JP5372226B2 - Outline font compression method - Google Patents

Description

  When displaying characters on a device having a display device such as an LCD (Liquid Crystal Device), high-quality characters can be displayed from a single data in a free size by using an outline font.

  Since outline fonts require calculation of outlines and fill calculations inside outlines, they have been used mainly on computers with high calculation processing power. Due to improvements, outline fonts have been used.

  However, the capacity of a memory mounted on an embedded device is often small, and in order to implement as many functions as possible in a small memory, it is required to reduce the size of data to be stored.

  Considering that there is an upper limit on the required character size in a display device installed in an embedded device, the present invention aims to reduce the size of conventional outline font data.

  Outline font data generated by the present invention can be installed in various devices that display characters and has a wide range of use. In particular, the present invention has a high utility value because the compression ratio is high with respect to typeface fonts that use a lot of curves that have been conventionally difficult to adopt due to the large data size.

  Conventionally, as a method of reducing the size of outline font data, a method of entropy compression using coordinate values and relative values of adjacent coordinates using Huffman coding (see Patent Document 1) or the order of outlines is arranged. There is known a lossless compression method such as encoding and compressing based on continuity of codes representing straight lines and curves (see Patent Document 2).

  On the other hand, as in the present invention, as a size reduction method using approximation, a resolution reduction method, for example, outline data designed with 10000 × 10000 pixels is reduced to a level that does not impair designability on a display device (for example, 256 × A method that reduces the number of bits required to represent each coordinate by reducing the resolution to 256 pixels), resulting in a smaller data size, and assigning an appropriate line / curve formula with a smaller size ( Patent Document 3) is known. In this method, the straight line from the coordinates (3000,5000) to the coordinates (3025,4990) is 10000 × 10000 pixels and the distance is 0 from the coordinates (77,128) to the coordinates (77,128) at the resolution of 256 × 256 pixels. Thus, the data size can be reduced because it can be omitted.

Japanese Patent Laid-Open No. 06-149215 JP 2001-044850 A Japanese translation of PCT publication No. 2004-51696

  In the above lossless compression method, there is a problem that a compression rate is limited, so that a large original data size becomes a large size after compression.

  Further, in the method of reducing the resolution, if the display device has a display capability of 256 gradations and the resolution of the outline data is reduced to 256 × 256 pixels, the deviation from the original design can be covered by the anti-aliasing technology. However, even if it is not necessary to draw characters larger than 40x40 pixels on an actual device (for example, screen display in some embedded devices), reducing the resolution of outline data to 40x40 pixels will result in anti-aliasing. Even if the technology is used, the deviation from the original design cannot be covered, and an unintended display result is obtained, and it can be said that there is a limit to the compression rate as in the case of the lossless compression method.

  In order to solve the above problems, the present invention provides an outline for reducing the size of outline font data by approximating a part of a Bezier curve constituting the outline font data at a level at which a human does not feel a difference in design. An object is to provide a font compression method.

  In order to solve the above problems, the present invention has been found that there is outline data of characters designed with accuracy of size X square, and the size of characters displayed on the screen is smaller than size Y (Y <X) square. In this case, among the data representing the Bezier curve included in the outline data, a first approximation step for approximating a certain distance L or less between a certain curve and the straight line with a straight line, and the curve and the straight line in the first approximating step. A second approximation step of approximating a Bezier curve having a distance of L ′ (L ′> L) or less with two straight lines, and the degree of curve of the Bezier curve in advance. When it is close to the defined curve pattern, it comprises any one or more of a third approximation step that approximates with the pattern.

  According to the present invention, the upper limit of the size of characters output to a display device is determined for a part of the Bezier curve constituting the outline font data even when the compression rate is limited in the conventional compression method. In this case, since the outline font data size is reduced by performing approximation at a level where humans do not feel the difference in design, there is an advantage that a particularly high compression effect can be obtained for a font having a design with many contours. is there.

