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Old 03 February 2020, 18:19   #21
number6
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@thread

Horizontal interrupts and vertical blank interrupts go way back:
[ Show youtube player ]

That uses 1 color register btw. You can see how having more (mentioned in the other postings) allowed expansion of this methodology.
Anyone who used dynamic hi-res will also remember that the image would not remain stable if the mouse was moved.
Ham-8 basically deprecated SHAM, but it's still very interesting.

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Last edited by number6; 03 February 2020 at 18:41. Reason: added notes
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Old 03 February 2020, 18:46   #22
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I'm interested in different colours on the Horizontal.
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Old 03 February 2020, 20:01   #23
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Originally Posted by coder76 View Post
It's of course also possible to use normal bitplane mode or extra halfbright (EHB), and then change palette on every scanline, but leave a set of colors unchanged; that way you can also move around blitter objects without distorting their colors when moving object horizontally or vertically.

The sliced EHB mode certainly works better for games than sliced HAM.

Sliced EHB technique was used by the game Universe. It changes only first 16 colors on a scanline basis and leaves the other 16 colors untouched for objects, and these colors are also used by sprites. It seems that not that many colors are changed per scaline (probably 2-8), as the game is said to have only 256 colors in all, so the copper doesn't also slow down CPU access to chip ram completely. But this mode is probably not suitable for fast action games on OCS.
If you go down on just 32 colors and change the first 16 (= 128 colors) would that reduce bandwidth for basic single screen platformers?
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Old 03 February 2020, 20:06   #24
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Originally Posted by Retro1234 View Post
I'm interested in different colours on the Horizontal.
You could actually do that even in the old days with 1 register.
Basically you had to know machine cycles and color clocks to do it though.

With Amiga it's obviously not necessary, but it is an interesting parallel about people pushing the limits of what a machine was originally designed to do.

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Old 03 February 2020, 20:28   #25
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@Retro1234, HAM was another innovation the Amiga had, to allow truecolor pictures (as opposed to palettized images) without hefty RAM requirements. Computers were just beginning to get more than 64K of RAM; the Mac cost $2500 and had 128K. The Amiga 1000 doubled that, allowing another 256K expansion. Within 2 years, needs had been found that required 1MB to leave enough for the OS and programs. A HAM image would be exactly the size on file and in RAM as a 64-color picture! And this meant that an OS and a program to handle these pictures could fit in RAM.

HAM was the predecessor of the "hi-color" modes found on PC graphics cards in 1995. 4K colors vs ~29K colors. When hi-color cards came out PCs weren't powerful enough to run games in hi-color mode. This took another 4 years, and in 1999, PCs could run games in truecolor mode (~16M colors), but many lowered it to hi-color to not get a slow framerate.

I have never heard of "sliced HAM" before either. To me, it's PCHG. It's an image format that increases the number of colors in palettized pictures, but not in HAM pictures. Rather, it brings the total counted differing colors on-screen closer to 4K and reduces the artifacts, and leaves enough CPU left to run the OS and programs, on Amiga 1000 and up.

By comparison, you had to wait a few seconds to display a truecolor picture on your PC in 1992, due to the palette remapping; what you would see would be 256 colors. The same year, HAM was upgraded on AGA to 256K colors - much better than hi-color, but not as good as truecolor.
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Old 04 February 2020, 00:40   #26
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Originally Posted by saimon69 View Post
If you go down on just 32 colors and change the first 16 (= 128 colors) would that reduce bandwidth for basic single screen platformers?

Well sure, when you reduce bitplanes and copper instructions, then you get more DMA time for blitter and also more CPU time if you run code from chip ram, or this pseudo fast ram 512k expansion which everyone had for A500.


Everything works of course much better under AGA, when you also have fast ram+more bitplanes, and the ability the swap an entire color bank with a single copper instruction.



There is no restriction on scrolling screen with the sliced EHB method (or other normal bitplane modes), it doesn't distort colors like HAM mode, and since you also have a fixed palette for blitter objects and sprites, you can move them also vertically without distorting their colors.
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Old 04 February 2020, 01:27   #27
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There is no restriction on scrolling screen with the sliced EHB method (or other normal bitplane modes), it doesn't distort colors like HAM mode, and since you also have a fixed palette for blitter objects and sprites, you can move them also vertically without distorting their colors.
If for some reason AGA did not come out i would have expected more adventure games using the same method that Universe did - well, there is still time (not the engine though)
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Old 04 February 2020, 12:24   #28
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HAM is AWESOME! Have you ever seen high-end HAM8 demos? They're easily the best of any on Amiga!

Sure, the HAM8 mode may just be a framebuffer with c2p used, and the resolution may be blocky, but the results are better than 256 colours any day.
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