Path: utzoo!mnetor!lsuc!jimomura From: jimomura@lsuc.uucp (Jim Omura) Newsgroups: comp.graphics Subject: Grey Scale Slope Content Index Message-ID: <1988Feb29.223957.20454@lsuc.uucp> Date: 1 Mar 88 03:39:56 GMT Reply-To: jimomura@lsuc.UUCP (Jim Omura) Organization: Consultant, Toronto Lines: 136 Summary: A Proposal -- Please Reply by Net Mail I don't read 'comp.graphics' regularly so please comment via Net mail. Cheers! -- Jim O. TITLE: Image Fidelity, Resolution, Grey Scaling, Slope A restatement of BIX messages graphic.disp/disc.bench #88-89 With the advances in display technology we can now consider using digital technology in normal consumer photographic applications, in effect, replacing the traditional family photo albums with computers. The current Amigas and Atari ST's are right on the border. We are faced with the problem of evaluating equipment on for their ability to represent continuous tone images with visually satisfying detail. Traditional photographic measures don't work well for digital display because photography assumes infinite grey scale and grey scale testing in photography is really contrast testing. Conversely, in digital technology the limits of grey scale repre- sentation of a given system can make a big difference in the fidelity of the resulting picture. For instance, if you have only black and white, you can simulate grey scale with dithering. However, by dithering you are, in effect treating 2 or more pixels as 1. Thus in terms of fidelity your picture is no better than that produced by a system with, perhaps half as many pixels, but 3 levels of brightness. The question is whether there's some index possible, which will reasonably represent the ability of a digital system to display acceptably fine detail of continuous tone. What I've tried to do is to integrate the characteristics of grey scale and resolution, resulting in what I feel will be a useful measurement for comparing and evaluting image related systems. It won't stand on its own, but it is useful as a starting point. It works like this: take 1/100th of the display in horizontal or vertical and determine the number of grey scale steps can be represented in that space. The result is a "grey slope content index". The reason for 1/100th the image size: Photographically, 1/100th is actually very crude. If you take a head and shoulder portrait, 8" * 10" and look for the detail level of this increment, you're dealling with about 1/10". In hard terms, you may be looking at the ability to reproduce eyelashes, or moles or skin creases. It is arguable that finer increments might be used as a baseline. However, if you try a smaller increment than than 1/100th, you may not get meaningful results until you're up in the 1K pixel range. Practically speaking, we find that image systems such as the Amiga can produce acceptable "portraits" and "snapshot" quality images with substantially less than 1K pixels. Most people will agree that the Amiga is visibly better than the ST in this application and somewhere in around the specs. of the ST is pretty much the bottom limit. Some people will include the ST as acceptable while others won't. This index seems to become useful at about the point where the imaging systems become useful. The index, being open ended, won't become obsolete with improving systems. Other measures will also be valuable, particularly where a specific factor is significant, but this index should remain a part of system comparison. Examples: For the Atari ST, we have a high res of 640 * 200 * 4 of 512 colors. 8 distinct grey scales are in the total palette, but only 4 are usable at once. The index works like this: 7 pixels are visible in 1/100th of the width (6 pixels in full and a bit one the 7th), but the limit is the 4 available colors. The index, excluding dithering, is therefore 4. The vertical limit is 2 because only 2 pixels are available at all in 1/100th of the image height. On the Amiga, with 16 grey levels and 16 colors available at one time and 640 * 400 res. the index for horizontal fidelity is 7 (7 pixels). However, the vertical index is 4 (4 pixels). These examples seem to show that the index succeeds in indicating superior image fidelity. The question remains whether close results show systems of roughly equivalent usefulness in this field. I think it does. Taking the ST again, and using the lower resolution 320 * 200 * 16 colors, we find that horizontally, we get 4 pixels in 1/100th the width (3 and a bit of the 4th showing). Of the 8 grey levels, therefore, only 4 are useful in this small space. Vertically, again you get an index of 2. Indeed, with the ST, in either mode you get roughly the same grey scale capabilities. In the high mode, you can reduce the screen's effective resolution by using dithering and come close to the same image. Likewise, you can "mix" the 4 colors of the high res. into 16 permutations and come up with roughly the same image in the lower res. Dithering: Dithering can be considered by using "pseudo-pixels" which are groups of adjacent pixels which represent a single pixel with a greater range of grey scale than each individual pixel. For example, we can again take the Atari ST in its monochrome display mode which had 640 * 400 * 2 colors (black and white). The basic usage of the display yields indexes of 2 in either axis because there are only 2 real grey values available. However, if you allow 2 pixels per pseudo-pixel, then you have 4 pseudo pixels in the X axis, but 3 grey values. This gives an index of 3 in the X axis and 3 again in the Y axis. Note that this is close to the 4 and 2 indices of the color screens. In fact, the pictures produced are fairly comparable. The problem with dithering is that it creates complexity in the analysis. Notice that you can also use groups of 3 pixels in the X axis for the Atari ST yielding 4 grey levels. However, the limit becomes the number of pseudo-pixels in the 1/100th measurement and again, you have a maximum of 3 grey values in the X axis. In the Y axis, the grouping limits the display to 2 pseudo-pixels for an index of 2. The best result is therefore not to group pixels in the Y axis. The determining of the absolute best result for dithered displays becomes even more difficult when each pixel can have more than the 2 simplest grey values. The difficulty of calculating the permutations increases with the number of pixels in the zone evaluated. This is another reason why a small zone (1/100th) is used rather than a larger one (1/16th the dimension tested, for example would yield 20 pixels in the X axis of the Atari ST at its lowest resolution). -- Jim Omura, 2A King George's Drive, Toronto, (416) 652-3880 ihnp4!utzoo!lsuc!jimomura Byte Information eXchange: jimomura