It is a figure which shows the structure of the outline font compression method of this invention. It is a figure which shows the data description method of a general outline font. It is a figure for demonstrating the linear approximation method of a Bezier curve. It is a figure for demonstrating the 2 straight line approximation method of a Bezier curve. It is a figure for demonstrating the curve pattern approximation method of a Bezier curve. It is a figure which shows the example at the time of making the curve pattern to approximate into a Bezier curve. It is a figure for demonstrating an approximate threshold, Comprising: (a) It is a figure which shows the correlation of spatial frequency and contrast sensitivity, (b) It is a figure explaining how to obtain | require the spatial frequency in LCD of a mobile telephone. It is an example of the table showing the correspondence between a pattern number and a patterned Bezier curve. It is a figure which shows the example at the time of making the curve pattern to approximate into a spline curve. It is a flowchart explaining the conversion processing operation | movement which concerns on embodiment of this invention. It is a flowchart explaining the conversion processing operation | movement which concerns on embodiment of this invention. It is a flowchart explaining the conversion processing operation | movement which concerns on embodiment of this invention.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Embodiment]
FIG. 1 is a block diagram showing a schematic configuration of an outline font compression method according to an embodiment of the present invention. In FIG. 1, an outline font compression method according to an embodiment of the present invention includes an outline data file 11 storing outline font data designed with, for example, 256 × 256 pixels, and an outline font of, for example, 256 × 256 pixels from the outline data file 11. An outline data file reading unit 12 that reads data, a coordinate resolution reduction unit 13 that reduces the coordinate resolution of the read 256 × 256 pixel outline font data, and outline font data that has been subjected to the coordinate resolution reduction process by approximation. An approximate line conversion unit 14 that converts part of the curve data into straight line data, an entropy compression unit 15 that performs entropy compression on all outline font data that has undergone approximate line conversion, and an entropy pressure Compressed outline data file output unit 16 for file output compressed outline data that are done, and a compressed outline data file 17 for storing the output compressed outline data. Of the above-described configurations, the other configurations except for the approximate line conversion unit 14 that converts a part of curve data into straight line data by approximation are substantially adopted in the above-described lossless compression method and resolution reduction method. it is conceivable that.

  FIG. 2 is a diagram showing a general outline font data description method. In general, outline font data is composed of a header portion describing the nature of data and a portion describing the outline of each character. In this embodiment, as shown in FIG. 2, it is assumed that the types of lines connecting the coordinate values and the coordinates are described in order from one point with a closed curve as data describing the outline of the character. In this embodiment, the original outline data is designed with a size of 256 × 256 pixels, and the display device draws only up to a size of 40 × 40 pixels.

  The processing shown in FIG. 1 is executed in order based on the premise as described above, and the outline font data whose coordinate resolution has been reduced by the coordinate resolution reduction unit 13 is approximated by the approximate line conversion unit 14 to obtain the curve data. A process for converting a part of the data into straight line data is executed. Data conversion by the approximate line conversion unit 14 is performed by first finding a Bezier curve portion of the character outline data for the character as shown in FIG. 2, determining whether approximation is possible, and performing conversion by approximation. In such a case, the Bezier curve portion is replaced with the approximated data and output. In the present invention, the conversion processing includes linear approximation, linear approximation, and curve pattern approximation for the Bezier curve.

  FIG. 3 is a diagram for explaining a linear approximation method of a Bezier curve. In FIG. 3, a Bezier curve constituting a part of the outline of the outline font is represented by four points a0, a1, a2, and a3, where a0 is a start point, a3 is an end point, and a1 and a2 are control points. When it is desired to approximate this with a straight line a0-a3, when the distance L to the point p farthest from the straight line a0-a3 in this Bezier curve is equal to or less than an “approximation threshold (described later)”, this Bezier The curve is approximated by one straight line a0-a3. This represented the Bezier curve, [Bézier curve] + [a0 X coordinate value] + [a0 Y coordinate value] + [a1 X coordinate value] + [a1 Y coordinate value] + [a2 X coordinate value] + [Y2 coordinate value of a2] + [X coordinate value of a3] + [Y3 coordinate value of a3] (see FIG. 2) is [straight line] + [X coordinate value of a0] + [a0 value of a0 It is replaced with Y coordinate value] + [X coordinate value of a3] + [Y coordinate value of a3] and output from the approximate line conversion unit 14. As a result, the size of the outline font data can be reduced.

