EP1196911A2

EP1196911A2 - Antialiased character reproduction with increased pixel density

Info

Publication number: EP1196911A2
Application number: EP00979588A
Authority: EP
Inventors: Albertus A. S. Sluyterman; Michiel A. Klompenhouwer
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1999-11-16
Filing date: 2000-11-15
Publication date: 2002-04-17
Also published as: WO2001037248A3; WO2001037248A2; KR20010101509A; JP2003515195A

Abstract

The reproduction of characters, in particular italic characters and characters like A,V,W,VA, on a CRT display can be improved by using non-square addressable pixels. The horizontal addressable pixel density is increased without increasing the number of scanning lines by a same factor.

Description

Character reproduction.

The invention relates to a method and device for character reproduction.

In usual electronic displays, the reproduction of slant lines is troublesome, showing staircase effects. In particular when lines are almost but not entirely vertical, like in an italic /, the staircase effect is very noticeable. As shown in Fig. 1, this is caused by the fact that when we follow the character upwards line by line, we sometimes make no pixel step in the horizontal direction and sometimes we do. The reason for the awkward looking italic / is that there is insufficient horizontal positioning flexibility for all the pixels. The usual approach to solving this problem is to increase the pixel density both horizontally and vertically, maintaining the pixel grid square. However, this does not solve the staircase problem, it only makes it less noticeable because the staircases become smaller. This improvement comes at a high price, because the increased vertical pixel density requires a higher scanning frequency. Moreover, the time per scan line is reduced, in the end leading to loss of sharpness because of limitation in the bandwidth of the video amplifier.

It is, inter alia, an object of the invention to provide an improved character reproduction. To this end, the invention provides a method, a device, a computer, and a monitor as defined in the independent claims. The dependent claims define advantageous embodiments. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

In the drawings:

Fig. 1 shows a prior art display of an italic character /, using a square pixel grid; Fig. 2 shows a reduction of the staircase effect by increasing the horizontal pixel density;

Fig. 3 shows an increase of the horizontal pixel density while maintaining the thickness in accordance with the present invention;

Fig. 4 shows a first embodiment of the invention; Fig. 5 shows a second embodiment of the invention; and Fig. 6 shows a third embodiment of the invention.

The invention is based on the recognition that, for example, doubling only the horizontal pixel density would solve the problem, as can be seen in Fig. 2. One can see that the wavelength of the stairs, measured along the character is halved. One also sees that in this example the character has become thinner. This is not the purpose of doubling the horizontal pixel density, and will also not happen in case of true type fonts. The result is shown in Fig. 3. In order to see if this would also be true in practice, we took a PC with a special video card, and programmed that video card to display 600 lines with 1600 (instead of 800) pixels per line. Initially this resulted in a doubled number of characters, which was not the goal. Then we horizontally stretched the text to regain the original character width. It was clearly visible that non-square pixel structure eliminates the staircase effects. It is important to understand, and this is not trivial, that this result could not have been obtained by using an anti-aliasing algorithm. The reason for this is that the video bandwidth of a CRT monitor is normally designed to resolve all addressable pixels. Therefor, an anti-aliasing algorithm would under these conditions cause a loss in sharpness. For instance, to simulate a pixel located exactly between two hardware pixels, an anti-aliasing algorithm would give two pixels side by side a 50% intensity. This gives the simulated pixel twice the width of a single pixel. And because the monitor bandwidth is sufficient for resolving a single pixel, the double width will be observed as a loss in sharpness. Once we have increased the addressable pixel density to such an extent that the individual pixels can no longer be resolved, an anti-aliasing algorithm can indeed result in further improvements without causing a loss in sharpness.

It is also important to know that increasing the horizontal addressable pixel density does of course require a higher clock frequency in the video-card, but does not require a higher video bandwidth in the monitor as long as we keep the character width the same.

Moreover, as long as we do not use the increased horizontal pixel density to reduce the width of the characters, we do not need a smaller screen pitch.

A simple embodiment of the invention can be summarized as follows. The reproduction of characters, in particular italic characters and characters like A,V,WNA, on a CRT display can be improved by using non-square addressable pixels. The horizontal addressable pixel density is increased without increasing the number of scanned lines by a same factor. The impact of this invention is that improved character quality can be obtained at very limited cost. It provides extra opportunities for video card manufacturers. By display driving with pixels that are smaller than they are high, a better picture quality can be realized at lower cost. While the pixel width is halved, the characters are doubled in width by means of software, e.g. by means of the driver software of the video graphics card. This allows for an improved display of the characters, especially of italic characters. In this embodiment, this improvement can be realized without requiring more expensive monitors that have higher scan frequencies and larger video bandwidths.

