meekins@CIS.OHIO-STATE.EDU (timothy lee meekins) (01/25/91)
Tim's guide to extended graphics on the IIgs. The Apple IIgs performs screen updates 60 times per second (double the television standard of 30 FPS). Since 30 FPS is all that is needed, we can use 60 FPS to our advantage. This includes alternating the color of a pixel during each screen update, giving the illusion of an interpolated color. Our eye does the blending for us. How practical is this method? For example, if a gray scale palette is created, by alternating colors, we can create 31 shades of gray from a base of 16 colors. Since the palette doesn't change, we are forced to modify those pixels which are to blended. If a large number of these pixels exist, then the pixels cannot be updated quickly enough. For a picture of reasonable complexity, it is most likely that the screen cannot be updated quickly enough for this new method to be practical. But is there a faster method? Yes. But using a faster method, like in any computer science problem, you lose the ability of doing certain things. If certain color entries in a palette are used specifically for the blended colors, then only the palette entry needs to be changed, but this limits us to 16 colors to palette, so we really don't gain anything. The best bet is to use the original method, but with little complexity so that CPU can keep up. What about the ACS demo, Gradient.32? The ACS demo pulls some simple tricks which isn't very practical unless you want to perform a vertical gradient shading similar to what ACS did. ACS used the palette updating methods using all 16 palettes. First a box was displayed on the screen using the colors 0 and 1, but with different palettes, as shown below: Screen Palette # 00000000 0 11111111 0 00000000 1 11111111 1 00000000 2 11111111 2 ...etc... Once this box has been drawn, all that is needed to do is to alternate the palette color entry 1. Even frames: Palette Color 0 Color 1 0 000 000 1 111 111 2 222 222 3 333 333 4 444 444 etc... Odd frames: Palette Color 0 Color 1 0 000 111 1 111 222 2 222 333 3 333 444 4 444 555 etc... So, as you can see, alternating colors gives us more colors, but at a high cost, which can't be paid for in many situations. Also, someone was asking if there are any mixed graphics mode programs. Yes, take a look at Crystal Quest. The game graphics are 320 mode and the score is 640 mode. +---------------------------S-U-P-P-O-R-T-----------------------------------+ |/ Tim Meekins <<>> Snail Mail: <<>> Apple II \| |> meekins@cis.ohio-state.edu <<>> 8372 Morris Rd. <<>> Forever! <| |\ timm@pro-tcc.cts.com <<>> Hilliard, OH 43026 <<>> /| +-------------------------S-H-A-R-E-W-A-R-E-!-------------------------------+
taob@pnet91.cts.com (Brian Tao) (02/09/91)
From meekins@CIS.OHIO-STATE.EDU (timothy lee meekins): > Also, someone was asking if there are any mixed graphics mode programs. > Yes, take a look at Crystal Quest. The game graphics are 320 mode and > the score is 640 mode. I think Thexder use 640-mode graphics to display the scrolling view of the game on top, and 320-mode for the stats and scores on the bottom. I could be wrong, but I recall seeing more than 4 pure colours on a line (check out the Energy bar graph). Brian T. Tao {taob@pnet91.cts.com} || Computer guru? Someone who got University of Metro Toronto || their computer a couple of weeks Scarberia, ON, MIC 3A8 *B-) || before you did. (Alvin Toffler)