1. Field of the Invention
This invention concerns a display equipment, which performs display at sub-pixel precision based on an original image. The image not a vector image but is a raster image (pixel precision: in the case of a font, not a vector font but a raster font), and art related to this display equipment. To be more specific, this invention concerns a filtering technique to be used in the process of performing sub-pixel display.
2. Description of the Related Art
Display equipment that employs various types of display devices is well known and used in the past. Included among such display devices are color LCD's, color plasma displays, and other display devices, in which three light-emitting elements, which respectively emit light of the three primary colors of R, G, and B, are aligned in a fixed order in a first direction to form one pixel. A plurality of such pixels are aligned in the first direction to form one line. A plurality of such lines are aligned in a second direction, which is orthogonal to the first direction, to form the display screen.
There are also many display devices, such as the display device in a portable telephone, mobile computer, etc., which have a relatively narrow display screen and in which detailed display is difficult to achieve. When the display of a small character, photograph, or complex picture, etc. is attempted with such a display device, part of the image tends to become smeared and unclear.
Literature (titled: “Sub Pixel Font Rendering Technology”) concerning sub-pixel display, which makes use of each pixel being formed of the three light-emitting elements for R, G, and B to improve the clarity of the display on a narrow screen, is being disclosed on the Internet. The present inventors have checked this literature upon downloading it from a website published by Gibson Research Corporation.
This art is described with reference to FIGS. 23 to 28. In the following description, the image of the alphabetic character, “A”, is used as an example of the image to be displayed.
Referring to FIG. 23, each single line is composed of a plurality of pixels, each of which is formed from three light-emitting elements aligned along the direction of the line. The horizontal direction in FIG. 23 (the direction in which the light-emitting elements of the three primary colors of R, G, and B are aligned) is referred to as the first direction. The orthogonal, vertical, direction is referred to as the second direction. Any order of alignment of the light-emitting elements besides R, G, and B is possible. The prior art and the present invention are applied likewise even if the order of alignment is changed.
A pixel (set of three light-emitting elements) is aligned in a single row in the first direction to arrange a single line. A plurality of lines are aligned in the second direction to arrange the display screen.
With this sub-pixel technology, the original image is, for example, an image such as shown in FIG. 24. In this example, the character, “A”, is displayed over an area of seven pixels each in the horizontal and vertical directions. Where each of the R, G, and B light-emitting elements is handled as a single pixel in order to perform sub-pixel display, a font, which has a definition of three times that of the above-described image in the horizontal direction, is prepared, as shown in FIG. 25, over an area of 21 (=7×3) pixels in the horizontal direction and 7 pixels in the vertical direction.
Then as shown in FIG. 26, a color is determined for each of the pixels in FIG. 24 (i.e. not the pixels of FIG. 25 but the pixels of FIG. 24). However, since color irregularities occur if display is performed as it is, a filtering process, using factors such as shown in FIG. 27(a), is applied. Factors concerning the luminance are shown in FIG. 27(a). The luminance values of the respective sub-pixels are adjusted by multiplying a factor, for example, of 3/9 in the case of the central target sub-pixel, of 2/9 in the case of an adjacent sub-pixel, and of 1/9 in the case of the sub-pixel next to the adjacent sub-pixel.
These factors are now described in more detail with reference to FIG. 28. In FIG. 28, the “*” indicates that the sub-pixel may be any of the three primary color light-emitting elements for R, G, and B. The determination of the factors is started from the first stage at the top and proceeds to the second stage and the third stage. The factor of the central target sub-pixel is determined at the center of the third stage.
In proceeding from the first stage to the second stage, energy is distributed uniformly among the three primary color light-emitting elements for R, G, and B, that is, the factor of the first stage is just ⅓. Likewise, energy is distributed uniformly in proceeding from the second stage to the third stage, that is, the factor of the second stage is also just ⅓.
Since the central sub-pixel is reached from the first stage via a total of three paths at the center, left, and right sides of the second stage, the synthetic factor (in which the factors of the first and second stages are synthesized) of the central sub-pixel is ⅓×⅓+⅓×⅓+⅓×⅓= 3/9. Also, since a sub-pixel adjacent the central pixel is reached via two paths, the synthetic factor thereof is ⅓×⅓+⅓×⅓= 2/9. Since there is only one path for a next adjacent sub-pixel, the synthetic factor thereof is ⅓×⅓= 1/9.
(1) First Problem
However in actuality, each of the three primary color light-emitting elements of R, G, and B differ in the degree that they contribute to luminance. Part of this difference is due to source brightness, and part is due to the response of the eye to different colors.
Thus when a filtering process for sub-pixel display by the prior art is performed, although color irregularities are eliminated, the entire image becomes blurry and the display quality is poor.
(2) Second Problem
With the prior art, since the denominator of a factor is 9, a factor cannot provide an integer aliquot in general (aliquot refers to a number that contains an exact number of some other number, i.e., one number exactly divisible by another number without a remainder). Thus when a factor is approximated by an integer, the error is too great to ignore.
Thus in performing the filtering process for sub-pixel display by the prior art, floating decimal point computation is necessary. Floating decimal point computation disables high-speed integer computation and makes it difficult to incorporate the process into hardware.
(3) Third Problem
Also conventionally, an anti-aliasing process is performed to improve the visibility of an image in a narrow display area. However, since the anti-aliasing process blurs the image as a whole in an attempt to alleviate jaggedness, image quality is degraded by the blurring of the image.
With regard to this point, visibility is improved by the application of the above-described sub-pixel technique.
However, there have been demands for even better visibility in the display results achieved by the application of the sub-pixel technique.