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Claims  |
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We claim:
1. A method for use in generating from an outline representation of a
character a bitmap representation therefor, at a selectable point size and
resolution, the outline representation being defined relative to a
high-resolution em-square and the bitmap representation being defined in
terms of a low resolution em-square, such method comprising the steps of:
a) defining, for at least one dimension, a plurality of control zones, each
having at least a starting control edge and an ending control edge, within
the high-resolution em-square, each zone preferably corresponding to a
typographic feature of the character;
b) defining, for at least one dimension, a plurality of interpolation
zones, each having at least a starting edge and an ending edge;
c) arranging said interpolation zones to produce the best typographic
quality in the bitmap representation of the character.
2. The method of claim 1 further comprising the step of:
constraining at least one of said control zones to a predetermined size
when the size of said bitmap representation of a character is smaller than
a threshold size.
3. The method of claim 1 further comprising the step of
simultaneously constraining at least one of said control zones of a
plurality of one or more characters to a predetermined size when the size
of said bitmap representation of said different characters is smaller than
a second size.
4. The method of claim 3 further comprising the step of:
for a character, constraining a plurality of control zones to a plurality
of respective predetermined sizes when the size of the bitmap
representation of the character is smaller than a threshold size.
5. The method of claim 4 further comprising the step of:
arranging the respective predetermined control zone sizes in a hierarchy.
6. The method of claim 4 wherein the hierarchy is arranged according to the
typographic importance of the respective control zone.
7. The method of claim 1 further comprising the step of:
establishing for each of said control edges, a correspondence between its
position in the high-resolution em-square and a position in the
low-resolution em-square.
8. The method of claim 7 further comprising the step of:
using each of said interpolation zones to establish a correspondence for
each point on said outline representation of a character that is contained
within an interpolation zone and does not coincide with a control edge,
between their positions in the high-resolution em-square and a position in
the low-resolution em-square.
9. The method of claim 7 or claim 8 wherein the low-resolution em-square
comprises a finite number of pixels, and including the additional step of:
rounding, to the nearest whole number of pixels, the number of pixels in
each control zone in the low-resolution em-square.
10. The method of claim 7 wherein the low-resolution em-square comprises a
finite number of pixels, and the step of establishing correspondence
includes the step of:
rounding, to the nearest whole pixel, the positions of the control edges in
the low-resolution em-square.
11. The method of claim 7 wherein the low-resolution em-square comprises a
finite number of pixels, and including the additional step of:
allocating the pixels in the low-resolution em-square according to the
relative sizes of the control zones for each dimension in the outline
representation of the character.
12. The method of claim 11 further comprising the step of:
rounding, to the nearest whole number of pixels, the number of pixels in
each control zone in the low-resolution em-square.
13. The method of claim 1 wherein characters are represented in the
high-resolution em-square by a combination of curves and straight lines.
14. The method of claim 13 wherein the curves comprises Bezier curves.
15. The method of claim 1 wherein the size of the character in the bitmap
representation is less than twenty five points.
16. The method of claim 1 wherein the resolution of the character in the
bitmap representation is less than six hundred resolution units per inch.
17. A digital font for use in generating a bitmap representation of a
selected character from a digital representation of the outline of said
character, comprising, in a digital storage medium having a plurality of
locations capable of storing digital information:
a first set of storage locations containing data representing x-oriented
control zones, each zone corresponding to a typographic feature of the
character; and
a second set of storage locations containing data representing y-oriented
control zones, each zone corresponding to a typographic feature of the
character.
18. The digital font of claim 17 further having:
a third set of storage locations containing data representing x-oriented
interpolation zones; and
a fourth set of storage locations containing data representing y-oriented
interpolation zones.
19. The digital font of claim 17, further comprising:
a fifth set of storage locations containing data representing constraint
functions, each constraint function being associated with an indicator of
a threshold size bitmap representation of the character for which the
constrain function is operative.
20. The digital font of claim 19 wherein the data for at least one
constraint function includes a control zone size, to which the control
zone is set when the constraint function is operative.
