WO1994006094A1 - Procede de conversion de glyphes sous forme d'images vectorielles en images en mode point combinant anticrenelage et ajustement de grille - Google Patents

Procede de conversion de glyphes sous forme d'images vectorielles en images en mode point combinant anticrenelage et ajustement de grille Download PDF

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Publication number
WO1994006094A1
WO1994006094A1 PCT/US1993/007956 US9307956W WO9406094A1 WO 1994006094 A1 WO1994006094 A1 WO 1994006094A1 US 9307956 W US9307956 W US 9307956W WO 9406094 A1 WO9406094 A1 WO 9406094A1
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WIPO (PCT)
Prior art keywords
pixel
fractional
width
glyph
value
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Application number
PCT/US1993/007956
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English (en)
Inventor
Edward Yee Hungshun
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Go Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Go Corporation filed Critical Go Corporation
Priority to AU50892/93A priority Critical patent/AU5089293A/en
Publication of WO1994006094A1 publication Critical patent/WO1994006094A1/fr

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    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G5/00—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators
    • G09G5/22—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators characterised by the display of characters or indicia using display control signals derived from coded signals representing the characters or indicia, e.g. with a character-code memory
    • G09G5/24—Generation of individual character patterns
    • G09G5/246—Generation of individual character patterns of ideographic or arabic-like characters
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T11/00—2D [Two Dimensional] image generation
    • G06T11/20—Drawing from basic elements, e.g. lines or circles
    • G06T11/203—Drawing of straight lines or curves
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G5/00—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators
    • G09G5/22—Control arrangements or circuits for visual indicators common to cathode-ray tube indicators and other visual indicators characterised by the display of characters or indicia using display control signals derived from coded signals representing the characters or indicia, e.g. with a character-code memory
    • G09G5/24—Generation of individual character patterns

