Bloody Blitter


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Bloody Blitter

Postby unseenmenace » Fri May 06, 2005 9:15 am

I am steadily getting to grips with how to use the Blitter chip but something seems a bit dodgy to me. Is it just me or to draw a masked sprite (without the use of a separate mask bitmap) do you have to use mode %0100 - "NOT Source AND Target" with each plane of the source onto all 4 planes of the destination, meaning 16 blitter operations just to cut the hole in the background? If I just do 4 blits with planes respectively (1 to 1, 2 to 2 etc) then it doesn't cut the whole thing out. Am I doing something stupid or do I have to have a separate mask to make using the Blitter worthwhile?
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Postby NiceGuyUK » Fri May 06, 2005 10:24 am

Already emailed you this, but for the benefit of other forum users...

This Blitter Manual might prove useful
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Postby Pink/RG » Fri May 06, 2005 1:05 pm

You should use a seperate mask bitmap for your sprites.

This also gives you the advantage of being able to use colour 0 in sprites (if you just automatically OR together your graphic bitplanes to construct the mask you won't have this possibility).

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Re: Bloody Blitter

Postby ijor » Fri May 06, 2005 3:14 pm

unseenmenace wrote:Is it just me or to draw a masked sprite (without the use of a separate mask bitmap)

You have to use a mask. This is not particular to Blitter, it is exactly the same if you want to do a transparent blit with VDI software. Check many of the VDI blitting examples on transparent blitting.

You don’t need a prebuilt separate mask, it is easy to generate it a runtime. Again, VDI samples show how to do it.

Disregarding hardware or system, it is plainly impossible to make a transparent blit from a simple graphic. How the hardware (or the software) can know what is transparent in the graphics and what is not? In Windows DirectX, for example, you use either a key color or alpha blending.

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