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Fixed bug 1424 - Handling of alpha channel in Altivec accelerated blit functions

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evilbite 2012-02-19 09:38:21 PST

There is only one Altivec accelerated blit function
(ConvertAltivec32to32_prefetch() or ConvertAltivec32to32_noprefetch(),
depending on the CPU used) that is supposed to handle all alpha combinations.
This works as follows for every pixel line:
1. Blit single pixels until an aligned address is reached
2. Accelerated blit as far as possible
3. Blit single remaining pixels
Part 2. is set up correctly to handle different combinations of the alpha
channels of the participating surfaces. Parts 1. and 3. only do a simple copy
of all the pixel's components from souce to destination. But when the source
surface has no alpha channel (Amask is 0, e.g. the video surface) the surface's
alpha value must be used instead. Otherwise crap (uninitialized data) is being
copied to the destiniation's alpha channel.

The attached patch is a quick'n'dirty solution to the problem. A more
sophisticated solution might require separate functions for different
combinations of the alpha channels of the participating surfaces.
This commit is contained in:
Sam Lantinga 2012-02-20 20:56:52 -05:00
parent 4fed7973b3
commit e4461d199e
1 changed files with 8 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

8
src/video/SDL_blit_N.c
View file

@ -707,6 +707,8 @@ ConvertAltivec32to32_noprefetch(SDL_BlitInfo * info)
while ((UNALIGNED_PTR(dst)) && (width)) {
bits = *(src++);
RGBA_FROM_8888(bits, srcfmt, r, g, b, a);
if(!srcfmt->Amask)
a = srcfmt->alpha;
*(dst++) = MAKE8888(dstfmt, r, g, b, a);
width--;
}
@ -734,6 +736,8 @@ ConvertAltivec32to32_noprefetch(SDL_BlitInfo * info)
while (extrawidth) {
bits = *(src++); /* max 7 pixels, don't bother with prefetch. */
RGBA_FROM_8888(bits, srcfmt, r, g, b, a);
if(!srcfmt->Amask)
a = srcfmt->alpha;
*(dst++) = MAKE8888(dstfmt, r, g, b, a);
extrawidth--;
}
@ -790,6 +794,8 @@ ConvertAltivec32to32_prefetch(SDL_BlitInfo * info)
DST_CHAN_DEST);
bits = *(src++);
RGBA_FROM_8888(bits, srcfmt, r, g, b, a);
if(!srcfmt->Amask)
a = srcfmt->alpha;
*(dst++) = MAKE8888(dstfmt, r, g, b, a);
width--;
}
@ -821,6 +827,8 @@ ConvertAltivec32to32_prefetch(SDL_BlitInfo * info)
while (extrawidth) {
bits = *(src++); /* max 7 pixels, don't bother with prefetch. */
RGBA_FROM_8888(bits, srcfmt, r, g, b, a);
if(!srcfmt->Amask)
a = srcfmt->alpha;
*(dst++) = MAKE8888(dstfmt, r, g, b, a);
extrawidth--;
}