  FIG. 4 is a diagram for explaining a two-line approximation method of a Bezier curve. In FIG. 4, when the Bezier curve approximation as shown in FIG. 3 cannot be performed, the Bezier curve is divided into two, ie, a Bezier curve from a0 to p and a Bezier curve from p to a3. When the above approximation condition shown in FIG. 3 is satisfied, a Bezier curve is represented by two straight lines a0-p and p-a3, and this is output from the approximate line conversion unit 14 as a two-line approximation of the Bezier curve. As in FIG. 2, the size of the outline font data can be reduced. Note that even if the Bezier curve is further divided and linear approximation is performed, the compression effect is lost, and therefore no further division is performed.

FIG. 5 is a diagram for explaining a curve pattern approximation method of a Bezier curve. In FIG. 5, when it is desired to approximate a Bezier curve constituting a part of the outline of the outline font to a pre-defined pattern curve (described later), the Bezier curve start point (x ₁ , y ₁ ) end point ( x _4, y ₄₎ and the distance L between the point p with distance most in the perpendicular direction of the straight line compared to the "approximation threshold (to be described later)", the distance L is "approximation threshold (described later shown in FIG. 5 ) ”, The Bezier curve is approximated by a pre-defined patterned curve. In approximation by a patterned curve, the coordinates of the two points, the start point and end point, and the pattern number are represented by 5-bit data. Accordingly, the size of the outline font data can be reduced even by the curve pattern approximation method of the Bezier curve.

FIG. 6 is a diagram illustrating an example in the case where the curve pattern to be approximated is a Bezier curve. When the curve to be patterned is a Bezier curve as shown in FIG. 6, the X coordinate of the two control points is a line segment connecting the start point (x ₁ , y ₁ ) and the end point (x ₄ , y ₄ ), respectively. Define a pattern that has 1/4 position and 3/4 position and 32 Y coordinates from -16 to 15. The defined 32 patterns are stored in a table (see FIG. 8) in association with pattern numbers and used. When the Bezier curve at the start point (x ₁ , y ₁ ), end point (x ₄ , y ₄ ), control point (x ₂ , y ₂ ), (x ₃ , y ₃ ) is approximated with a patterned Bezier curve Is expanded, reduced, and rotated so that the start and end points of the patterned curve are the same as the Bezier curve, and compared with the 32 patterns stored in the table.

  FIGS. 7A and 7B are diagrams for explaining the approximate threshold described above. When outline font data designed with 256 x 256 pixels is displayed at a size of 40 x 40 pixels, antialiasing is a technique that eliminates jagged edges of the outline by representing the pixels that pass through the outline with gradation values. This anti-aliasing technique is well known. For example, when the upper half of a contour line passing through the center of one pixel in 40 × 40 pixels is a filled region, the gradation value of this pixel is 128 in 256 gradations. However, when a person looks at a pixel corresponding to the outline of a character drawn on the LCD of a mobile phone, there is no sense of incongruity even if the gradation value changes to some extent. Conventionally, “contrast sensitivity value” has been used as a guideline for the amount of change, and a difference between gradation values that cannot be distinguished by humans can be expressed as “256 gradations ÷ contrast sensitivity value”. As shown in FIG. 7 (b), the LCD of the mobile phone is about 170dpi and can be seen from a position about 30cm away from the human eye, so the viewing angle is 7 degrees and 120 cycles, that is, the spatial frequency is 17 (cycle / cycle). Degrees). Therefore, it is derived from the graph shown in FIG. 7A that the contrast sensitivity is about 20, and the gradation value error that cannot be distinguished by humans is 12.8. When the positional deviation of the coordinate value expressed by 256 × 256 is 1, the gradation value deviation at 40 × 40 is 40. Accordingly, the allowable displacement of the coordinate value expressed by 256 × 256 = “approximate threshold” is 12.8 ÷ 40 = 0.32. If you want to know about "contrast sensitivity value" and "spatial frequency", please refer to the document "K.T. Spare, S.W. Remcourt," Visual Information Processing ", 1986, published by Science Inc.".

