JPH08110957A - Device and method for shading picture pattern - Google Patents

Abstract

PURPOSE: To provide a shading device capable of easily determining a correction value and executing quick shading processing for a picture pattern. CONSTITUTION: A pattern memory 1 storing original data is connected to a reading circuit 2. The circuit 2 is connected to a frame memory 5 through a saturation processing circuit 4. A correction value generating circuit 3 divided into R-component, G-component and B-component generating parts 3a to 3c is connected to the circuit 2. The circuit 3 is connected to the circuit 4 through a a correction value calculating circuit (DDA) 6. Each of R-component, G- component and B-component saturation processing circuits 4a to 4c constituting the saturation processing circuit 4 is provided with an adder 8, an overflow/ underflow signal generating circuit 9, an overflow processing circuit 10, and an underflow processing circuit 11. The frame memory 5 is connected to a display.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image pattern shading apparatus and method for performing a shading process on an image pattern such as a sprite used for displaying characters in a game.

[0002]

2. Description of the Related Art Arcade games and home-use video games enjoy the operation of a specific character displayed on the screen of a display with a joystick or push button switches. Therefore,
Introducing many characters with excellent display technology,
If you can make various changes, the interest in the game will increase. Characters in such games are typically displayed by sprites. What is a sprite?
This is an image pattern in which a figure repeatedly used in screen display is divided into rectangles or the like in advance and registered as original data, and the original data is displayed by being superimposed on another image. Since sprites can be moved or redrawn at high speed on the background screen (scroll screen), some sprites are prepared, and the game character moves around by redrawing while moving the specified coordinates. You can create animation effects.

Such a sprite has an infinite number of pixel data (pixel data) represented by a predetermined number of bit strings.
In the case of expressing a solid body having a shadow, it is necessary to store in advance the original data having a shadow (the brightness at the time of displaying on the display is determined) in the memory. However, since the brightness of such original data is fixed, there is no flexibility in the expression of light reflection due to the movement or change of the light source, the movement or change of the sprite, etc. Accordingly, a lot of original data must be stored, which is a burden on the memory.

To cope with this, a shading device has been developed which changes the luminance of each pixel by adding a correction value to the pixel data in the original data. This shading device has a pattern memory, a read circuit, a correction value generation circuit, an addition circuit, a frame memory, etc., and performs shading work as follows. That is, first, the read circuit reads the original data from the pattern memory. The pixel data in this original data is RGB data composed of an R component (red), a G component (green), and a B component (blue). The correction value generation circuit
A correction value corresponding to each of these RGB components is generated. This correction value is represented by the same number of bits as the pixel data. Then, the correction value is added to the RGB data of the original data by the adder circuit, and this data is written in the frame memory. Further, the data written in the frame memory is displayed on the display.

[0005]

However, the conventional shading device for sprites as described above has the following problems. That is, since it is necessary to add the correction value to all the pixels of the original data, the work of determining the correction value for each of a large number of pixels is very troublesome and the shading processing takes time.

Further, since the number of bits of displayable data is limited, when the correction value is added to the original data, the value becomes smaller than the value expressible by the bit string (called underflow). ) Or a large value (called overflow) makes it difficult to display on the display.

The present invention has been proposed in order to solve the above problems, and an object thereof is to provide a shading device for an image pattern, which can easily determine a correction value for original data. That is.

A second object is to provide a shading device for an image pattern, which can quickly determine a correction value for original data.

A third object is to provide a shading device for an image pattern that can generate image data that can be displayed on a display even when an underflow or an overflow occurs.

A fourth object of the present invention is to provide a shading device for an image pattern which can rapidly perform underflow processing and overflow processing.

A fifth object of the present invention is to provide a shading method for an image pattern in which a correction value for original data can be easily determined.

A sixth object is to provide a shading method for an image pattern capable of generating image data which can be displayed on a display even when an underflow or an overflow occurs.

[0013]

In order to achieve the above object, the invention according to claim 1 is a pattern memory storing original data for displaying an image pattern on a screen of a display device, and the original data. In a shading device for an image pattern, which has a correction value generating means for generating a correction value for changing the luminance in each pixel and an adding means for adding the correction value to the original data, It is characterized in that a correction value calculating means for determining a correction value for a pixel on a straight line connecting the two points based on the correction value for the pixel is provided.