21. The digital font of claim 19, wherein the constraint functions are
arranged in a hierarchy.
22. The digital font of claim 21 wherein the hierarchy is organized
according to typographic importance of character features.
23. The digital font of claim 17 wherein the storage locations are embodied
in magnetic media.
24. The digital font of claim 17 wherein the storage locations are embodied
in semiconductor memory media.
25. The digital font of claim 24 wherein the semiconductor memory media
comprises a read-only memory.
26. A method for converting a two-dimensional high-resolution outline
representation of a character to a two-dimensional low-resolution bitmap
representation of the character at a selectable point size and resolution
including the steps of:
a) breaking the character into a plurality of control zones oriented in
each in each of the two dimensions, each control zone corresponding to a
typographic feature; and
b) mapping the typographic features, as a function of the information
contained in the control zones in each of the two character dimensions,
from the high resolution representation to the bitmap representation of
the character at a selected size.
27. The method of claim 26 wherein the step of mapping includes selecting,
for each control zone, a scaling factor appropriate to map the extent of
the control zone to an integral number of pixels in the bitmap
representation.
28. The method of claim 26 further comprising the step of:
providing a plurality of constraint functions, each constraint being
operative for sizes below a threshold size bitmap representation of the
character and for respectively constraining a size of a control zone
corresponding to a typographic feature of the character to a predetermined
size when the bitmap representation of the character is smaller than the
threshold size for which the constraint function is operative.
29. The method of claim 28 further comprising the step of:
arranging the plurality of constraint functions in a hierarchy according to
the relative importance of the typographic feature respectively
corresponding to each control zone.
30. The method of claim 29 further comprising the step of:
simultaneously constraining to a predetermined size at least one control
zone for each of a plurality of characters when the bitmap representation
of one of the characters is smaller than the threshold size for which the
constraint function is operative.
31. A method for converting a two-dimensional high-resolution outline
representation of a character to a two-dimensional low-resolution bitmap
representation of the character at a selectable point size and resolution
including the steps of:
a) defining for the character, in at least one of the two dimensions, at
least one control zone, each control zone corresponding to a typographic
feature; and
b) mapping the typographic features, as a function of the information
contained in the control zones, from the high resolution representation to
the bitmap representation of the character at a selected size, said
mapping step including the steps of (1) scaling the control zone from its
size in high-resolution outline units to a corresponding number of pixels
in bitmap space, and (2) rounding said number of pixels to the nearest
integer number.
32. The method of claim 31 wherein the size of the character in the bitmap
representation is less than twenty-five points.
33. The method of claim 31 wherein the resolution of the character in the
bitmap representation is less than six hundred resolution units per inch.
34. In a computer processing system, a method for generating a
two-dimensional low-resolution bitmap representation of a character at a
selectable point size and resolution, from a two-dimensional
high-resolution outline representation of the character including
information defining for the character, in at least one of the two
dimensions, at least one control zone, each control zone corresponding to
a typographic feature, comprising mapping each typographic feature, as a
function of the information contained in the corresponding control zone,
from the high resolution representation to the bitmap representation of
the character at the selected size, said mapping including the steps of
(1) scaling the control zone from its size in high-resolution outline
units to a corresponding number of pixels in bitmap space, and (2)
rounding said number of pixels to the nearest integer number.
35. The method of claim 34 wherein the size of the character in the bitmap
representation is less than twenty-five points.
36. The method of claim 34 wherein the resolution of the character in the
bitmap representation is less than six hundred resolution units per inch.
37. A computer processing system for generating a two-dimensional
low-resolution bitmap representation of a character at a selectable point
size and predetermined resolution, from a two-dimensional high-resolution
outline representation of the character, comprising:
a. memory media providing, for each such character, information defining
for the character, in at least one of the two dimensions, at least one
control zone, each control zone corresponding to a typographic feature;
and
b. means for mapping each typographic feature, as a function of the
information contained in the corresponding control zone, from the high
resolution representation to the bitmap representation of the character at
the selected size, including means for (1) scaling the control zone from
its size in high-resolution outline units to a corresponding number of
pixels in bitmap space, and (2) rounding said number of pixels to the
nearest integer number.
