each such region is identified corresponding to the portion of the page representation contained in that region. Data identified for at least one of the regions is then compressed, using at least one compression algorithm
and stored. For producing the page representation after storing the compressed data, the compressed data is decompressed and transmitted to the visual-output device.
In another aspect of the invention, at least one compression factor and a plurality of compression algorithms are provided. The compression factor has a determinable value that is related to a reference value. A compression algorithm is then
chosen based on the relationship of the determined value of the compression factor to the reference value.
More specifically, the preferred embodiment of the invention is an apparatus for printing a two-dimensional page representation composed potentially of text, graphic and image objects (object representations) individually, and in combination. A
print device is responsive to raster data for printing a page containing the page representation. An input device, such as a personal computer or workstation, is used for inputting data defining the page representation. A program memory stores program
instructions including a plurality of different algorithms for compressing the data associated with corresponding different representation types and their combinations. The selection of compression algorithms is based in part on balancing the
compression factors of compression ratio, computational complexity, and visual quality. A processor is coupled to the input device, print device, program memory and a data memory for executing the stored program instructions.
The processor is responsive to the data input in the form of descriptive commands for identifying data for each of a plurality of ordered regions or bands containing collectively at least a portion, and preferably, all of the page representation. The data for each region corresponds to the portion of the page representation contained in that region. Some regions may not contain data. The descriptive commands, which are not necessarily band limited, are converted into lists of primitive elements
selected from a set of primitive elements. Each primitive element represents at least a portion of an object representation. These lists are referred to as display lists. There is preferably a display list for each region, although the display list
could be for the entire page, or for other defined regions.
The types (and combinations of types) of representations and boundaries (referred to as bounding boxes) of each type contained in each region are determined. The display list data associated with the determined types of representations for each
region is rasterized into an uncompressed band and then compressed using algorithms corresponding to the analysis of certain compression factors. Rasterizing refers generally to the conversion of high-level descriptive commands into rasters. Data
associated with primitive elements is often already in raster form. However, for purposes of this discussion, rasterizing refers to the conversion of display list data for a region into raster form without regard for whether or not the data associated
with the corresponding primitive elements is already in raster form.
The compression factors may, and preferably do include compression goals specifying target visual quality, compression ratio, and computational complexity. Compression ratio refers generally to the bytes of memory required to hold the compressed
data relative to the bytes of memory required to hold the same data uncompressed. Additionally considered are such factors as the type of representation, content of individual bounding boxes, overall content of the page representation, estimated versus
actual compression being achieved, and the number of passes or attempts made at compressing the data. Other factors may also be used, and some of these factors may not be used in all situations. For example, the factors could be prioritized so that
some are given more weight than others. As an extension of this, in certain situations some factors could be given no weight at all relative to other factors.
Some of these factors inherently have values that are readily determined. Others relate to characteristics or features the state of which is determined and a value assigned accordingly. For instance, the three representation types of text,
graphics and images could be assigned arbitrary respective identifier values 1, 2, and 3.
An algorithm, generally speaking, refers to a particular algorithm or combination of algorithms with particular parameter values. Thus, a change in parameter values results in a change in the algorithm.
The compressed data is stored sequentially by region. In the preferred embodiment, when required for printing, data for a region is read and decompressed. Depending on the system configuration, the compressed data may be transmitted to an
external printer or stored pending requirement of the data by the print device. The data is then transmitted to the print device for printing. Producing data (display lists) for each region and defining the regions to conform to the sequential output
of raster data to an output device minimizes the number of times the data is decompressed, data added, and then recompressed. During this overall process "data" defining the page representation takes the form of descriptive commands, display lists and
associated information, and raster data.
Data representative of the page representation is thus compressed and held in memory until such time as it is required by the print device for printing, or until the content of a region is changed. The data for the regions are swapped in and out
of the compressed-data memory using the selected compression and corresponding decompression algorithms, thereby reducing substantially the buffer memory requirements. This and other features and advantages of the present invention will be apparent from
the following detailed description of the preferred embodiment of the invention and as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an apparatus made according to and for practicing the method of the invention.