FIG. 8 is an example of a table showing the correspondence between pattern numbers and patterned Bezier curves. As described above, the patterned Bezier curve is the position of 1/4 of the line segment connecting the start point (x ₁ , y ₁ ) and the end point (x ₄ , y ₄ ) with the X coordinate of the two control points. Since the pattern is defined as having 3/4 positions and 32 Y coordinates from -16 to 15, pattern numbers are assigned accordingly. The Bezier curve is expressed as Equation 3 described later.

FIG. 9 is a diagram illustrating an example in which the approximate curve pattern is a spline curve. As shown in FIG. 9, in spline curve approximation, the X coordinate of one control point is set to the half of the line segment connecting the start point (x ₁ , y ₁ ) and the end point (x ₄ , y ₄ ). , Define 32 patterns with Y coordinates from -16 to 15. The 32 patterns defined are stored in a table (not shown) in association with the pattern number for use.

10A to 10C are flowcharts for explaining the conversion processing operation according to the embodiment of the present invention. The conversion processing operation will be described based on the flowcharts of FIGS. 10A to 10C. In step S1, the coordinate value (X, Y) is read from the head of the outline data. In step S2, the coordinate value read in step S1 is set as the start point (x ₁ , y ₁ ). In step S3, the line type is read from the outline data. In step S4, if the line type read in step S3 is a Bezier curve, the process proceeds to step S5. Otherwise, the coordinates of the end point are read from the outline data, and then the process proceeds to step S36.

In step S5, the coordinate value (X, Y) is read from the outline data. In step S6, the coordinate value read in step S5 is set as a Bezier curve control point 1 (x ₂ , y ₂ ). In step S7, coordinate values (X, Y) are read from the outline data. In step S8, the coordinate value read in step S7 is set as the control point 2 (x ₃ , y ₃ ) of the Bezier curve. In step S9, coordinate values (X, Y) are read from the outline data. In step S10, the coordinate value read in step S9 is set as the end point (x ₄ , y ₄ ) of the Bezier curve.

  In step S11, (1) of the Bezier curve is derived from the four points of the start point, the control point 1, the control point 2, and the end point.

  In step S12, the equation (2) of the straight line S connecting the start point and end point of the Bezier curve is derived.

  In step S13, a point P on the Bezier curve that is the farthest distance between the straight line S and the Bezier curve in the perpendicular direction of the straight line S is calculated from the equations (1) and (2). In step S14, the distance L between the straight line S and the point P is calculated from the equation (2) and the coordinate value of the point P obtained in step S13. In step S15, the distance L is converted into a distance l at the maximum size drawn on the display device. In step S16, when the distance l = the gradation error value l is equal to or less than the limit value M of the gradation error that can be distinguished by the human eye, the process proceeds to step S17, and if not, the process proceeds to step S18. In step S17, since the Bezier curve is approximated by a straight line S, the original outline data of the line type “Bézier curve” coordinate values “control point 1” and “control point 2” is replaced with the line type “straight line” and output. Proceed to step S33.

In step S17, since the Bezier curve is approximated by a straight line S, the original outline data of the line type “Bézier curve” coordinate values “control point 1” and “control point 2” is replaced with the line type “straight line” and output. Proceed to step S33. In step S18, the expression (3) for approximating with the Bezier curve when the start point of the pattern curve is placed at the start point (x ₁ , y ₁ ) and the end point at (x ₄ , y ₄ ), or a spline Formula (4) in the case of approximating with a curve is derived.