38. The method of claim 37 wherein the size of the character in the bitmap
representation is less than twenty-five points.
39. The method of claim 37 wherein the resolution of the character in the
bitmap representation is less than six hundred resolution units per inch. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the art of generating digital representations of
alpha numeric characters or other symbols. More particularly, it relates
to the generation of bitmap representations of characters at selectable
resolutions and point sizes, from digital representations of their
outlines.
2. Discussion of the Prior Art
The field of electronic typographic and image composition has given birth
to a large variety of display devices and printing systems for making
visible in printed or other form (e.g., on a video display screen)
alphanumeric and other characters and symbols. (Hereafter the term "output
device" will be used generically, to include all types of print and
non-print output devices.) This field includes, but is not limited to,
electronic typesetting and publishing systems, full page composition
systems, word processing systems, and all of their output devices. In such
systems, each character or symbol must be supplied, at some point in the
systems, as digital information from which the character form may be
constructed on each output device. The digital data will generally have to
be compiled or generated in a way which takes into account specific
characteristics of the output device, such as its resolution, and specific
characteristics of the image to be displayed, such as the point size
desired for typographic material.
A large number of typefaces are in popular use. The generation of each
character or symbol in each typeface desired, at each point size to be
used, can be a substantial undertaking. For a small manufacturer, the cost
of developing all of this material could make the effort quite
impractical. And when a new output device is developed, with different
characteristics, it could be necessary to prepare a new digital data set
for characters adapted thereto. This can make it quite expensive to take
advantage of improvements in output technology as they become available.
Fortunately, it is not necessary to start "from scratch" each time a
character set, font, or symbol is needed. Digital forms of a large number
of typefaces and other symbols are available from a number of suppliers.
However, these suppliers frequently will not have a digital representation
in a form which meets the user's requirements or the characteristics of
his output device. The typeface supplier generally wants to establish as
its "master" digital version of a character a form which is as close to an
analog form as possible--that is, a form which is virtually independent of
output device characteristics and the point size at which characters are
to be viewed. The most frequently employed approach is to store an encoded
representation of the outlines of each of the characters. The location and
shape of the outline is defined using a normalized coordinate system whose
resolution is considerably finer than the resolution of any conceivable
output device for any reasonable selection of point size. Several
techniques are available for encoding character outlines. These range from
storing every point on the outline to storing closely spaced points which
can be connected by straight lines to storing only selected "control
points" which define the curves and straight lines of a character
according to a known set of rules. The use of outline encoding reduces the
amount of memory needed to store a typeface and provides a
size-independent character representation. However, most output devices
will not accept the encoded outline character data; they require that
characters be represented in a so-called "bitmap" form, as a list of
numbers locating the position of each point in the character shape (i.e.,
not only every point on the outline, but every point in the interior of
the character, as well). A translation or transformation is therefore
required from the outline form to the bitmap form.
To understand the nature of this transformation, start by considering the
display medium as a rectangular grid of N.times.M locations, each location
representing a single picture element (or "pixel") which may be turned on
(to display a part of a character) or left turned off (to represent
background). The state of each picture element can be represented by a
single bit whose value is either zero or one (of course, additional bits
can be added to represent color, intensity or other attributes). When a
character is stored in bitmap form, it is represented in a memory as a bit
pattern corresponding to the intended illumination of picture elements on
the display. The whole display field is represented in a memory array as
an assemblage of bitmap characters.
Smaller-sized characters are made up of fewer pixels than larger characters
and have smaller bitmap representations. It is thus necessary to make
available for each character which is to be displayable, a separate bitmap
representation in each required size. Further, since each bit in the
memory corresponds to a selectable pixel location on the display, and
pixel locations depend on display resolution, a separate bitmap
representation is required for each different resolution output device.