FIG. 2 is an illustration of a page having different types of two-dimensional representations.
FIG. 3 is a flow diagram summarizing a method of practicing the invention, 4 is a flow diagram of step 58 of the diagram of FIG. 3.
FIG. 5 illustrates visually the development of non-intersecting bounding boxes from bounding boxes of different representation types that overlap according to the method of the diagram of FIG. 4.
FIG. 6 is a simplified graphic example of overlapping bounding boxes with identifying coordinates used in the method of the diagram of FIG. 4.
FIG. 7 is a flow diagram illustrating the development of non-intersecting bounding boxes corresponding to step 122 in FIG. 4.
FIG. 8 is a flow diagram of step 78 of the diagram of FIG. 3.
FIG. 9 is a flow diagram of step 80 of the diagram of FIG. 3.
FIG. 10 is a functional block diagram corresponding to the apparatus of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD
Referring initially to FIG. 1, a generalized visual-representation-generating system made according to the present invention is shown generally at 10. It includes a visual information source 12 connected via a communication link 14 to an
output-data generator 16. Generator 16 is connected to a visual-output device 18 via a communication link 20. As will be seen, various embodiments are realizable from this general structure. Output data generator 16 can be resident within a host unit
including source 12, can be resident within an output unit including visual-output device 18, or can be functionally split between a host unit and an output unit.
In the typical instance when data generator 16 and output device 18 comprise a laser or other raster printer, information source 12 is a conventional work station or other computer-based system, such as an Apple Macintosh or IBM PC. The term
print device is also used herein as an example of a visual-output device. In the preferred embodiment, this term applies to the electromechanical apparatus responsive to raster data for producing a printed page. Generator 16 could also be incorporated
in a computer or workstation, such as a computer-based system as has just been mentioned, programmed to function as described herein, for controlling a raster display or printing device, as has also been mentioned. Further, as is discussed with
reference to FIG. 10, a host unit could generate and output the compressed data and an output unit could receive the compressed data, decompress it and transmit it to a resident visual-output device.
The preferred embodiment of the invention is thus directed to the printing of two-dimensional pixel representations. The general concepts are equally applicable to three (or more) dimensional representations to the extent they are realizable in
the system of FIG. 1.
Generator 16 includes an input.backslash.output controller 22 coupled to communication links 14 and 20. A conventional CPU (central processing unit) or processor 24 is coupled to controller 22, as well as to a read/write or random access memory
(RAM) 26, used partially as a buffer memory, for storing data, and a read only memory (ROM) 28 for storing program instructions and fixed information, such as nonvariable data and compression and decompression algorithms, as is discussed in further
detail with reference to FIGS. 3, 5, and 7-10. Any of a variety of conventional CPU's may be used, depending on the actual application. Further, other forms of hardware that accomplish the same functions can be used.
FIG. 2 illustrates a page 30 having a page representation 34 that could be defined by data input by the input device using a conventional page-description language, such as the language available from Adobe Systems Incorporated known by the name
PostScript. In the printer environment, as a PostScript file is created on source 12 (FIG. 1 ), objects can be created in any arbitrary order or fashion on a page. The objects are defined by one or more descriptive commands. As used herein, then, an
"object-defining command" is the command or collection of commands that define an object.
Different compression schemes have been found to be preferable for the different representation types of text, graphics, and images. For instance, the human eye is often less sensitive to changes in images than to degradations in something as
well defined as text. Thus, technically lossy compression schemes such as JPEG, when used at reduced levels of compression for images, can be visually lossless. Further, by the nature of graphics objects, some otherwise lossy schemes may be usable
without compromising spatial resolution. The LZW technique has been found to work well on text, run-length coding is effective for graphics and text/graphics combinations, and the JPEG technique is useful for images. It is therefore advantageous to
identify the different types of objects in a page representation.