In the above equation (3), the X coordinates of the two control points are respectively the 1/4 position and 3/4 of the line segment connecting the start point (x ₁ , y ₁ ) and the end point (x ₄ , y ₄ ). When a position is set, and a Y coordinate is −16 to 15 and q is regarded as a pattern number, a patterned Bezier curve corresponding to the number is obtained, the table shown in FIG. 8 is obtained.

  In step S19, 0 is substituted for pattern number q. In step S20, the distance L when the Bezier curve and the approximate curve are farthest in the perpendicular direction of the straight line S is calculated from the Bezier curve equations (3) and (1) corresponding to the pattern number q. In step S21, the value of pattern number q is increased by 1. In step S22, when the value of the pattern number q is 32, the process proceeds to step S23, and if not, the process proceeds to step S20. In step S23, Q is the pattern number when L is the smallest value in the loop of steps S19 to S22. In step S24, the distance L in the pattern Q is converted into the distance l in the maximum size drawn on the display device. In step S25, when the distance l = the gradation error value l is equal to or less than the limit value M of the gradation error that can be distinguished by human eyes, the process proceeds to step S26. Otherwise, the process proceeds to step S27.

In step S26, since the Bezier curve is approximated by a patterned approximate curve, the original outline data of the line type “Bézier curve” coordinate values “control point 1” and “control point 2” is used as the line type “pattern curve”. And "pattern number Q" is output and the process proceeds to step S37. In step S27, the equation (5) of the straight line S ₁ connecting the start point (x ₁ , y ₁ ) and the point P (x _p , y _p ) is derived.

At step S28, in the perpendicular direction of the straight line S _1, the distance L ₁ of the case where the start point (x _1, y ₁₎ the point from _P (x _p, y _p) is the Bezier curve and a straight line S ₁ connecting to farthest Calculate from Equation (1) and Equation (5). In step S29, it converted to a distance l ₁ at a maximum size drawn a distance L ₁ calculated in step S28 on the display device. In step S30, when the distance l ₁ = the gradation error value l ₁ is equal to or less than the limit value M of the gradation error that can be distinguished by the human eye, the process proceeds to step S31. Otherwise, the process proceeds to step S36.

In step S31, it derives the point _P (x _p, y _p) and the end point (x _4, y ₄₎ wherein the straight line S ₂ connecting (6).

In step S32, in the perpendicular direction of the straight line S _2, the point _P (x _p, y _p) end point from the (x _4, y ₄₎ the distance L ₂ in the case of the Bezier curve and the line S ₂ is farthest connecting to Calculate from Equation (1) and Equation (6). In step S33, it converted into the distance l ₂ at a maximum size drawn a distance L ₂ obtained in step S32 on the display device. In step S34, when the distance l ₂ = the gradation error value l ₂ is equal to or less than the gradation error limit value M that can be distinguished by the human eye, the process proceeds to step S35, and if not, the process proceeds to step S36.

In step S35, since the Bezier curve is approximated by two straight lines, the original outline data of the line type “Bezier curve” coordinate values “control point 1” and “control point 2” is converted into the line type “straight line” and the coordinate values. The output is replaced with “point P (x _p , y _p )” and the line type “straight line”, and the process proceeds to step S37. In step S36, since the Bezier curve conversion process does not occur, the original data is output as it is. In step S37, the coordinate value (x ₄ , y ₄ ) of the end point is substituted as the start point (x ₁ , y ₁ ) of the next line. In step S38, the contour line has been written. If the closed curve is completed, the process ends. If not, the process returns to step S3.

  By performing this flow for all characters, the entire outline font file can be compressed. In the above-described flow, the example in which the approximation process is executed using all the first to third approximation steps according to the present invention has been described. However, any one of the first to third approximation steps according to the present invention is described based on this flow. It is also possible to change to execute one or more approximation processes.

  After this processing is performed, statistics on the appearance frequencies of all coordinate values and the types of lines connecting the coordinates are obtained, and existing entropy compression processing such as Huffman coding is performed.