Therefore, although the outline representation of a character can be done
at a sufficiently high resolution as to be independent of size and display
resolution for all practical intents, the bitmap representation is
necessarily dependent on the point size to be displayed and the resolution
of the output device.
Frequently, important points on the ideal outline locus of a character will
fall between the available pixel locations on the display field, requiring
that choices be made as to which of the available pixels to use to
represent those portions of the outline. And as a character is scaled up
or down in size, certain adjustments may have to be made to account for
the human visual perception mechanisms which cause the character to be
viewed as having specific qualities. For example, as point size is
changed, it is often appropriate that the height and width of the
character be varied by slightly different scaling factors.
A still further complicating factor is the fact that for typographic
consistency, certain characterizing features and relationships between
features of different characters must be preserved through the outline-to
bitmap conversion process. For example, the heights of certain portions of
specified letters must be kept the same, the widths of certain portions of
letters must be maintained in a desired relationship, and so forth.
Failure to preserve these relationships will cause loss of the typeface's
design identity and typographic quality.
The promotion of the appearance of unity between the characters in a
typeface has sometimes been accomplished by using conventional signal
processing techniques such as filtering. That approach, however, provides
improved uniformity (e.g., in the thickness of lines) only at the expense
of diminished resolution. This is a direct consequence of performing
operations on character spectra rather than operating on character
features.
The conversion of outline representations of typefaces to bitmap
representations of selectable point size and resolution presents
difficulties when achievement with an automatic system is sought,
particularly where typographic design integrity is to be maintained in the
bitmap product. The key to preserving typograpic consistency at different
resolutions and point sizes lies in maintaining typograpically significant
intracharcter and intercharacter relationships. Until very recently, high
quality conversion has required considerable human "polishing" or editing
of any bitmap which has been generated from an outline form by an
automated system. This manual editing is, of course, expensive and time
consuming. Moreover, since editing is required for each point size, a
system cannot be provided with an ability to select an arbitrary point
size for type. This restricts the user to just those point sizes for which
edited type has previously been created.
In order to overcome these limitations, a number of methods have been
developed for automated bitmap character generation from outlines. For
example, U.S. Pat. No. 4,785,391, which is incorporated herein in its
entirety by reference for background and which is assigned to the same
assignee as the present invention, describes a method which uses a
symbolic feature specification system which establishes a bridge between
the outline form of the character and the bitmap form. This system links
typographic features (both within individual characters and between
different characters) and selectively arranges them hierarchically in
order of importance.
The feature specification taught in U.S. Pat. No. 4,785,391 treats the
em-square containing a character as being composed of a hierarchy of zones
formed in both the horizontal and vertical directions. The zones are
completely character-specific. Each zone defines the extent of some
feature of the character, as selected by the operator of a computer-aided
design (CAD) station. The available pixels are allocated first to the most
important features, and then to successively less important features.
Although the method described in U.S. Pat. No. 4,785,391 maintains high
typographic quality and consistency, it may, at times, employ
computationally intensive techniques.
Another method for producing a scaleable typeface data is taught by U.S.
Pat. No. 4,675,830. The method stretches and compresses character contours
to bring specified contour points into proper alignment with a bitmap
grid. The method breaks the character contour into segments which connect
a pair of specified contour points. The method then offsets all the
coordinates in the segment in order to bring one of the specified contour
points into alignment with the bitmap grid. Finally, the method effects
linear scaling of the segment with a scale factor in order to bring the
second specified contour point into alignment with the bitmap grid.
Since the specified contour points described in U.S. Pat. No. 4,675,830 do
not necessarily relate to typographic features, normally this means that
there are more contour segments when compared to the method disclosed in
U.S. Pat. No. 4,785,391. Consequently, since the method of U.S. Pat. No.
4,675,830 requires a separate transformation for each segment defined on
the character outline, it can be extremely computationally slow when
compared with either the method taught in U.S. Pat. No. 4,785,391 or the
method of the present invention.