Continuing to refer to FIG. 2, page 30 has defined boundaries represented by border line 32. The boundaries thus represent the maximum area within which page representation 34 is to be produced. Page representation 34 includes the following
objects on a background of a single color. A title or main heading 36 is formed of text in different colors (represented by the different tones). A subheading 38 identifies a text representation 40; a subheading 42 identifies a graphics representation
44; and a subheading 46 identifies an image representation 48. These subheadings are text representations in relatively large fonts and, along with text representation 40, are all a single color different than the background color. Text representation
40 is in a reduced font. Graphics representation 44 has grid identifiers 50 in the form of alphanumeric characters (text), and a bar chart section 52 composed of bars of different colors. Image representation 48 is simply an array of pixels of varying
colors.
Page representation 34 incorporates separate examples of large and small text, graphics, and image representations or objects. In a more complex page representation various of the different objects could overlap. That is, they could be printed
at least partially on a common area. The preferred method of the present invention is designed to take such overlapping areas into consideration, as is discussed in greater detail below.
A generalized flow diagram chart of a process or method 54 according to the invention is provided in FIG. 3. Processor 24 of FIG. 1 executes instructions to function as an interpreter that recognizes the PostScript descriptive commands input as
page data from source 12, as is provided by step 56 in the flow diagram of FIG. 3. In the general method of the invention, the page description data is divided into at least one, and preferably R different data regions at step 58. The regions can be
determined in advance, as is the case with the preferred embodiment and therefore be determined arbitrarily with reference to a particular page representation. They could also be determined dynamically for each page representation. Referring to FIG. 2,
an example of a dynamic determination would be to divide the page into separate regions corresponding to title heading 36, text subheading 38, text section 40, graphics subheading 42, each of grid identifiers 50, bar chart section 52, image subheading 46
and image section 48. In the preferred embodiment of the invention, however, in which raster data is produced for printing a page, the page area is divided into a plurality of fixed regions in the form of parallel bands. The bands are chosen and
ordered to correspond to the generation of raster data for output to a print device, and is therefore related to the resolution and scan order of the output device.
Referring again to FIG. 2, page 30 is shown for purposes of illustration divided into sixteen bands 60-75. When it is time to print, data for band 60 is read out and decompressed first and the data progresses sequentially through the bands until
data for the last band 75 is output. These bands each correspond to multiple raster "scans" of the page and provide for ordering the data in a way that will make the data readily available for printing. An actual letter-size page having a resolution of
300 dpi may be divided into about 20 to 40 bands.
As has been mentioned, the present invention provides for a reduction in the amount of memory required through the use of compression techniques, as is represented by step 78 in FIG. 3. By compressing a rasterized version of the descriptive data
of the page representation, according to the preferred embodiment, and decompressing it as needed by the printer or display, the amount of RAM needed may be drastically reduced.
This memory reduction is achieved by storing in the working RAM a compressed representation of what is conventionally stored uncompressed in a frame buffer. Raster data is created for one region at a time and stored uncompressed in RAM 26. This
data is then compressed and re-stored in RAM 26 until needed. The data for all the regions is ultimately processed in this way until data for essentially all the regions is compressed and stored. In the preferred embodiment of the present invention, as
data for each region is requested for output (printing), it is decompressed and output to the output device (print device), as represented by step 80 in FIG. 3.
By compressing the data for the regions in the reverse order required for output to the output device, the compression/decompression cycle of the last region may be avoided, since it can simply be rasterized and output directly. Also, depending
on the circumstances, the decompression algorithm typically is, but may not be exactly the reverse of the compression algorithm. When the output (page printing) is completed, process 54 ends for that page.
The following description of this process is directed to processing data for a single page. It will be understood that multiple pages may also be processed at a time using a similar system, so that different ones of the steps take place
simultaneously for various regions of the same or different pages.
Step 58 is shown in further detail by the flow diagram of FIG. 4. High level descriptive (such as PostScript) commands are input into data generator 16 from a source 12 as shown in FIG. 1. As has been described, these commands define, usually
in no particular order, where text, graphics, and image objects appear. Some of the commands do not define a particular object. These commands may be directed to identifying locations on a page, what color to use, and the like. Text typically includes
definitions of font and character size, as well as character identifiers and other information, such as the color of the text. Graphics are defined by area fills and strokes of arbitrary color, and images are usually provided by bit or byte patterns.