According to a second aspect of the present invention, in the shading apparatus for the image pattern according to the first aspect, the correction value calculating means is a digital differential analyzer constituted by a digital circuit.

According to a third aspect of the present invention, a pattern memory in which original data for displaying an image pattern on a screen of a display device is stored in a representation format of a predetermined number of bit strings, and a luminance in the original data for each pixel. A shading device for an image pattern, comprising: a correction value generating means for generating a correction value to be changed in the expression format of the bit string; and an adding means for adding the correction value to the original data. When the addition result with the correction value is smaller than the minimum value that can be expressed by the bit string, the minimum value is output, and when it is larger than the maximum value that can be expressed by the bit string, the maximum value is output. A processing means is provided.

According to a fourth aspect of the present invention, in the shading apparatus for the image pattern according to the third aspect, the saturation processing means, when the addition result of the addition circuit is smaller than the minimum value that can be represented by the bit string. An overflow / underflow signal generation circuit that generates an underflow signal and generates an overflow signal when it is larger than the maximum value that can be expressed by the bit string, and all bits in the bit string when the overflow signal is input. Is output as 1, and an overflow processing circuit that outputs the bit string of the addition result as it is when the overflow signal is not input, and outputs all bits in the bit string as 0 when the underflow signal is input, If the underflow signal is not input Characterized by comprising a underflow processing circuit directly outputs a bit string of the addition result to.

According to a fifth aspect of the present invention, a correction value for changing the brightness of the original data for displaying the image pattern on the screen of the display device for each pixel is generated, and the correction value is added to the original data. In the shading method for the image pattern, any 2 of the original data is used.
It is characterized in that the correction value for the pixel on the straight line connecting the two points is determined based on the correction value for the pixel at the point.

According to a sixth aspect of the present invention, the original data for displaying the image pattern on the screen of the display device is represented by a predetermined number of bit strings, and the correction value for changing the luminance of the original data for each pixel is set. In a shading method for an image pattern that is generated in a bit string representation format and that adds the correction value to the original data, if the addition result of the original data and the correction value is smaller than the minimum value that can be expressed by the bit string. Is generated, and the maximum value is generated when it is larger than the maximum value that can be represented by the bit string.

[0019]

The operation of the present invention having the above construction is as follows. That is, according to the first aspect of the present invention, the user determines the correction value for any two points of pixels in the original data stored in the pattern memory. The correction value calculation means determines the value of the correction value for the pixel on the straight line connecting the two points, based on the correction values of the two points. The correction value determined as described above is added to the original data by the adding means, and the shading processing is performed.

According to the second aspect of the invention, when the correction value between any two points is determined as described above, the correction values of the two points are input to the correction value calculating means which is a digital differential analyzer. Then, the correction values for the pixels on the straight line connecting the two points are calculated by substituting the correction values for the two points into the predetermined function.

According to another aspect of the present invention, when the addition result of the original data and the correction value by the addition circuit is smaller than the minimum value that can be expressed by a predetermined number of bit strings, the saturation processing circuit performs the minimum value. Is output. Then, when the addition result of the original data and the correction value by the addition circuit is larger than the maximum value that can be expressed by a predetermined number of bit strings, the saturation processing circuit outputs the maximum value. Therefore, regardless of the value of the addition result, the sprite data shaded within the limit of the maximum and minimum values that can be displayed is generated.

According to the fourth aspect of the invention, when the addition result of the adder circuit is smaller than the minimum value that can be expressed by a predetermined number of bit strings, the overflow / underflow processing circuit outputs an underflow signal. When the addition result is larger than the maximum value that can be represented by a predetermined number of bit strings, the overflow / underflow processing circuit outputs an overflow signal.

When the overflow signal is input to the overflow processing circuit, all bits in the bit string are output as 1, and when the overflow signal is not input, the bit string of the addition result is output as it is. When an underflow signal is input to the underflow processing circuit, all bits in the bit string are output as 0, and when an underflow signal is not input,
The bit string of the addition result is output as it is. Therefore, even if the addition result exceeds the expression limit of the bit string, a correction value corrected to a value that can be expressed by the bit string is generated.

According to the invention of claim 5, the correction values for pixels at arbitrary two points in the original data are determined by the user. Next, the value of the correction value for the pixel on the straight line connecting the two points is determined based on the correction values of the two points.
The correction value determined as described above is added to the original data, and shading processing is performed.