It is therefore an object of the present invention to provide a system
which can more efficiently convert outline representations of individual
characters and typefaces to corresponding bitmaps of those characters and
typefaces within a wide range of sizes and display resolutions.
A further object of the invention is to provide a system for conversion of
character outline data to corresponding bitmap data with only minimal
human intervention or involvement, while maintaining a high level of
typographic quality.
Still another object of the invention is to provide a simpler and faster
mechanism for converting into bitmap form character information stored in
outline form, while at the same time having sufficient flexibility in the
mechanism to allow high typographic quality to be maintained
Yet another object of the invention is to provide an improved method for
converting into bitmap form character information stored in outline form,
based on knowledge of the features of the characters rather than their
spectral properties.
SUMMARY OF THE INVENTION
The present invention achieves these objectives through the use of a
symbolic feature specification system which establishes a bridge between
the outline form of the character and the bitmap form. All the information
needed to scale a character is contained in the data structure which
describes the character. The description of a particular character is
completely independent from any other characters. The method for
identifying features provides a size-independent constraint description of
characters, which is then usable in conjunction with outline character
data to translate size- and resolution independent outline
characterizations into character bitmaps at any desired size and
resolution.
The feature specification system of the present invention treats the
em-square containing the character as being composed of a set of control
edges formed in both the horizontal and vertical directions. The edges are
character-specific. The zones that are formed between any two paired
horizontal or any two paired vertical control edges define the
relationship or extent of a particular typographic feature of the
character. Arbitrarily-sized interpolation zones are then assigned and
interpolation is performed in order to assign the number of output device
pixels to be allocated between paired control edges in the character.
The foregoing and other objects, features, and advantages of the present
invention will be more readily understood and apparent from the following
detailed description of the invention, which should be read in conjunction
with the accompanying drawing, and from the claims which are appended at
the end of the detailed description
BRIEF DESCRIPTION OF THE DRAWING
In the drawing,
FIG. 1 is a diagrammatic illustration of a high resolution outline of an
exemplary letter "m" laid against a much lower resolution grid;
FIG. 2(a-d) is a pictorial illustration of the visual effects of different
display resolutions and character sizes;
FIG. 3 is a pictorial illustration of an outline for an upper case letter
"O" superimposed on an outline for an upper case letter "D" in the same
typeface, as they appear on a 100 dpi output device, where the outlines
have been encoded at 52,000 dpi effective resolution;
FIG. 4(A) is a pictorial illustration of conventionally generated bitmaps
for the letters O and D of FIG. 3;
FIG. 4(B) is a pictorial illustration of bitmaps for the letters O and D of
FIG. 3, generated according to the present invention;
FIG. 5 is a diagrammatic illustration of a high resolution outline
representation for the letter "n";
FIG. 6 is a diagrammatic illustration of the x-direction and y-direction
control edges, superimposed on the character outline of the letter "n" of
FIG. 5;
FIG. 7 is a diagrammatic illustration of the x-direction and y-direction
control zones between the control edges of the letter "n" of FIG. 6;
FIG. 8 is a diagrammatic illustration of the interpolation zones in the
x-direction and y-direction, defined for the letter "n" of FIG. 7;
FIGS. 9A and 9B together comprise a listing of an exemplary form of a
source file data structure according to the present invention, depicting a
listing of the control zones, the interpolation zones, and constraint
functions for the letter "n";
FIG. 10 is a diagrammatic illustration of a lower case "o" shown having an
outer closed curve which is separate and independent from the inner closed
curve; and
FIG. 11 is a listing of an exemplary form of a source file data structure
and data therefor according to the present invention, depicting a listing
of the control zones, the interpolation zones, and constraint functions
for the lower case letter "o"; and
FIG. 12 is a listing of an exemplary form of a source file data structure
for a constraint table and data therefor according to the present
invention.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
For purposes of illustration, reference is now made to the lower case
letter "m" shown in FIG. 1 on a rectangular coordinate grid 10. The grid
10 is intended to represent a medium to low resolution display field for
an output device which might be capable of showing, for example, 100-300
dots per inch (dpi). By contrast, the letter "m" is shown as a high
resolution outline. Typical outline encoding systems define character
features on a coordinate definition system providing several thousand
lines to the em. It is not feasible, of course, to show in the drawing a
coordinate grid of such density; consequently, only the resulting
character outline is illustrated. The assignee of the present invention,
Bitstream, Inc., Cambridge, Mass., for example, uses a system providing
8,640 lines to the em in both the horizontal and vertical directions. The
outline of the letter "m" (and each other symbol which is to be made
available) is encoded at this high resolution and stored in digital form.