Referring again to FIG. 4, the first object-defining command is selected at step 82. The intersection of the object defined by the command with each of the R regions (bands) is then determined, as shown generally at 84. Iterative loop step 86
symbolizes the sequential determinations made for each region. When the first region is selected, a determination is made at 88 as to whether the object described by the command intersects the region. If there is no intersection, the next region is
selected at step 86 and the determination repeated for that region.
If there is an intersection, primitive elements, collectively referred to as a display list, are generated for the portion of the object in the region at step 90. Primitive elements are basic object portions or "building blocks" that, when
combined, form a new definition of an object. Character masks are used to define text. Geometric shapes, such as trapezoids and run arrays (bit patterns) are both graphics primitive elements. Because of the random color and intensity changes, images
are defined by the actual image descriptions. In some instances, these primitive elements are stored in the display list indirectly via pointers. Preferably, a single display list is generated for each region. As has been mentioned, it would also be
possible to have a single display list for a page with allocation of data to a region taking place as raster data for the region is stored prior to compressing.
Each high level input object-defining command implicitly has a corresponding representation type, such as text, graphics or image. Other ways of classifying the object-defining commands may also be used. In this embodiment, the primitive
element has an associated representation type corresponding to this implicit type. A bounding box is also determined for each region in which a portion of an object exists. A representation type is assigned to each bounding box based on the associated
primitive element type. In the preferred embodiment, the representation type is one of the three preferred types of text, graphics and image.
A bounding box is a defined area containing an object or object portion in the region. In an X-Y coordinate system, a bounding box is preferably a rectangle defined by the coordinates of the lower left and upper right corners. Other definitions
for bounding boxes, such as trapezoids, could also be used. There is thus potentially a single bounding box for each representation type in each region, referred to as a regional bounding box. As is discussed below, as an object is added to a region,
the regional bounding box of the same type is preferably expanded to include the area of the new primitive element(s) and the display list for the region is updated, as represented by step 92.
Collecting only one text, one graphic and one image bounding box for each region satisfies a requirement of computational simplicity while processing the primitive elements. However, it does so at the expense of lost local information. For
example, if an "a" is marked on the left side of the region and a "b" is marked on the right side of the region, there would be a text bounding box that spans the width of the region although characters do not fill this entire span. Consequently, a
logical extension of this invention would be to perform this information collection on a smaller region basis, such as having two or three regions across the width of a page. This would provide more local tracking of the objects in the regions. In
general, as has been discussed, the page can be divided into any arbitrary regions.
FIG. 5 shows visually the development of bounding boxes. Bounding boxes are also referred to herein, in a general sense, as regions. Each bounding box identifies a specific region of a page in which an object, collection of objects, or portions
thereof, exists. As was described with reference to page 30 shown in FIG. 2, the selected regions can be set dynamically to correspond to the objects in each page representation. A bounding box is thus a specific example of this concept. However, in
the following discussion, the term region refers to the bands as shown in FIG. 2 and not to bounding boxes.
The left-most representation in FIG. 5 illustrates a regional graphics bounding box 100 and a regional text bounding box 102. The text bounding box was formed by combining all the bounding boxes (not shown) for characters 104 (t), 105 (e) and
106 (x). Bounding box 102 is enlarged by adding bounding box 107 associated with the addition of a new character 108 "t" to "tex", resulting in enlarged bounding box 110 shown in the center of FIG. 5 containing the word "text".
Referring again to FIG. 4, this process of building regional bounding boxes of each type continues until the descriptive command is processed for each region, or until intersection with all regions has been determined as described. After the
last region is checked, a determination is made at step 112 if there is another command. If there is, the next command is selected at step 114 and process 84 is repeated. If there are no other commands, then intersecting bounding boxes for each region
are determined, as shown generally at 116.
In the discussion to follow, it will be seen that different compression algorithms are applied to the different types and combinations of types of representations. The following procedure divides the bounding boxes into nonintersecting bounding
boxes that are exclusively of a particular type or a particular combination of types. This allows different objects of different types to have the associated raster representation compressed with different algorithms. During this procedure, the size of
bounding boxes that contain a combination of overlapping objects of different types are minimized.
