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Old 26 September 2021, 13:43   #1
Markowitch
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Blitter line draw BLTADAT

I'm trying to draw multiple lines using only one line draw - and it's not working. I hope some of you can help. Btw. I'm using WinUAE.

I'm setting BLTADAT to 0x8888 which should draw one line as four parallel lines. However, the output is not as I expect.

Instead of seeing four parallel lines I see lines interrupted by a saw-tooth pattern. Here shown using the simple minterm D=A (0xf0) in BLTCON0 bits 0-7.

Click image for larger version

Name:	muli-line.png
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ID:	73324

I also tried another minterm D=!A (0x0f) so that I paint everything but the line.

Click image for larger version

Name:	multi-line2.png
Views:	80
Size:	1.3 KB
ID:	73325

This last image reveals the areas that are blitted during the line draw. It sure looks like the blitter never can draw four parallel lines, because that would require the blit areas to be different.

And if that's true, then what's the point of setting BLTADAT to anything else than 0x8000?

EDIT: Standard Amiga 500 1.3 kickstart using WinUAE.

Last edited by Markowitch; 26 September 2021 at 15:12.
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Old 26 September 2021, 17:03   #2
Thomas Richter
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Quote:
Originally Posted by Markowitch View Post
And if that's true, then what's the point of setting BLTADAT to anything else than 0x8000?
There is none. Just set it to the requested value and the blitter will draw a line. That it needs this particular setting is documented, and everything else is "implementation defined", i.e. causes side effects due the inner working of the blitter.



Apparently, and UAE seems to emulate this properly, it is a state-dependent right-shift, which sounds quite plausible.


Quote:
Originally Posted by Markowitch View Post
EDIT: Standard Amiga 500 1.3 kickstart using WinUAE.
This is hardware, not kickstart related. Don't bang the hardware, please.
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Old 26 September 2021, 17:21   #3
Rotareneg
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Quote:
Originally Posted by Thomas Richter View Post
Don't bang the hardware, please.
Yeah, you might break compatibility with that new chipset Commodore is coming out with real soon....
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Old 26 September 2021, 17:25   #4
Toni Wilen
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Result is like that because blitter is always word based, even in line mode. Linedraw mode is basically a hack.

(And do keep banging the hardware unless you want to do something serious and boring )
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Old 26 September 2021, 20:16   #5
Markowitch
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Thanks for the replies - much appreciated.
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