Various techniques are described in the literature for effecting
appropriate digital coding of character outlines; see, for example, J.
Flowers, "Digital Type Manufacture: An Interactive Approach," I.E.E.E.
Computer, Vol. 17, No. 5, May 1984, at 40-48; W. Richmond, "Digital
Masters," Communication Arts, May-June 1984, at 78-81.
As stated above, the mapping of a character outline from a high resolution
form to a lower resolution form is a complex task. With great frequency,
features of the outline will not fall directly on available pixels but
will, instead, fall between them. This is a direct consequence, of course,
of a substantial reduction of resolution from the 8640.times.8640 line
em-square. Consider, for example, the left stem 22 of the letter "m" in
FIG. 1. Ideally, the width of the stem might be as represented at brace
24. However, due to the coarseness of the display resolution, it may not
be possible to display a segment of that precise width. There are only two
alternatives: the stem 22 may be wider or it may be narrower. In the
extreme, the choice may have to be made between using one pixel or no
pixels (possibly losing the feature entirely, or at least somewhat
altering its shape). Thus in FIG. 1, if each box 25 represents one pixel,
the stem 22 may be made two pixels wide, as shown by the brace 26, or it
may be only one pixel wide. And at the lowest resolutions, curves become
virtually non existent and straight lines dominate. FIG. 2 shows another
series of illustrations which demonstrate the inverse relationship between
the quality of a digital type image and the output resolution of the
output In FIG. 2(a), the images 31 and 32 were set at 96 points on a 650
dots-per-inch (dpi) output device; the images appear virtually analog to
the naked eye. More and more typographic detail is lost as the digital
bitmap becomes coarser. FIG. 2(b) shows the same characters at 33 and 34,
set at eight points on the same 650 dpi display. At twelve points on a 300
dpi display, the corresponding images 35 and 36 are shown in FIG. 2(c).
Finally, at six points on a 300 dpi display, the images are as presented
at 37 and 38 in FIG. 2(d), where a great deal of the detail of the analog
form has been lost.
Another problem which occurs when mapping characters from one resolution to
another is to maintain those correspondences between features which give a
typeface its visual identity. For example, referring back to FIG. 1, it
might be desired that the left and right stems 22 and 28 of the letter "m"
have the same relative width (or thicknesses) in the low resolution form
as they did in the high resolution form. When a number of such criteria
apply to a given character, it may not be possible, as the resolution is
decreased, to maintain all of these desired relationships. At that point
it is necessary to sacrifice some relationships in order to preserve
others.
It may also be desired to preserve relationships between the features of
different characters. For example, it may be desired to make the width of
the left stem 22 of the letter "m" the same as the width of the
corresponding portion of the letter "n" (or some other letter). Or one
might wish to ensure that the heights of two letters such as a lower case
"a" and a lower case "o" are the same. The list of potential relationships
is limitless.
Of course, the maintenance of typographic quality is not simply a matter of
mechanically preserving correspondences. In the example shown in FIG. 3,
the goal is to generate bitmaps for the analog "O" 42 and "D" 44. At
first, the procedure appears to involve a straightforward task of forcing
the heights, arch thicknesses and widths of the "O" and "D" to correspond.
Consider, however, bitmap versions of those letters which have been
produced using customary prior art techniques, as in FIG. 4(A).
Distortions are apparent even to an untrained eye. For example, the bottom
segment 45a of the O (42'), is of a different length than the top segment,
45b. And the lower arch 46a is shaped differently from the top arch 46b.