As controlled by iterative loop 118, the regions are sequentially checked to see if the bounding boxes of the different types intersect, as shown in step 120. If not, the next region is checked. If the bounding boxes do intersect, the bounding
boxes for the region are divided at step 122 into non-intersecting bounding boxes. This process is illustrated visually in the right illustration of FIG. 5 with shaded boxes representing different non-intersecting bounding boxes. These non-intersecting
bounding boxes may also be referred to as subregions. Regional bounding box 110 is divided into non-intersecting bounding boxes 124, 125 and 126. Similarly, regional graphics bounding box 100 is divided into non-intersecting bounding boxes 125, 128,
and 129. Nonintersecting text bounding boxes 124 and 126 have only text representations in them. Non-intersecting graphics bounding boxes 128 and 129 have only graphics representations in them. Remaining bounding box 125 has a combination of both text
and graphics representations, as noted in the figure.
FIG. 6 is a simplified representation of a combination of three different original bounding boxes 130, 132 and 134 in a region 136 of types text (T), graphics (G), and image (I), respectively, as shown. This figure does not represent the same
bounding box configuration as FIG. 5, but is used to illustrate the method of dividing intersecting bounding boxes of all three representation types. In order to simplify the explanation, adjacent coordinate values are used and only represent relative
pixel coordinates. Actual values are typically much higher and are typically not adjacent. Bounding box 130 is defined by the lower left and upper right coordinates having the (X, Y) values (0,2)(6,3). Similarly, bounding boxes 132 and 134 have the
coordinate values (1,1)(2,4) and (3,0)(5,5), respectively. In order to delineate between adjacent bounding boxes, the pixels along the left and bottom boundaries are included in the bounding box and the pixels along the right and top boundaries are
excluded. Thus, the upper right coordinate identifying each bounding box is not included in the bounding box.
The process of dividing these regional bounding boxes into nonintersecting bounding boxes may be thought of as a distilling operation. The method is illustrated in the flow diagram of FIG. 7 with reference to the chart of FIG. 6. Starting with
step 140, the bounding box coordinates are listed and the Y coordinates are ordered from lowest to highest. This procedure may also be performed in reverse using the highest coordinates first instead of the lowest. In the following discussion, the
reference to X and Y coordinates may be reversed and the results will be the same. In this case the Y coordinates are: 0, 1, 2, 3, 4 and 5. The two lowest Y coordinates (0 and 1) are selected initially, and the X coordinates that are associated with
the selected Y coordinates are ordered during step 142. Thus, Y=0,1 and X=3,5 as provided by bounding box 134.
X coordinates 1 and 2 for Y=1, which define the bottom boundary of bounding box 132, are not used since this forms the top of what would otherwise be a test box (1,0)(2,1). This top line is not included in the test box, so there is no
intersection with bounding box 132.
The next two lowest X coordinates are selected initially, during step 144. In this case there are only two X coordinates. A test is then made during step 146 to determine if the resulting test box is in one of the original bounding boxes. Test
box (3,0)(5,1), the top of which is shown by a dotted line in FIG. 6, is in bounding box 134 and is identified as bounding box A.sub.l. The subscript l refers to the bounding box type, which in this case is image. This representation type and the
coordinates of bounding box A are stored during step 148.
If the test box is not in an original bounding box, it is ignored and step 148 bypassed. The next step 150 is a determination as to whether there are more X coordinates for the existing pair of Y coordinates, 0,1. If so the next two X
coordinates would be 5 and whatever the next one is. Since there are no more X coordinates, a test is made at step 152 to determine if there are any more Y coordinates. If not, the distilling procedure is completed. However, in this example, we are
just beginning. There being another Y, the procedure returns to step 142 and repeats the above steps for Y=1,2.
Rather than go through the process step-by-step verbally, the following table is used to show the various steps involved in identifying all the nonintersecting bounding boxes A through M.