As illustrated in Fig. 4, the invention can be used in video graphics cards, and in driver software for video graphics cards. Fig. 4 shows a monitor Mon, the hardware HW of which is not changed by the invention. The monitor Mon is driven by a personal computer PC comprising an application program AP, a software driver of the video card DVc, and a video card Nc. The software SW of the application program AP is not changed by the invention. However, the video card driver software and the video card hardware are changed by the invention. The video card driver software is changed in that the character fonts are characterized as large in the horizontal direction. The video card hardware is changed in that the ratio between the horizontal pixel number to the vertical scan line number is substantially larger (e.g. by a simple integer factor of 2 or 3) than the ratio between the width and the height of the image. Preferably, the mouse driver software is changed as well to prevent the mouse from being less sensitive for horizontal movements as for vertical movements as a result of the video card hardware and video card driver software adaptations.

Fig. 5 shows an embodiment Nc' of a video card that has the advantage that no adaptation in the video card driver software is required. First, by means of a first frame memory buffer FM-4x that is four times as large as a reference size, the number of pixels is doubled both horizontally as vertically, whereby the character magnitude is set at 200%. Next, the line number that has been doubled as a result of this vertical pixel number doubling, is halved to the original line number by means of a vertical sealer VS. The result is applied to a second frame memory buffer FM-2x that is two times as large as the reference size, and thereafter to a digital-to analog converter DAC. The first frame memory buffer FM-4x and the second frame memory buffer FM-2x may be part of a single memory unit. As a result, the video card driver software does not need to take the non-squareness of the addressable pixels into account; it only needs to set the character magnitude at 200%. Fig. 6 shows a modification of the embodiment of Fig. 4. The differences are as follows: in the PC, the video card driver software DVc (SW) is set at 200% magnitude, just like in Fig. 5; in the PC, the video card hardware Vc-4x (HW) is arranged for handling data that is four times enlarged as compared to the reference magnitude; and in the monitor Mon', the vertical sealer NS is present, while in the Fig. 5 embodiment, the vertical sealer VS was part of the video card Nc' in the PC. The vertical sealer NS is placed between an analog-to-digital converter ADC and a digital-to-analog converter DAC. An output of the DAC is applied to the analog remainder (remaining analog circuitry R and a display D) of the monitor circuitry. Any other digital monitor circuitry would be placed between the vertical sealer VS and the DAC.

From the examples shown in Figs. 5 and 6 it appears that the step of increasing a horizontal pixel density of the image to obtain an intermediate image having an increased horizontal pixel density without increasing a number of scanning lines (at all or by a same factor), may very well be carried out by first increasing both the horizontal pixel density and the vertical pixel density, followed by a reduction of the vertical pixel density.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. Interchanging the method steps falls within the scope. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method of character reproduction of an image, the method comprising the steps of: increasing a horizontal pixel density of the image to obtain an intermediate image having an increased horizontal pixel density without increasing a number of scanning lines by a same factor; and horizontally stretching characters in the intermediate image.

2. A method as claimed in claim 1, wherein the number of scanning lines is not increased.

3. A video device for reproducing images, the device comprising: means for increasing a horizontal pixel density of the image to obtain an intermediate image having an increased horizontal pixel density without increasing a number of scanning lines by a same factor; and means for horizontally stretching characters in the intermediate image.

4. A video device as claimed in claim 3, wherein the horizontal pixel density is increased by an integer factor of 2.

5. A computer (PC), comprising: a video device (Vc) as claimed in claim 3 to obtain output images; and an output for supplying the output images.

6. A computer (PC) as claimed in claim 5, wherein a mouse driver is adapted to compensate for the increased horizontal pixel density.

7. A computer (PC, Mon), comprising: a video device (Vc) as claimed in claim 3 to obtain output images; and a monitor (Mon) for displaying the output images.

8. A monitor (Mon), comprising: means (ADC) coupled to receive a first image having increased horizontal and vertical pixel densities; means (VS) for reducing a vertical pixel density of the image to obtain a second image the horizontal pixel density of which has been more increased than its vertical pixel density; and means (R, D) for displaying the second image.