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Fixed bug 1424 - Handling of alpha channel in Altivec accelerated bli…

…t functions

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.
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slouken committed Feb 21, 2012
1 parent 7ced736 commit 77de4c697adf6c1b74e06add594a44f74ba55817
Showing with 8 additions and 0 deletions.
  1. +8 −0 src/video/SDL_blit_N.c
@@ -689,6 +689,8 @@ static void 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--;
}
@@ -716,6 +718,8 @@ static void 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--;
}
@@ -769,6 +773,8 @@ static void ConvertAltivec32to32_prefetch(SDL_BlitInfo *info)
vec_dstst(dst+scalar_dst_lead, DST_CTRL(2,32,1024), 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--;
}
@@ -798,6 +804,8 @@ static void 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--;
}

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