In summary, the present invention has the following configuration.
(Supplementary note 1) When there is outline data of a character designed with accuracy of size X square and the size of the character displayed on the screen is known to be less than or equal to size Y (Y <X) square,
Of the data representing the Bezier curve included in the outline data,
A first approximating step of approximating a straight line with a distance between a certain curve and a straight line that is less than a certain value L;
A second approximating step of approximating a Bezier curve having a length equal to or less than L ′ (L ′> L) but a length of the distance between the curve and the straight line by two straight lines in the first approximating step; and,
An outline font compression method comprising: one or more of a third approximation step for approximating a curve degree of the Bezier curve close to a predefined curve pattern.

(Appendix 2) The first approximating step is:
When a character designed with size X square accuracy is displayed on the screen with size Y square, it is assumed that the contour part of the character is expressed by an anti-aliasing method,
The character is represented by a size Y square, and the gradation value of the contour pixel when a part of the contour of the character is drawn as a Bezier curve according to the original outline data,
When the contour line is drawn as a straight line connecting the start and end points of the Bezier curve,
When it is less than the difference between the gradation values that are considered indistinguishable when viewed with the human eye,
The outline font compression method according to appendix 1, wherein the Bezier curve data is converted into straight line data connecting a start point and an end point of a Bezier curve.

(Appendix 3) The second approximation step is
When a character designed with size X square accuracy is displayed on the screen with size Y square, it is assumed that the contour part of the character is expressed by an anti-aliasing method,
The character is represented by a size Y square, and the gradation value of the contour pixel when a part of the contour of the character is drawn as a Bezier curve according to the original outline data,
When A is the point where the distance between the Bezier curve start point and end point and the Bezier curve is the farthest, the straight line connecting the Bezier curve start point and A and the straight line connecting A and the Bezier curve end point are When the contour line is drawn as two straight lines, the difference from the gradation value of the contour pixel is
When it is less than the difference between the gradation values that are considered indistinguishable when viewed with the human eye,
The outline font compression method according to attachment 1, wherein the Bezier curve data is converted into two pieces of straight line data.

(Appendix 4) The third approximation step is
The normal line of the point where the distance between the straight line connecting the start point and the end point of the Bezier curve and the Bezier curve is farthest passes through the midpoint of the straight line, and the Bezier curve having N patterns as the curvature is previously set as the pattern number. It has a table to define with correspondence,
When a character designed with size X square accuracy is displayed on the screen with size Y square, it is assumed that the contour part of the character is expressed by an anti-aliasing method,
The character is represented by a size Y square, and the gradation value of the contour pixel when a part of the contour of the character is drawn as a Bezier curve according to the original outline data,
The difference from the gradation value of the contour pixel when the contour line is drawn with one of the N pattern curves defined in the table,
When it is less than the difference between the gradation values that are considered indistinguishable when viewed with the human eye,
The outline font compression method according to appendix 1, wherein the Bezier curve data is converted into a pattern number corresponding to the pattern curve.

(Supplementary Note 5) The difference between the gradation value of the contour pixel before approximation and the gradation value of the contour pixel after approximation is
The above first to second conditions are met on condition that the distance between the Bezier curve and the approximate line does not exceed the distance after the maximum gradation value error that cannot be distinguished by the human eye is converted into a distance of size X square accuracy. 5. The outline font compression method according to any one of appendices 2 to 4, which is approximated in any of the three approximation steps.

  (Supplementary Note 6) After a part of the Bezier curve is converted into a straight line or a curve using any of the first to third approximation steps, the converted start and end points and the Bezier curve control points are represented. The outline font compression method according to appendix 1, wherein the coordinate values are collectively compressed by entropy coding.

(Appendix 7) In the third approximation step,
The outline font compression method according to appendix 4, wherein the curve defined in advance is a spline curve in which the control point is on a perpendicular passing through the midpoint of the straight line connecting the start point and the end point, and the curvature thereof is N patterns. .

  In the present invention, the Bezier curve can be expressed with a small amount of data by effectively using the above three approximation steps, and therefore, the data can be greatly reduced particularly for a font having a design with many curve portions. Therefore, it is effective for display devices installed in embedded devices.

