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.
libsdl-org · Feb 21, 2012 · 3aead2b · 3aead2b
1 parent 5d2fff5
commit 3aead2b
Showing 1 changed file with 8 additions and 0 deletions.
diff --git a/src/video/SDL_blit_N.c b/src/video/SDL_blit_N.c
@@ -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--;
         }