In the D (44'), there is a break 47 in the connection between the upper
end of the arch and the stem. And the ends of the lower arch 48a and upper
arch 48b do not terminate symmetrically.
By contrast, use of the present invention yields the bitmapped O (42") and
D (44") of FIG. 4(B). There, symmetry has been maintained and the shapes
of the characters correspond as nearly as possible to their analog forms
42 and 44.
According to the present invention, the preservation of correspondences
between features in the higher resolution outline version of characters
and their lower resolution bitmap counterparts starts with identification
of those typographically significant features and labelling them in the
file which defines outline versions of the characters. Thus, the initial
feature labelling is, at least for most practical purposes,
resolution-independent.
A character consists of one or more closed contours defined in a high
resolution coordinate system, which usually is a cartesian coordinate
system. The present invention makes use of character outlines that consist
of vectors and Bezier curves, however, the method of the present invention
would work with other different representations of character outlines. A
typical high resolution character outline 50 for the letter "n" is shown
in FIG. 5.
Features are identified within the high resolution character through the
use of control edges. Control edges identify those x and y coordinates
that should fall on pixel boundaries after transformation into output
device coordinates. Control edges in the x-dimension can be visualized as
vertical lines. Control edges in the y-dimension can be visualized as
horizontal lines. A set of x control edges 52 and y control edges 54 which
define selected typographically important characteristics of selected
letter "n" is shown in FIG. 6.
As depicted in FIG. 5, there is one x and one y control edge which form the
basic control edges for each character with respect to the high-resolution
em-square. These control edges are always located at the coordinates x=0
and y=0 in the high-resolution em square. In FIG. 6, control edge 56
represents the basic x control edge for the letter "n". Control edge 70
represents the basic y control edge for the letter "n". Further, control
edges 58, 60, 62, and 64 represent the x-coordinate locations of the
letter "n" which should fall on pixel boundaries when the transformation
is made from the high resolution outline character to the lower resolution
output device bitmap format. At the same time, control edges 72, 74, and
76 represent the y-coordinates of the high-resolution outline "n" which
should fall on pixel boundaries when the transformation is made from the
high resolution outline format to the lower resolution output device
bitmap format.
It should be noted that control edges do not have to coincide with any
extrema, horizontal, or vertical lines of the outline form of the
character. Control edges may be placed, for example, at a predetermined
distance from a particular feature of a character if the type designer
deems that it is important to control the distance between the control
edge and the particular feature of a character.
Having now identified the features within the high-resolution outline
character through the use of x and y control edges, the relationship
between the control edges needs to be established. In the illustrative
example, the relationship between control edges is established by control
zones. A control zone is created by established a pairing between two
control edges. X control zones must connect pairs of x control edges. Y
control zones must connect pairs of y control edges. The high resolution
character in the em-square containing the character is sliced into "zones"
of differing lengths in both the horizontal and vertical directions Each
zone establishes a run-length segment which defines the span and placement
of the selected features within the em-square. It should be carefully
noted that all control zones are derived from the basic control edges at
(x, y) coordinates of (0, 0) which are present in all characters.
Therefore, the relationship between control edges in both the x and y
direction is maintained, as the character is scaled in size or transformed
into any desired output resolution.
As an example, FIG. 7 illustrates how control zone assignments might be
made, using the lower case "n" as test case. To make the zone assignments,
the outline form of the character 50 is displayed on the CAD workstation
screen, and the operator moves a pointer to successive x and y locations
which he then labels as the control edges. The operator then pairs control
edges to define, or assign, the beginnings and ends of control zones. In
the example of FIG. 7, control zone 80, which spans the distance between x
control edge 56 and x control edge 58, would be the first x control zone
to be defined. Control zone 82, which spans control edges 58 and 60, would
be the second x control zone to be assigned. Next, control zone 84, which
spans control edges 58 and 64, would be the next control zone to be
assigned. Finally, control zone 86, which spans control edges 64 and 62,
would be the final control zone assigned in the x direction. The
assignment of control zones normally continues until all important
typographic features have been determined. Some control zone assignments
can be automated by using expert systems or other knowledge based
processing systems.