Definitions

  • the present invention relates generally to computer graphics, and in particular, to the display of ideal glyphs using rasterization techniques for text rendering. Still more particularly, the present invention relates to a method for glyph rasterization that employs both grid fitting and anti- aliasing techniques.
  • Scan conversion converts an ideal description of a character referred to as a glyph description to a pixel image.
  • a glyph description of a Kan i ideograph is shown.
  • a glyph description is typically a series of curves and points that define an ideal outline of the area forming the character. All existing methods for converting ideal glyph descriptions to pixel images use generalized techniques that introduce imperfections in the resulting pixel images.
  • Figure IB when the ideal glyph representation of Figure 1A is converted to a 95 x 95 pixel matrix, the character remains very readable despite the imperfections. However, these imperfections become progressively more severe as the size of the ideal glyph is reduced.
  • Figure 1C shows an enlarged illustration of the glyph representation of Figure 1A converted to a 14 x 14 pixel matrix. As can be seen, the 14 pixel version is unreadable.
  • Common computer display technology ranges from 70 to 90 dots per inch. At these resolutions, text at relatively common sizes (10 to 20 typographer's points) can be difficult to read and aesthetically inferior.
  • One approach used in the prior art to resolve this problem is a "two color grid fitting" method. This method produces pixel images using only black or white pixels .
  • Grid fitting is a technique used to modify the ideal glyph shape to match the pixel grid in an effort to produce more aesthetically pleasing and readable results.
  • Figures 2A-2D are a graphical representation of a pixel grid, the black and white pixels, and the glyph representation.
  • Figures 2A and 2B illustrate the pixel conversion results for the Latin character "0" using standard scan conversion techniques. As shown in Figure 2A, the scan conversion process yields satisfactory results if the ideal outline happens to align with the pixel grid in a harmonious way. However, as shown in Figure 2B, when the ideal outline does not align with the pixel grid, the scan conversion produces undesired artifacts such as the two additional pixels 10 shown in Figure 2B.
  • Figure 2B While the scan conversion shown in Figure 2B is mathematically correct, the process yields an aesthetically unpleasing inconsistency between the left and the right sides of the character.
  • the prior art grid fitting method attempts to overcome this problem by providing "hints" for special portions of the glyph. These hints identify important portions of the glyph that should be adapted to fit in the pixel grid to produce an aesthetically pleasing result.
  • Figures 2C and 2D show the effects of grid fitting.
  • Figure 2C illustrates the glyph representation identical to Figure 2B.
  • the grid fitting method uses the hint to control the rounding and alignment process of the outline to yield a distorted outline shown in Figure 2D with the portion of the glyph between the hint aligned with the pixel grid.
  • Figure 2D the right side of the "0" is shifted about a half a pixel to the left so that the undesired artifacts 10 generated in Figure 2B are not produced.
  • Hinting removes detail from the shape of the ideal outline to render a different shape, which is not ideal, but is more pleasing under (sub-optimal) conditions where there are too few pixels to represent the ideal accurately.
  • Anti-aliasing techniques have been used primarily in television and video production to enhance the quality of pixel images.
  • the various anti-aliasing techniques use pixels of intermediate densities or color to create the illusion of higher resolution.
  • details such as curvature and varying stroke thickness that are lost with the two color grid fitting method can be rendered with a high degree of visual fidelity as shown in Figure 3A.
  • Figure 3B even when anti-aliasing techniques are applied, the pixel image will still have undesired distortion and artifacts 12 due to misalignment with the pixel grid. Therefore, neither approach of the prior art is effective at providing pixel images of complex ideographic characters at small sizes.
  • the present invention overcomes the limitations and shortcomings of the prior art with an improved method for generating aesthetically superior and more readable pixel images, even at small sizes, from ideal glyph representations.
  • the present invention advantageously combines a two color grid fitting technique and an anti-aliasing technique to produce both ideographic and Latin glyphs with uniformly pleasing aesthetic characteristics.
  • the preferred embodiment of the present invention comprises the steps of: retrieving an ideal glyph representations and hints from a font; adapting the ideal glyph representation to the pixel grid using a grid fitting technique; dividing the adapted glyph representation into a plurality of non-overlapping zones using the hints; applying an anti-aliasing technique to each zone without considering the pixel values outside the zone; and processing the zones to produce a final gray-scaled bit map.
  • Figure 1A is a glyph of the prior art represented in ideal outline form
  • Figure IB illustrates a prior art pixel conversion of the conventional glyph of Figure 1A to a matrix of 95 by 95 pixels;
  • Figure 1C illustrates a prior art pixel conversion of the conventional glyph of Figure 1A to a matrix of 14 by 14 pixels
  • Figures 2A-2D are graphical representations of the prior art pixel conversion results, a glyph representation, and a grid representing pixels
  • Figures 3A and 3B are graphical representations of pixel images enhanced using anti-aliasing and a pixel grid;
  • Figure 4 is a flow chart of the general method of the present invention for performing glyph rasterization;
  • Figures 5A and 5B are a flow chart of a preferred method for hint conversion
  • Figures 6A-6E are graphical representations of a portion of a stroke and pixel boundaries
  • Figure 7 is a flow chart of a preferred method for control point transformation
  • Figure 8 is a graphical representation of the preferred method for anti-aliasing of the glyph zone by zone to pixel space
  • Figures 9A and 9B are a flow chart of a preferred method for sampling and processing the sampled points for scan conversion
  • Figure 10 is a graphical representation of contour lines, pixel boundaries, a sample interval and a table of fractional pixel contributions.
  • Figure 11 is a graph of the transformation from font units to pixel units.
  • the method of the present invention advantageously yields pixel images that are more aesthetically pleasing and readable than the prior art for both Latin characters and Asian ideographs.
