A conventional character display apparatus for displaying characters with high definition using a display device capable of color display, is disclosed in, for example, Japanese Laid-Open Publication No. 2001-100725.
In the character display apparatus of Japanese Laid-Open Publication No. 2001-100725, the color element levels of subpixels corresponding to a basic portion of a character are set to a predetermined color element level. The color element levels of subpixels adjacent to the subpixels corresponding to the basic portion of the character are set to color element levels other than the predetermined color element level, based on at least one correction pattern. The set color element levels are converted to brightness levels based on a predetermined table. As a result, the character is displayed on a display section (display device).
In this conventional technology, the basic portion of a character refers to a core (central backbone) of the character.
Data, such as RGB, CYM, or the like, are assigned as color elements to individual subpixels contained in pixels. A color element level indicates how much a color element contributes to a character color. In this conventional technology, a color element level is represented by a value of “0” to “7”. “7” indicates a character color. “0” indicates a background color. Thus, by using color element levels assigned on a subpixel-by-subpixel basis, it is possible to construct a logical model which does not rely on a combination of actual character and background colors.
In order to actually display characters on the character display apparatus, color element levels need to be converted to brightness values. To achieve this, a brightness table for use in converting color element levels to brightness values is provided, depending on a combination of a character color and a background color. For example, when a black character is displayed in a white background, the color element level “7” is converted to a set of brightness values for R, G and B, which are all “0” while the color element level “0” is converted to a set of brightness values for R, G and B, which are all “255”.
FIG. 16 is a diagram showing an example, in which the color element levels of subpixels corresponding to the basic portion of a character “/” are set to a predetermined value, and the color element levels of subpixels adjacent to the basic portion of the character are set based on a certain correction pattern.
Each rectangle corresponds to a single subpixel. A hatched rectangle is a subpixel, the color element level of which is represented by the concentration thereof. The color element level is increased with an increase in the concentration. In this example, there are four color element levels, i.e., “0”, “1”, “2” and “3”. When a luminous level ranges from “0” to “255”, the color element levels are converted to respective luminous levels “255”, “170”, “85” and “0”, which are displayed on a display section.
Thus, by controlling the color element levels of subpixels separately, a resolution can be apparently improved in a direction along which the subpixels are arranged. Further, by appropriately controlling the color element levels of subpixels adjacent to subpixels corresponding to the basic portion of a character, colors other than black imparted to a character cannot be easily recognized by human eyes. As a result, the contour of a character as well as the character itself can be displayed with high definition on a display screen.
Another conventional technology for displaying a character by controlling subpixels separately is disclosed in Japanese Laid-Open Publication No. 2002-91369.
In a conventional display apparatus disclosed in Japanese Laid-Open Publication No. 2002-91369, for the size of a character to be displayed, a rasterized character image has a 3-fold size in a longitudinal direction of a subpixel and a 3-fold size in an arrangement direction of subpixels. A subpixel is associated with each picture element array of three picture elements (pixels) successively arranged in the longitudinal direction of a subpixel contained in the character image. The brightness value of a subpixel is calculated based on the picture element values of its associated picture elements successively arranged in the longitudinal direction.
FIG. 17 is a diagram for explaining a specific operation of the conventional display apparatus of Japanese Laid-Open Publication No. 2002-91369.
Generally, an image, such as character or graphics, is represented by binary picture element values. As shown in FIG. 17(a), for example, when a slant line is displayed on a display screen, one of two brightness values is simply mapped to each picture element (pixel). In FIGS. 17(a) and 17(b), each rectangle represents a pixel constituting a display screen, and a filled portion corresponds to a slant line.
In this case, in the conventional display apparatus disclosed in Japanese Laid-Open Publication No. 2002-91369, simple mapping to each picture element on a display screen is not performed. Initially, a character image having a resolution three times that of the display apparatus is produced. For example, when it is assumed that one picture element of the display apparatus is composed of a 3×3 matrix, a rasterized character image has a size three times larger than an image to be displayed. For example, when a slant line as shown in FIG. 17(a) is rasterized with a resolution three times that of the display apparatus, a rasterized image as shown in FIG. 17(b) is obtained.
An average value of a plurality of picture elements contained in the above-described character image having the 3-fold size, which are associated with each subpixel of the display apparatus, is mapped to the picture element. For example, the character image of FIG. 17(b) is mapped to subpixels shown in FIG. 17(c). In FIG. 17(c), a rectangle corresponds to a subpixel. A subpixel with a character R thereabove exhibits a red color. A subpixel with a character G thereabove exhibits a green color. A subpixel with a character B thereabove exhibits a blue color. In addition, a filled portion indicates six subpixels to which an average value of picture element values is mapped. Each of the six subpixels, which are longer than are wide, corresponds to three adjacent picture elements in the vertical direction of FIG. 17(b).
As a result, the resolution of the arrangement direction of subpixels can be improved. In addition, since the intensity of color exhibited by each subpixel is determined depending on how much a portion of a character generated with a 3-fold resolution is associated with a single subpixel, the resolution in the longitudinal direction of the subpixel can be apparently improved.
However, in the above-described conventional technology of Japanese Laid-Open Publication No. 2001-100725, the resolution in the longitudinal direction of subpixels is not taken into consideration. Therefore, when a slant line is displayed, jaggies are significant depending on the degree of the slant angle.
In the above-described conventional technology of Japanese Laid-Open Publication No. 2002-91369, a rasterized character image has a 3-fold resolution in the process. Therefore, a problem arises that a large amount of working memory is required. Moreover, there is a limitation such that the width or font of characters cannot be freely changed.
The present invention provides a solution to the above-described conventional problems. An object of the present invention is to provide a character display apparatus and a character display method, which apparently improve resolutions of subpixels in an arrangement direction and a longitudinal direction without a large amount of working memory and are capable of freely changing the width of a character; and a character display program for causing a computer to perform the steps of the method; and a computer readable recording medium.