According to the sixth aspect of the invention, when the addition result of the original data and the correction value is smaller than the minimum value that can be represented by a predetermined number of bit strings, the minimum value is generated.
Then, when the addition result of the original data and the correction value is larger than the maximum value that can be represented by a predetermined number of bit strings, the maximum value is generated. Therefore, no matter what value the addition result is, sprite data shaded within the limits of the maximum value and the minimum value that can be represented by a predetermined number of bit strings is generated.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment corresponding to the present invention described in claims 1 to 6 will be described below with reference to the drawings. In addition,
Each means described in the claims is a circuit realized on a predetermined IC chip. Further, among the numerical expressions shown below, the ones with H added at the end indicate that they are hexadecimal numbers. The sprite data is composed of RGB signals represented by 5 bits without a sign, and RGB can take values from 00H to 1FH, respectively.
Values above H are not expressible and cannot be displayed.

(1) Structure of the Embodiment First, the structure of this embodiment will be described below. That is, this embodiment includes a pattern memory 1, a read circuit 2, a correction value generation circuit 3, a saturation processing circuit 4, a frame memory 5, a correction value calculation circuit 6 and a drawing command generation circuit 7, as shown in FIG. ing. The pattern memory 1 stores original data of sprite and is connected to the reading circuit 2. The reading circuit 2 is connected to the frame memory 5 via the saturation processing circuit 3. Readout circuit 2
Is to read original data from the pattern memory 5 and transfer the data to the saturation processing circuit 4 for each dot.

The correction value generating circuit 3 includes an R component generating section 3a,
It is divided into a G component generating section 3b and a B component generating section 3c, which are connected to the reading circuit 2. The R component generation unit 3a, the G component generation unit 3b, and the B component generation unit 3c generate correction values for the R component (red), G component (green), and B component (blue), respectively, and calculate the correction value. It is connected to the saturation processing circuit 4 via the circuit 6. The R component generation unit 3a, the G component generation unit 3b, and the B component generation unit 3c respectively include an R component (red) and a G component.
The correction values of the component (green) and the B component (blue) are generated. The generated data is 5 bits with a sign of -10H ~
Values in the range up to + 0FH are possible. Here, 5
In order to handle positive and negative numbers uniformly in bit data, complement expression is used to represent negative numbers. Therefore, when the 5 most significant bits in the correction value are 1, it becomes a negative value, and when the 5 most significant bits are 0, it becomes 0.
Or a positive value.

The correction value calculation circuit 6 is a digital differential analyzer (DDA: Digital) using a digital circuit.
Differentiation Analyzer) is realized, and the amount of change in luminance between two vertices is calculated based on the luminance of each vertex of the sprite. For example, let each vertex of a rectangular sprite be a, b,
c and d, ab, bc, cd, d-
It has a function of obtaining the amount of change in the luminance of the pixel data between a.

The saturation processing circuit 4 is an R component saturation processing circuit 4.
a, G component saturation processing circuit 4b, B component saturation processing circuit 4c
R component generator 3 of correction value generator 3
It corresponds to the a, G component generator 3b and B component generator 3c. Each saturation processing circuit 4a, 4b, 4c includes an adding circuit 8, an overflow / underflow signal generating circuit 9,
An overflow processing circuit 10 and an underflow processing circuit 11 are configured. As shown in FIG. 2, the adder circuit 8 adds a correction value corresponding to each component of the original data to calculate a sum (S), and also a carry signal (C
R) is output. The correction value is 5 bits with a sign, but in this addition, it is calculated as 5 bits without a sign. The CR value is 1 when the above S is 20H or more.
And is set to 0 for other values.

As shown in FIG. 3, the overflow / underflow signal generating circuit 9 is a logic circuit composed of two NOT circuits, one AND circuit and one NAND circuit, and has the following functions. . That is, as shown in the combination table of FIG. 4, when CR is 1 and the most significant bit of the correction value is 0, that is, when the correction value is a positive value, an overflow occurs, so a value of 1 is output as the overflow signal. Further, when CR is 0 and the most significant bit of the correction value is 1, that is, when the correction value is a negative value, underflow occurs, so a value of 0 is output as the underflow signal. In cases other than the above, 0 is output as the overflow signal and 1 is output as the underflow signal. Therefore, in the output from the overflow / underflow signal generation circuit 9, the overflow signal is never 1 and the underflow signal is never 0.