The position of x control edge 58 is defined relative to x control edge 56.
X control edge 60 is then defined by x control zone 82, relative to x
control edge 58. X control edge 64 is then defined by x control zone 84,
relative to x control edge 58. Finally, x control edge 62 is defined by x
control zone 86, relative to x control edge 64. Consequently, the
relationship of typographic features to one another will be maintained,
since the control zones and control edges are derived from known control
edges.
For proper completion of the zone assignments, it is necessary that (1)
there is a control edge at the origin of the character, (2) the number of
control zones must be one less than the number of control edges, and (3)
every control edge has at least one control zone connected to it. Once
these conditions are met, the pixel position of all control edges can be
determined. The number of output device pixels in a control zone is
computed by scaling the length of the control zone in outline resolution
units into pixels in the lower resolution output device format. The value
resulting from the scaling operation is normally rounded to the nearest
whole number of pixels. Thus, the number of pixels in the control zone for
the required output resolution and size can be determined. Consequently,
the number of pixels contained in a control zone enables the position of
the control edge at one end of the control zone to be calculated when the
position of the control edge at the other end of the control zone is
known.
After the positions of all the x control edges have been computed by
determining the size in output device pixels of the control zones
connecting all of the control edges, a similar process is used to compute
the positions of all of the y control edges. With reference to FIG. 7, y
control edge 70 is at the origin, whose position is defined as y=0 in
output device coordinates. The position of control edge 74 relative to
control edge 70 is determined by the length of control zone 90. The length
of control zone 90 is scaled into output device pixels and rounded to the
nearest whole number. Next, the position of control edge 76 may be
calculated from the length of control zone 92. The length of control zone
92 connecting control edges 70 and 76 is scaled into output device pixels
and rounded to the nearest whole number. Finally, the position of y
control edge 72 is determined by the length of control zone 94, which
spans control edges 76 and 72. Since the position of control edge 76 has
been determined by the length of control zone 92, the length of control
zone 94 is calculated from control edge 76 and scaled into output device
pixels, which are then rounded to the nearest whole number.
There are many possible ways of setting up control zones to meet all of the
conditions outlined above. Some arrangements produce better typographic
output quality results than others. Considerable typographic experience is
required to choose an arrangement of control edges and control zones that
produces high-quality output for the required size and resolution range.
The use of control zones ensures that equal sized parts of characters
always get an equal number of pixels after scaling. While this is one of
the most important requirements when effecting intelligent character
scaling, it alone is not sufficient in practice to produce high-quality
output typography.
Typographic characters are frequently drawn with subtle variations to
produce apparent optical consistency. For example, round bottomed
characters dip below the baseline in order to make them appear to be
sitting on the baseline. Given a particular output resolution for the
typographic characters, a control zone defining the extent of a particular
typographic subtlety may give unwanted results at smaller point sizes. For
example, consider the vertical stems of the letter "n" as depicted in FIG.
7. For type design purposes, control zones 82 and 86 may be of slightly
different lengths, thus making the left and right stems of the letter
slightly different in width. As the size of the letter "n" is decreased,
the effect of rounding errors is to make single pixels increasingly more
significant as a proportion of feature size. For example, there will come
a character size at which the difference between the width of the two
stems, given the constant output resolution, will become objectionable.
Consequently, in order to maintain typographic consistency, control zones
of possibly differing lengths in high resolution form must be constrained
to have equal lengths below a certain character size. However, by making
the constraint apply only below a specified size, typographic subtlety is
allowed to exist in large characters at high resolution, but inhibited for
small characters at low resolutions. This avoids the situation where a
small difference may be magnified to a whole pixel for very small
characters and consequently appear as a huge relative difference.
Therefore, constraints on control zones can be used to guarantee that
groups of zones receive equal numbers of pixels at any scaling factor,
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