______________________________________ Y X TEST BOX ID STORE? TYPE ______________________________________ 0,1,2,3,4,5 -- -- -- -- -- 0,1 -- -- -- -- -- 3,5 -- -- -- -- 3,5 (3,0)(5,1) A YES I 1,2 -- -- -- -- -- 1,2,3,5 -- -- -- -- 1,2
(1,1)(2,2) B YES G 2,3 (2,1)(3,2) C NO -- 3,5 (3,1)(5,2) D YES I 2,3 -- -- -- -- -- 0,1,2,3,5,6 -- -- -- -- 0,1 (0,2)(1,3) E YES T 1,2 (1,2)(2,3) F YES T,G 2,3 (2,2)(3,3) G YES T 3,5 (3,2)(5,3) H YES T,I 5,6 (5,2)(6,3) I YES T 3,4 -- -- -- --
-- 1,2,3,5 -- -- -- -- 1,2 (1,3)(2,4) J YES G 2,3 (2,3)(3,4) K NO -- 3,5 (3,3)(5,4) L YES I 4,5 -- -- -- -- -- 3,5 -- -- -- -- 3,5 (3,4)(5,5) M YES I ______________________________________
FIG. 10 is a functional block diagram of the preferred embodiment of system 10 shown in FIG. 1. As shown in FIG. 10 and as has been discussed, a descriptive command receiver 154 assigns display list objects to the incoming commands, divides the
display list objects into regions, defines the regional bounding boxes, and stores the resulting information by region in a portion of RAM 26 referred to functionally as a working memory 156. A distilling process unit 158 then divides the regional
bounding boxes as required into nonintersecting bounding boxes and stores it in memory 156 as has been described with reference to FIGS. 4-7. The regional bounding boxes are preferably retained in case further processing is required in a region.
After dividing the regional bounding boxes into non-intersecting bounding boxes, the display list is "rasterized" for each region and compressed. Then, the compressed data is stored along with bounding box and representation type information,
compression algorithm identifiers, and any compression parameters, as provided by step 78 shown in FIG. 3. This step is shown in further detail in FIG. 8. The compression algorithms are selected, by what is referred to as "consultant" 166 (FIG. 10), on
the basis of the previously mentioned compression factors. For instance, high speed operation and high quality may be required. In such a printer, computationally simpler algorithms and lower compression ratio requirements might be used.
Alternatively, lower speed, lower quality, and higher compression ratio requirements would allow for more computationally complex algorithms.
Some compression schemes, such as one-color encoding, two-color encoding, run-length encoding, and subsampling, are computationally simple. They provide limited compression of visually active objects, particularly images. However, with simple
objects, very high compression is realized. More complex compression schemes, such as those conventionally known as LZW and JPEG, provide varying levels of compression, depending upon the data processed and the values assigned to parameters. The amount
of compression is also controlled by controlling the parameters associated with such schemes, as is well known in the art. In some cases, then, it may be possible to obtain poor visual quality quickly or high quality slowly. The particular compression
algorithms used are determined by the compression factors.
Combinations of these techniques can be used to achieve even different levels of compression, computational complexity and visual quality. The characteristics of the contents of the bounding boxes may be identified to determine whether yet other
techniques would be appropriate. If a given bounding box has, for example, a graphic representation, then perusal of it may show that it consists of only two different colors. For a continuous tone device, a savings of 8-to-1 for each colorant is
achieved by representing the pixel map of the bounding box with a one-bit-per-pixel map without sacrificing quality. If the contents have a single color, it is sufficient to store only the color and bounding box information. The compression realized
depends upon the size of the bounding box.
As another example, the local activity or rate of change in the content of a bounding box could be determined. If there is only slight activity, a run-length coder technique may be used. If it is highly active, the JPEG technique may be
employed. Further, if a representation is only a small portion of a region, then the raster contents of the bounding boxes may be simply copied. This provides RAM savings by not saving a raster representation of the unmarked background. This is also
desirable for some bounding boxes that contain combinations of objects or representation types. Thus, copying the raster data associated with a bounding box may be an important compression algorithm for s | | |