As shown in FIG. 5, the overflow processing circuit 10 is composed of five OR circuits corresponding to each bit of 5-bit data. Therefore, when the overflow signal value is 0, the overflow processing circuit 10 outputs the S value of the addition result as it is to R0 to R4, and when the overflow signal is 1, all bits of R0 to R4 are forced to 1 Set to.

The underflow processing circuit 11 is composed of five AND circuits corresponding to each bit of 5-bit data. Therefore, when the underflow signal is 0, all bits of R0 to R4 are forcibly set to 0, and when the underflow signal is 1, R0 to R4, which is the S value of the addition result, is directly stored in the frame memory 5. Of the write data W0 to W4. As described above, W0
The values of to W4 are all 1s when there is an overflow, all 0s when there is an underflow, and otherwise have the same value as the addition result S. An example of such a calculation result is shown in FIG.

Further, the frame memory 5 is a memory interposed between the saturation processing circuit 4 and a display (not shown), and the write data from the saturation processing circuit 4 is written and written in a predetermined area. It outputs data to the display. The writing area is designated by the drawing command generating circuit 7.

(2) Operation of the Embodiment The operation of this embodiment having the above-mentioned structure will be described below. In order to simplify the description, the original data and the write data drawn in the frame memory 5 are simple figures as follows. That is, as shown in FIGS. 7A and 7B, it is assumed that the original data and the write data are rectangular with horizontal H dots and vertical V dots, and are composed of three colors. Also, the original data shall be copied to the frame memory at the same size without enlargement or reduction. The apex of the original data is a,
b, c, d, and the vertices of the write data drawn in the frame memory are a ', b', c ', d'. To draw the write data, the original data is read from the vertex a to the vertex b, and one dot is drawn in the direction from a'to b'of the frame memory. This operation is called line drawing. The line drawing is repeated by moving one dot downward in the figure from the side a-d to the side bc, which is the end point of the line, and from the side a'-d 'to the side b'-c'. If the above operation is completed at the vertices d and c and the vertices d ′ and c ′, the drawing for one sprite is completed.

Both the original data and the write data are RGB data, and the values of the RGB components are 00H to 1FH as described above.
Takes the value of. The correction value is -10H to + 0FH as described above.
Takes a value, and a negative value plays a role of reducing the brightness when drawing the original data, and a positive value plays a role of increasing the brightness. Both the original data and the correction value are gray codes in which the R, G, and B components have the same value.

Given the above settings, the operation of this embodiment will be described below with reference to the flowchart of FIG. 8 showing the processing procedure. First, the original data is read from the pattern memory 1 by the read circuit 2 (step 70).
1). As shown in FIG. 9, the user selects the vertex a of the original data,
The correction values for the respective R, G, B components of the four points b, c, d are determined and output from the correction value generation circuit 3 (step 702).

Next, the correction value calculation circuit 6 (DDA) obtains RGB correction values for each of the RGB data between a and d and between b and c. This is performed based on a graph in which the correction value is the y coordinate and the position on the display screen is the x coordinate. For example, as shown in FIG. 10, two points P and Q or two points V and D can be given on the coordinate based on the correction values and the positions of the two points a and b on the graph. Then, according to the value of y in the straight line graph (proportional function) connecting the two points, each R between a and b is
A correction value for GB data is obtained. Similar to the above, FIG.
As shown in FIG. 1, a correction value is also obtained between bc (step 703). Further, as shown in FIG. 12, the correction value calculating circuit 6 similarly obtains the correction value of the end portion (e-f) of an arbitrary horizontal line in the H direction in the V direction in the same manner as described above (step 704).

When the correction values for all the dots of the original data are calculated as shown above (shown in FIG. 13), the correction values are added to the original data in the adder circuit (step 70).
5). At the same time, in the adder circuit 8, the addition result S is 20
It is determined whether or not it is H or more. If it is 20H or more, 1 is output as the carry signal CR, and 0 otherwise.
Is output (step 706). In the overflow / underflow signal generation circuit 10, when CR is 1, it is determined whether the most significant bit of the correction value is 0, that is, the correction value is a positive value (step 707). 1 is output (step 708). When CR is 0, it is determined whether the most significant bit of the correction value is 1, that is, the correction value is a negative value (step 70).
9) If it is negative, 0 is output as an underflow signal (step 710).