  • the method of the present invention is preferably a computer implemented process that can be used by a conventional computer to display characters on a conventional display device such as a monitor or CRT, or to print characters on a conventional laser printer.
  • FIG. 4 provides an overview of a preferred embodiment for the method of the present invention.
  • the preferred method begins in step 400.
  • a computer retrieves character information including ideal glyph representations and hints .
  • the character information is a number of closed contours and hinting data structures .
  • Each contour preferably comprises a starting point, and a number of connecting lines and curves identified by control points.
  • This character information specifies an outline of the area for the character, and is specified in font units of an abstract coordinate space.
  • the hint data may apply to all contours and control points of a character or only a subset of contours. Each hint identifies the parts of the character that should exactly fit the pixel grid boundaries.
  • step 402 the hint portion of the character information is converted from font units of the abstract coordinate space to pixel units.
  • the pixel units will be set according to operational limitations of the printer or display device being used.
  • the scaling factor is also determined by the global metrics of a type face and the desired pixel size.
  • Step 402 scales the hints using special grid fitting techniques as will be described below. However, those skilled in the art will realize that the various grid fitting techniques of the prior art could also be used.
  • the control points are transformed to pixel units in step 403.
  • the hint transformation effectively divides the image into a group of non-overlapping zones. Within the each zone, the scaling factor is slightly different from other zones which fits the hints to the pixel grid.
  • step 403 the scaling information from the hint transformation step (step 402) is used to convert the control points of the contours into pixel space to produce a series of scaled line segments and curves.
  • step 404 points at fixed intervals along each curve and line segment are sampled.
  • step 405 the sampled points are processed zone by zone to determine the fractional coloring for each pixel and produce a gray-scaled image.
  • step 406 The present invention uses this method to produce sharper transitions at those points of the glyph that have been indicated to be of aesthetic importance. Conversely, adjacent features are prevented from affecting the glyph simply because they are near a point of importance.
  • each hint preferably comprises a starting point ( s) and an ending point (e) that are designated to be "snap points”.
  • the hint also includes a canonical width (c) for the interval between the snap points.
  • the canonical width indicates the preferred distance between the snap points at low resolution.
  • the hint transformation process also uses two metrics global to the typeface: the standard horizontal and vertical stroke widths ( W) that indicate dominant stroke widths in their respective dimensions.
  • the preferred method begins in step 501 of Figure 5 by scaling the starting point (s) to the fractional value s ' .
  • the canonical width (c) is scaled to the fractional value c'.
  • the actual stroke width, the absolute value of (s-e) is scaled to the fractional value w'
  • the standard stroke width ( W) is scaled to the fractional value W .
  • the fractional values for the canonical, actual and standard stroke widths are compared. If the fractional value of the actual stroke width, w' , and the fractional value of the canonical stroke width, c' , differ by more that 0.5 of a pixel, w' is used as the desired width.
  • c ' is used as the desired width, and w ' is set equal to c ' .
  • W fractional value of the standard stroke width
  • w ' is set equal to WJ and used as the desired stroke width.
  • This step establishes either the standard or the canonical stroke width as the desired stroke width, unless they differ from the actual stroke width by more than 0.5 pixel. In such a case, the actual stroke width is used for the desired stroke width.
  • the scaling is further refined in step 506, in which the method determines whether w ' is wider than one pixel.
  • step 507 the fractional values of the starting point s ' and the actual stroke width w ' are rounded to the nearest integer.
  • step 508 the fractional value of the actual stroke width w" is added or subtracted from the fractional value of the starting point s ' depending on whether s > e to produce the transformed value of e.
  • the transformation of the hint is complete and the process terminates in step 514.
  • step 509 the fractional value of the actual stroke width w' is set to the larger of its actual value and 0.25 pixel.
  • step 510 the fractional value of the actual stroke width w' is added or subtracted from the fractional value of the starting point s' depending on whether s > e to produce the fractional value for the end point e' .
  • the method fits these points onto the pixel grid.
  • step 511 the values of s' and e' are compared to the pixel boundaries. Since it was previously determined in step 506 that ⁇ v' is less than one pixel wide, the distance spanned by s ' and e' crosses at most one pixel boundary.
  • step 512 the method determines whether s ' and e ' are in the same pixel . If they are contained in the same pixel, the stroke is enclosed in a single pixel and the values for s ' and e' are left unchanged. The process then ends in step 514. However, if a pixel boundary is spanned by the area between s ' and e', the values of s ' and e' are adjusted in step 513 to shift the stroke in the direction of the lesser error to force s ' and e' to reside in the same pixel. The shifting of the stroke in step 513 can best be understood with reference to Figures
  • FIGS. 6A-6E which show the three possible cases when the s ' and e ' are compared to the pixel grid.
  • Figure 6A no adjustment of the stroke is necessary when s ' and e' are enclosed in a single pixel.
  • FIGs 6B and 6C it can be seen that the method of the present invention shifts the stroke of Figure 6B to the left by subtracting or reducing s ' and e ' by the same value to force e ' in alignment with closest pixel boundary.
  • Figures 6D and 6E A similar adjustment in the opposite direction is shown in Figures 6D and 6E.
  • the method of the present invention shifts the stroke of Figure 6D to the right by increasing s ' and e ' by the same value to force s ' in alignment with closest pixel boundary.
  • the method of the present invention advantageously includes the benefits of stroke shifting such as the removal of undesired artifacts from the image that are caused when the ideal glyph representation does not correspond precisely to the pixel grid.
