From:	"A. Jans-Beken" <jansb000 AT wxs DOT nl>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Very fast allegro blitting
Date:	Mon, 03 Jan 2000 23:56:44 -0800
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SimonK wrote:

> The blit function is pretty fast, damn thing is all in asm i think. I
> wouldn't bother rewriting it.

In this case a low-resolution screen is used, and perhaps there is room in
the video memory for an alternate screen. I have understood that allegro
supports video-cards with hardware-acceleration, and blitting from
video-memory to video-memory (done by accelerated video hardware) should be
considerable faster then the build-in blit_from_memory_to_video_memmory
function.

A second idea that I once proposed in this:

When doing animations with sprites or bitmaps:
1 Create a BITMAP
2 Draw the OLD situation in this bitmap
3 Create a second BITMAP
4 Draw the NEW situation in this bitmap
5 Create a third BITMAP = OLD - NEW
6 Masked-blit this bitmap to the screen

The trick is in the OLD-NEW operator. It blanks all pixels that are the same
color in both the NEW and OLD situation. Only pixels where really new
content appears will be blitted.
The OLD-NEW operator should be written in assembler (to difficult for me).
Maybe it is even possible to use MMX or hardware accelerated functionality.
However I tested this algo in plain C some time ago and it really works very
smooth.

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