In the overflow processing circuit 10,
When the overflow signal is 0, the addition result S is directly output to R0 to R4 (step 711). When the overflow signal is 1, all bits of S are forced to 1 (step 712). In the underflow processing circuit 11, when the underflow signal is 0, R
All bits of 0 to R4 are forcibly set to 0 (step 713). When the underflow signal is 1, R0-
R4 is directly output as write data (step 711).

The write data created by repeating the above work is written in the area designated by the drawing command generating circuit 7 in the frame memory 5, as shown in FIG. 14 (step 714). By outputting this write data from the frame memory 5 to the display as sprite data, a sprite screen with smooth shading is displayed on the display as shown in FIG. 14 (step 715).

(3) Effects of Embodiments The effects of the present invention as described above are as follows. That is, when the correction value is determined, the correction value calculation circuit 6 is used to set the correction values of the RGB data at the vertices of the original data of the sprite, and the correction values of the RGB data at all other points are set. I can decide. Therefore, the correction value can be easily determined. Further, since the digital differential analyzer is used as the correction value calculation circuit 6, the correction value between arbitrary vertices can be quickly determined by a predetermined function.

Even if overflow or underflow occurs when the correction value is added to the original data, the saturation processing circuit 4 performs saturation processing to correct the value so that it can be displayed. It will not cause any problems. Further, when overflow or underflow occurs, all bits are forcibly set to 1 or all bits by the overflow / underflow signal generation circuit 9, the overflow processing circuit 10, and the underflow processing circuit 11 regardless of the degree of the overflow or underflow. Since it becomes 0, rapid saturation processing can be realized.

(4) Other Embodiments The present invention is not limited to the above embodiments, and the contents of data processing of each part can be changed as appropriate. That is, in the above embodiment, RGB in each pixel is
However, if the luminance of each pixel is changed to a different value, the hue can be changed. In addition, although the pixel data is represented by 5 bits in the above embodiment, the number of bits is not limited to 5,
It may be 5 or more if it is desired to make the change in brightness or hue more complicated, and may be 5 or less if it is desired to make it more singular. Furthermore, although the frame data is copied at the same size as the original data in the above embodiment, the present invention can be applied even when the frame data is copied after being reduced, enlarged or modified.

In the above embodiment, when the correction value calculation circuit calculates the correction value between two vertices, the graph of the proportional function connected by the straight line is used. A variety of changes can be given to, and subtle expressions are possible.

Further, although an example of smooth shading for a sprite is shown in the present embodiment, the present invention, for example, is a line which is a straight line connecting two points, a quadrilateral polyline in which four lines are connected, Shading can be applied to a polygon that is a quadrangle without an original picture.

[0047]

The effects of the present invention as described above are as follows. That is, by providing the correction value calculation means, it is possible to provide a shading device for an image pattern that can easily determine a correction value for original data.

By using a digital differential analyzer as the correction value calculation circuit, it is possible to provide a shading device for an image pattern which can quickly determine a correction value for original data.

By providing the saturation processing means, it is possible to provide a shading device for an image pattern capable of generating image data that can be displayed on a display even when an underflow or an overflow occurs.

By providing the saturation processing means with an overflow / underflow signal generating circuit, an overflow processing circuit and an underflow processing circuit, a shading device for an image pattern capable of rapidly performing underflow processing and overflow processing is provided. can do.

Since the value of the correction value for the pixel on the straight line connecting the two points is determined based on the correction value of the two points determined by the user, it is possible to easily determine the correction value for the original data. A shading method for an image pattern can be provided.

In the case of underflow, the minimum value that can be displayed on the display is generated, and when the overflow occurs, the maximum value that can be displayed on the display is generated, so what value is the addition result of the original data and the correction value? Even so, it is possible to provide a so-called shading method for an image pattern that can generate image data that can be displayed on a display.

[Brief description of drawings]

FIG. 1 is a circuit block diagram showing an embodiment of a shading device for an image pattern of the present invention.

FIG. 2 is a circuit block diagram showing an adder circuit in the embodiment of FIG.

FIG. 3 is a circuit block diagram showing an overflow / underflow signal generation circuit in the embodiment of FIG.

FIG. 4 is a table showing combinations of CR and overflow signals and underflow signals in the embodiment of FIG.

5 is a circuit block diagram showing an overflow processing circuit and an underflow processing circuit in the embodiment of FIG.