  • stroke shifting such as the removal of undesired artifacts from the image that are caused when the ideal glyph representation does not correspond precisely to the pixel grid.
  • the removal of such artifacts results in a much sharper image.
  • step 700 the process tests whether there are any remaining control points in the glyph that need to be converted from font units to pixel units. If there are no more control points to convert, then this conversion process is complete and ends in step 706. Otherwise, the preferred method will loop through steps 702-705 until each control point is transformed. In step 702, the control point to be transformed is retrieved.
  • step 703 the hint information is retrieved.
  • the method retrieves the information necessary to determine the bounds for each region, if any, that have been defined by conversion of the hints in step 402.
  • the scaling factor for each region is also determined.
  • step 704 the value of the control point being converted is compared to the hint information retrieved in step 703 to determine which zone or region contains the control point, and therefore, the appropriate scaling factor for converting the control point from font units to pixel units .
  • the control point is then scaled to pixel units in step 705 using the scaling function associated with its region or zone. Once the control point has been scaled, the method loops to step 701 and repeats the process until all the control points have been scaled.
  • Figure 11 is a graph of the transformation with pixel units along the Y-axis, font units on the X-axis and the ideal transformation (i.e., linear scaling without hinting) represented by the dashed line. Without hints, the values of the control points would simply be multiplied by a scaling factor and rounded. The introduction of hints distorts the transformation space into a discontinuous, but piece-wise linear one.
  • the width of the stroke is selected to be the canonical width, 0.9, and the ending coordinate, e ' , thus scales to s ' + w ' - 3.3.
  • the hint triple therefore divides the interval [0,500] font units into three regions or zones as shown in Figure 11.
  • the three zones range from 0 to 200, from 200 to 275 and from 275 to 500, respectively.
  • Each zone provides its own scaling function.
  • the three scaling functions are represented by the three solid line segments. Given a coordinate of X, a control point in font units, the method then determines which of the three ranges contains that value, and then applies the scaling function associated with that range or interval.
  • the three scaling functions generated by the example hint are:
  • T 0 (x) (2J/200)x, 0 ⁇ x ⁇ 200,
  • the method of the present invention performs anti-aliasing to obtain an even sharper image.
  • Anti-aliasing involves fitting the character to a source pixel grid which is larger than actually desired.
  • Each pixel in the source grid is either black (off) or white (on) , and is represented mathematically as either a minimum (0) or maximum (1) value.
  • the destination pixel grid allows the pixel values to range between 0 and 1.
  • samples When the source grid is actually reduced to the desired size, there are several pixels in the source grid (called “samples”) that correspond to a single pixel on the destination grid. These samples, whose values are either 0 or 1 are averaged together, and the result is a single final value that ranges from 0 to 1.
  • the present invention applies anti-aliasing steps to the glyph on a zone by zone basis.
  • the hinting process divides the glyph (the letter "0") into a series of zones which are defined by the starting and stopping points of the hints.
  • Each zone defines a portion of the pixel image that is predominately black or white, and the edges of - li ⁇ the zones mark areas where there is likely to be a significant color transition in the pixel grid.
  • the present invention advantageously uses the zone information created by grid fitting (i.e., hints) to influence the anti-aliasing averaging process.
  • the hints are preferably used to divide the glyph into several non-overlapping zones.
  • the glyph is divided into three vertical zones (A, B and C) by the two horizontal hints.
  • the glyph would be divided into several non-overlapping zones in both the vertical and horizontal directions.
  • step 901 the curves, if any, that form the glyph are decomposed into line segments.
  • the glyph will be represented only with line segments.
  • step 902 one of the line segments of the glyph is retrieved.
  • step 903 points are sampled on the line segment at 0.25 pixel intervals in the Y-dimension.
  • the value for the sampling intervals may be selected according to the level of gray scaling permitted by the display or printing device. In this example, 0.25 has been selected for 16 shades of gray.
  • step 903 sampling the line segment from (0.7, 0.7) to (1.7, 1.7) according to step 903 would require that the values of Y be selected as 0.75, 1.0, 1.25 and 1.5.
  • step 904 the corresponding X coordinate for each sampled value of Y is calculated. For the line segment from (0.7, 0.7) to (1.7, 1.7) this would produce sampling points (0.75, 0.75), (1.0, 1.0) , (1.25, 1.25) and (1.5, 1.5) .
  • step 905 determines whether there are any more line segments left to sample. The method preferably traverses each of the line segments of a contour until all segments of the contour have been sampled.
  • step 907 a polygon filing algorithm is used to determine the area of each fractional contribution.
  • the algorithm preferably defines the fractional contributions by specifying a beginning point and an ending point (or plurality thereof) on each scan line with the area between the points on or off. Each pair of points in a scan line represents an interval contributing to the fractional coloring of the pixels the interval crosses.
  • step 910 the contributions to the fractional coloring are determined by measuring the area of each pixel that is covered by the interval . After the contributions for each pixel have been determined, they are summed in step 911 to produce the fractional coloring value.
  • C(L 0 J, ly]) C( J, LyJ) + 0.25(Tx 0 l- ⁇ c 0 ),
  • C(bt,J, LyJ) C(L ,J, LyJ) + 0.25 for all X, such that btj ⁇ X, ⁇ Lc/J,
  • each C (p) represents the fractional coloring for each pixel which is used in step 912 to produce the image.
  • rf (l-r)jb where r is the sum of each pixel computed above.
  • the gray scale contributions for an example contour are shown.
  • the interval pixel by pixel is summed according to the above equations to produce the values in each row of the table in Figure 10.
  • the four rows for each pixel have been determined, they are tabulated to determine the gray scale coloring of each pixel .

