1. Field of the Invention
This invention generally relates to processing of digital images which are displayed on a screen, and more particularly to screen displays for a computer or television screen.
2. Description of the Prior Art
In general, the displaying of computer generated graphics on a low-resolution television screen with interlaced refresh has always presented major technical difficulties without achieving genuinely satisfactory quality for users by reason of a flicker effect. On a television screen of standard type, the refreshing of the lines is performed in an interlaced manner. Stated otherwise, the lines of odd rank are refreshed firstly and the lines of even rank subsequently and so on and so forth. If two consecutive lines exhibit very different brightness levels, the human eye of the user will perceive locally a flicker at a frequency of 50 or 60 hertz according to the television standard adopted. This flicker effect is relatively tiresome and in any event very detrimental to the image quality perceived by the user. In actual fact, it turns out that computer generated images such as graphics, windows or sub-windows typically exhibit strong contrasts from one line to another.
At present, the portable computer and television markets are about to merge, under the driving effect of the development of the Internet and of interactive television which requires associated technologies. It is therefore important to be able to display images with strong contrasts from one line to another such as the pages broadcast by the Internet network, on a standard television screen with visual comfort which is acceptable to the user.
The systems used hitherto to suppress or limit this flicker effect are often referred to as flicker filters. Since the flicker effect is due to the contrast between two consecutive lines, stated otherwise to high frequencies on a vertical column of the image, the basic anti-flicker filter systems implement a low-pass vertical digital filtering. After the filtering, two consecutive lines are closer together in terms of color and the flicker effect is diminished.
The effectiveness of a digital filter is tightly bound to the number of points taken into account. This type of filtering consists in fact in replacing a point or pixel by the average of the neighboring points and the larger the number of points to which the average pertains, the more effective is the filtering. This has an important influence on the slope of the transition between the passing part of the filter and the blocked or filtered part. In most graphics architectures, the data relating to the color of a point or of a pixel are read from a buffer memory according to a horizontal scanning order, this requiring the implementation of a number of line memories equal to n−1 for a filter with n steps, this translating into a very high cost of the line memories whose capacity must be large. These line memories are so expensive because they require large areas of silicon, because the anti-flicker filters are generally three-step, that is to say with storage of two lines, this translating into mediocre filtering performance. In point of fact, the filtering takes place during the steps of displaying the image, this requiring high throughput performance even if the filtering is mediocre.
Conventionally, an ideal filter makes it possible to completely suppress the flicker effect due to high frequencies, whilst preserving the low-frequency image details. Real filters are a compromise between flicker reduction and preservation of image precision. Either the entire image seems to be relatively stable but is scrambled on account of the filter, or the entire image is richer in details but is at risk of flickering. The user can be allowed to adjust the filtering according to the compromise which he deems best.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.