FIG. 6 is a diagram showing an example of a data value obtained by performing a saturation process by adding a correction value to the original data in the embodiment of FIG.

FIG. 7 is a diagram showing an example of a sprite used in the embodiment of FIG. 1, where (A) is original data and (B) is write data.

FIG. 8 is a flowchart showing a processing flow of the embodiment of FIG.

9 is a diagram showing an example of correction values for the four vertices of the sprite of FIG.

10 is a diagram showing an example of a function graph used in the correction value calculating means in the embodiment of FIG.

11 is a diagram showing an example in which a correction value between two vertical vertices of a sprite is calculated based on the correction value of FIG.

12 is a diagram showing an example of calculating a correction value between apexes of a sprite in the horizontal direction based on the correction value of FIG.

13 is a diagram showing an example in which all correction values for the sprite of FIG. 7 are calculated.

14 is a diagram showing an example in which the correction value of FIG. 13 is added to the sprite of FIG.

FIG. 15 is a diagram showing an example in which a flat sprite is shaded according to the present embodiment.

[Explanation of symbols]

 1 ... Pattern memory 2 ... Reading circuit 3 ... Correction value generating circuit 3a ... R component generating circuit 3b ... G component generating circuit 3c ... B component generating circuit 4 ... Saturation processing circuit 4a ... R component saturation processing circuit 4b ... G component saturation processing Circuit 4c ... B component saturation processing circuit 5 ... Frame memory 6 ... Correction value calculation circuit (DDA) 7 ... Drawing command generation circuit 8 ... Addition circuit 9 ... Overflow / underflow signal generation circuit 10 ... Overflow processing circuit 11 ... Underflow generation Circuit 701 and later ... Each step of the procedure

Claims (6)

[Claims] 1. A pattern memory for storing original data for displaying an image pattern on a screen of a display device, a correction value generating means for generating a correction value for changing the luminance of the original data for each pixel, and the original data. In a shading device for an image pattern having an adding means for adding the correction value to data, a correction value for a pixel on a straight line connecting the two points is calculated based on a correction value for any two points of the original data. A shading device for an image pattern, comprising a correction value calculating means for determining. 2. The shading device for an image pattern according to claim 1, wherein the correction value calculation means is a digital differential analyzer configured by a digital circuit. 3. A pattern memory in which original data for displaying an image pattern on a screen of a display device is stored in a representation format of a predetermined number of bit strings, and a correction value for changing the brightness of the original data for each pixel. In a shading device for an image pattern, which has a correction value generation unit that generates a bit string representation format and an addition unit that adds the correction value to the original data, a result of addition of the original data and the correction value by the addition circuit. Is provided with a saturation processing unit that outputs the minimum value when it is smaller than the minimum value that can be expressed by the bit string, and outputs the maximum value when it is larger than the maximum value that can be expressed by the bit string. A shading device for image patterns. 4. The saturation processing means generates an underflow signal when the addition result of the adding circuit is smaller than the minimum value that can be expressed by the bit string, and is larger than the maximum value that can be expressed by the bit string. Overflow generating an overflow signal in case
An underflow signal generation circuit, and an overflow processing circuit that outputs all the bits in the bit string as 1 when the overflow signal is input, and outputs the bit string of the addition result as it is when the overflow signal is not input, An underflow processing circuit that outputs all bits in the bit string as 0 when the underflow signal is input, and outputs the bit string of the addition result as it is when the underflow signal is not input. The shading device for an image pattern according to claim 3. 5. A shading method for an image pattern, which generates a correction value for changing the brightness of original data for displaying an image pattern on a screen of a display device for each pixel and adds the correction value to the original data. A shading method for an image pattern, wherein a correction value for pixels on a straight line connecting the two points is determined based on correction values for pixels at arbitrary two points of the original data. 6. The original data for displaying an image pattern on a screen of a display device is expressed by a predetermined number of bit strings, and a correction value for changing the brightness of the original data for each pixel is generated in the bit string expression format. Then, in the shading method for the image pattern in which the correction value is added to the original data, when the addition result of the original data and the correction value is smaller than the minimum value that can be expressed by the bit string, the minimum value is set. A shading method for an image pattern, which is characterized by generating the maximum value and generating the maximum value when the maximum value is larger than the maximum value that can be expressed by the bit string.