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
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  • Computer Hardware Design (AREA)
  • Controls And Circuits For Display Device (AREA)
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  • Image Processing (AREA)

Abstract

L'invention concerne un procédé de conversion en mode point de glyphes en représentations idéales permettant d'améliorer la précision et la lisibilité de glyphes chinois et japonais. Dans un mode préféré de réalisation, ledit procédé combine avantageusement des techniques d'anticrénelage et d'ajustement de grille en vue de produire des images. Il consiste à produire des représentations idéales de glyphes et de repères, à convertir les informations de repère en provenance des jeux de caractères de l'espace d'abstraction à coordonnées en unités pixel, à transformer les points de contrôle en unités pixel en utilisant les informations de repère afin de produire une série de segments et de courbes gradués, et à traiter les points échantillonnés afin de déterminer la coloration fractionnaire pour chaque pixel et produire des transitions plus nettes au niveau des points du glyphe désignés comme revêtant une importance esthètique.
PCT/US1993/007956 1992-08-28 1993-08-24 Procede de conversion de glyphes sous forme d'images vectorielles en images en mode point combinant anticrenelage et ajustement de grille WO1994006094A1 (fr)

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AU50892/93A AU5089293A (en) 1992-08-28 1993-08-24 Glyph rasterization method combining anti-aliasing and grid fitting

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US93796292A 1992-08-28 1992-08-28
US07/937,962 1992-08-28

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EP0843283A1 (fr) * 1994-07-19 1998-05-20 Microsoft Corporation Procédé d'affichage de police de caractères avec traitement de niveaux de gris des caractères adaptés à une grille
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JPH06208370A (ja) 1994-07-26
AU5089293A (en) 1994-03-29
CN1083605A (zh) 1994-03-09

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