11 Outline Data File 12 Outline Data File Reading Unit 13 Coordinate Resolution Reduction Unit 14 Approximate Line Conversion Unit 15 Entropy Compression Unit 16 Compressed Outline Data File Output Unit 17 Compressed Outline Data File

Claims (3)

If there is outline data of characters designed with size X square accuracy and the size of the characters displayed on the screen is known to be less than size Y (Y <X) square,
Among the data representing the Bezier curve included in the outline data, a straight line that has a distance of a certain value L or less between a certain curve included in the data representing the Bezier curve and a straight line connecting the start point and the end point of the Bezier curve A first approximation step approximated by
In the first approximating step, a Bezier curve whose distance between the curve and the straight line is larger than L but whose distance is L ′ (L ′> L) or less, and the start point of the Bezier curve and two Bezier curves are used. A second approximation step of approximating with two straight lines connecting a dividing point to be divided into two and a straight line connecting the dividing point and the end point of the Bezier curve;
When the degree of curve of the Bezier curve is close to a predefined curve pattern, it comprises any one or more of a third approximation step for approximating with the pattern,
The second approximation step includes
When a character designed with size X square accuracy is displayed on the screen with size Y square, it is assumed that the contour part of the character is expressed by an anti-aliasing method,
The character is represented by a size Y square, and the gradation value of the contour pixel when a part of the contour of the character is drawn as a Bezier curve according to the original outline data,
When the point where the distance between the start point and the end point of the Bezier curve and the Bezier curve is the farthest is the dividing point, the straight line connecting the start point and the dividing point of the Bezier curve, the dividing point, and the end point of the Bezier curve The difference from the gradation value of the contour pixel when the contour line is drawn as two straight lines to be connected,
When it is less than the difference between the gradation values that are considered indistinguishable when viewed with the human eye,
An outline font compression method characterized by converting Bezier curve data into two straight line data. If there is outline data of characters designed with size X square accuracy and the size of the characters displayed on the screen is known to be less than size Y (Y <X) square,
Among the data representing the Bezier curve included in the outline data, a straight line that has a distance of a certain value L or less between a certain curve included in the data representing the Bezier curve and a straight line connecting the start point and the end point of the Bezier curve A first approximation step approximated by
In the first approximating step, a Bezier curve whose distance between the curve and the straight line is larger than L but whose distance is L ′ (L ′> L) or less, and the start point of the Bezier curve and two Bezier curves are used. A second approximation step of approximating with two straight lines connecting a dividing point to be divided into two and a straight line connecting the dividing point and the end point of the Bezier curve;
When the degree of curve of the Bezier curve is close to a predefined curve pattern, it comprises any one or more of a third approximation step for approximating with the pattern,
The third approximation step includes
The normal line of the point where the distance between the straight line connecting the start point and the end point of the Bezier curve and the Bezier curve is farthest passes through the midpoint of the straight line, and the Bezier curve having N patterns as the curvature is previously set as the pattern number. It has a table to define with correspondence,
When a character designed with size X square accuracy is displayed on the screen with size Y square, it is assumed that the contour part of the character is expressed by an anti-aliasing method,
The character is represented by a size Y square, and the gradation value of the contour pixel when a part of the contour of the character is drawn as a Bezier curve according to the original outline data,
The difference from the gradation value of the contour pixel when the contour line is drawn with one of the N pattern curves defined in the table,
When it is less than the difference between the gradation values that are considered indistinguishable when viewed with the human eye,
An outline font compression method, wherein Bezier curve data is converted into a pattern number corresponding to the pattern curve. The difference between the tone value of the contour pixel before approximation and the tone value of the contour pixel after approximation is
The above first to second conditions are met on condition that the distance between the Bezier curve and the approximate line does not exceed the distance after the maximum gradation value error that cannot be distinguished by the human eye is converted into a distance of size X square accuracy. The outline font compression method according to claim 1, wherein the outline font compression method is approximated in any one of the three approximation steps.

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