Device dependent rendering

Details Device dependent rendering Device dependent rendering

1999-08-19

2003-05-13

Zimmerman, Mark (Department: 2671)

Computer graphics processing and selective visual display system

Computer graphics processing

Character generating

C345S467000, C345S611000

Reexamination Certificate

active

06563502

ABSTRACT:

This invention relates generally to computer systems, and more particularly to the rendering of characters to be displayed on computer output devices.
BACKGROUND OF THE INVENTION
Computer systems typically include some form of visual output device. For example, in
FIG. 1
, a prior art computer system
10
includes a computer
12
, a keyboard
14
, a disk storage unit
16
, a video display
18
, and a printer
20
.
Computer
12
includes a central processing unit (CPU)
22
, read only memory (ROM)
24
, random access memory (RAM)
26
, and input/output (I/O)
28
. The various components
22
-
28
of the computer
12
are coupled by a bus system
30
. As will be appreciated by those skilled in the art, the representation of the computer
12
has been highly simplified for the purposes of this discussion, but includes the fundamental components of a computer and their interconnections.
Devices such as keyboard
14
, disk storage
16
, video display
18
, and printer
20
are often referred to as “peripheral” devices. When coupled to the computer
12
they become part of computer system
10
. Some peripheral devices are primarily for inputting information to the computer
12
, other peripheral devices are used for outputting information from the computer
12
, and some computer peripherals are used for both inputting and outputting information.
Video display
18
and printer
20
are common output devices for the computer
12
. Such output devices typically form temporary or permanent images to convey information to a user of computer system
10
. For example, video display
18
includes a screen that can display character, graphical, and other types of images.
A number of problems arise when displaying characters on a computer output device. The term “character”, as used herein, is not limited to alphanumeric characters, but includes any form of character, number, symbol, or other coded device that can be represented or displayed on a computer output device. Some problems relate to the resolution of the output device, and other problems relate to human visual perception. “Resolution”, as it is used herein, refers to the size of individual pixels of a computer output device, and the spacing between the pixels of the output device.
The problems inherent in character imaging tend to be more pronounced for small characters than for larger characters. This is because small characters are made up of fewer pixels and, therefore, even a distortion of a few pixels is readily apparent in the smaller characters. Some of the distortions that may occur, particularly in these smaller characters, are: unbalanced “stem” widths, thin feature disconnection, over/under filling, and inaccurate weighting. One of the most noticeable of these defects is unbalanced stem widths, where vertical and horizontal strokes of characters may be of varying widths due to the character rendering process. Another very noticeable defect is the inaccurate weighting effect of small characters due to a quantization effect. Adding a pixel (the “quantum”) to a character stem that is only a few pixels wide can distort the “weight” given to that character, e.g. it may appear to be “bold” even when it is a normal weight character.
In
FIG. 2A
, the prior art problem of unbalanced stem widths is illustrated. In this instance, a lower case letter “m” is shown superimposed over a high-resolution grid
32
both in an outline form
34
, and a “bitmap” form
36
. “Bitmap” refers to a one-to-one mapping between data stored in the memory of the computer
12
and an image of the character to be displayed, i.e. each pixel of the output device corresponds to a bit of information stored in the computer. The outline
34
is typically provided by a parsed font program such as described in Adobe Type I format, Version 1.1, available from Adobe Systems, Inc. of San Jose, Calif., often referred to as the “Black Book” due to the color of its cover. Character outline
34
is converted or “rendered” into the bitmap
36
by a rendering program or “renderer” available from a variety of sources, including Adobe Systems, Inc.
In
FIG. 2A
, the letter “m” includes some distortion that is attributable to the conversion from font outline
34
to bitmap
36
form. In particular, the three vertical legs or “stems” of the letter “m” are of different widths. As noted, the left stem
38
a
is two pixels wide, the middle stem
38
b
is one pixel wide, and the right stem
38
c
is two pixels wide.
While characters are often rendered on a high-resolution grid as illustrated in
FIG. 2A
, they must be converted into a “coarse grid” representation before they can be displayed on the output device. The resolution of the coarse grid corresponds to the resolution of the output device.
In
FIG. 2B
, the conversion from a high-resolution grid to a coarse grid permits the renderer to re-balance the stems. More particularly, the system uses “hints” provided by the parsed font program to stretch and then align the stems of the character to the coarse grid. As noted, in
FIG. 2B
, the stems
40
a,
40
b,
and
40
c
are all the same width and, therefore, the unbalanced stem problem can be resolved for black-and-white types of output devices.
While the prior art has solved the problem of unbalanced stem widths for black and white output devices, the problem has re-emerged for greyscale and color output devices. Some output devices use a technique known as “anti-aliasing” in order to provide the illusion of smoother curves and less jagged diagonal lines. This is accomplished by varying greyscale or color values of the coarse grid. However, this manipulation can re-introduce the problem of unbalanced stem widths to anti-aliased characters.
In
FIG. 2C
, the lower case letter “m” is shown against a coarse grid
42
and corresponding high-resolution grid
44
. The high-resolution grid
44
is shown in only one of the cells of the coarse grid
42
so as not to confuse the drawings, although it will be appreciated that the high-resolution grid
44
is associated with all the cells of the coarse grid
42
. The coarse grid and the high-resolution grid are related as follows. If the output device has 2
n
+1 greyscale values, each “cell”
46
of the coarse grid
42
will have
2
n
pixels
48
. To provide a more concrete example, if 17 greyscale levels are provided from pure white to pure black, then 2
4
or 16 pixels of the high-resolution grid are in each cell of the coarse grid
42
. This means that the resolution of the high-resolution grid
44
is four times the resolution of the coarse grid
42
. As before, the font outline
50
for the letter “m” is obtained, and a bitmap
52
is developed at the resolution of the high-resolution grid from the font outline
50
by the renderer.
In
FIG. 2D
, the bitmap
52
has been converted into a greyscale “pixel map” which can be sent to the output device. The conversion is usually accomplished in a straightforward fashion. The number of pixels for a particular cell are counted, and this number is mapped to the greyscale level for that cell. Therefore, cells having more pixels will correspond to a darker greyscale value than cells having fewer pixels. This technique, referred to as “anti-aliasing”, greatly smooths the appearance of curves and reduces the jaggedness of diagonal lines, as is well known to those skilled in the art.
As used herein, a “pixel map” is much like a “bitmap” except that multiple values are stored in the memory of the computer system
12
which correspond to each pixel of the output device. For example, for each pixel on a greyscale or color video monitor, multiple sub-pixels can be defined each including a numeric value that is stored on the computer.
However, as seen in
FIG. 2D
, the unbalanced stem problem has reoccurred as a result of the application of the “anti-aliasing” process. More particularly, stem
54
a
appears to be two pixels wide, while stems
54
b
and
54
c
appear to be about one pixel wide. Again, this stem unbalance phenomenon is quite apparent in small characters and reduces the quali

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