blit: improve BlitNtoNSurfaceAlphaKey for 16bpp

diff --git a/src/video/SDL_blit_A.c b/src/video/SDL_blit_A.c
index 4ecb521..2c52209 100644 (file)
--- a/src/video/SDL_blit_A.c
+++ b/src/video/SDL_blit_A.c
@@ -2641,6 +2641,34 @@ static void BlitNtoNSurfaceAlphaKey(SDL_BlitInfo *info)
        unsigned sA = srcfmt->alpha;
        unsigned dA = dstfmt->Amask ? SDL_ALPHA_OPAQUE : 0;
 
+       if (srcbpp == 2 && srcfmt->Gmask == 0x7e0 && dstbpp == 2 && dstfmt->Gmask == 0x7e0) {
+           Uint16 *src16 = (Uint16 *)src;
+           Uint16 *dst16 = (Uint16 *)dst;
+           sA >>= 3;   /* downscale alpha to 5 bits */
+           while ( height-- ) {
+               DUFFS_LOOP4(
+               {
+                   Uint32 s;
+                   Uint32 d;
+                   s = *src16;
+                   if(sA && s != ckey) {
+                       d = *dst16;
+                       s = (s | s << 16) & 0x07e0f81f;
+                       d = (d | d << 16) & 0x07e0f81f;
+                       d += (s - d) * sA >> 5;
+                       d &= 0x07e0f81f;
+                       *dst16 = (Uint16)(d | d >> 16);
+                   }
+                   src16++;
+                   dst16++;
+               },
+               width);
+               src16 += srcskip / 2;
+               dst16 += dstskip / 2;
+           }
+           return;
+       }
+
        while ( height-- ) {
            DUFFS_LOOP4(
            {