From ce329d60e469f9653b18ddcd317e1aede50933ac Mon Sep 17 00:00:00 2001
From: Sam Lantinga <[EMAIL REDACTED]>
Date: Thu, 28 Dec 2023 09:33:02 -0800
Subject: [PATCH] Added support for alpha blending using palette alpha
Fixes https://github.com/libsdl-org/SDL_image/issues/409
---
src/video/SDL_blit_1.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)
diff --git a/src/video/SDL_blit_1.c b/src/video/SDL_blit_1.c
index 2c54a2e84a51..1a398da6a252 100644
--- a/src/video/SDL_blit_1.c
+++ b/src/video/SDL_blit_1.c
@@ -435,7 +435,7 @@ static void Blit1toNAlpha(SDL_BlitInfo *info)
const SDL_Color *srcpal = info->src_fmt->palette->colors;
int dstbpp;
Uint32 pixel;
- unsigned sR, sG, sB;
+ unsigned sR, sG, sB, sA;
unsigned dR, dG, dB, dA;
const unsigned A = info->a;
@@ -449,8 +449,9 @@ static void Blit1toNAlpha(SDL_BlitInfo *info)
sR = srcpal[*src].r;
sG = srcpal[*src].g;
sB = srcpal[*src].b;
+ sA = (srcpal[*src].a * A) / 255;
DISEMBLE_RGBA(dst, dstbpp, dstfmt, pixel, dR, dG, dB, dA);
- ALPHA_BLEND_RGBA(sR, sG, sB, A, dR, dG, dB, dA);
+ ALPHA_BLEND_RGBA(sR, sG, sB, sA, dR, dG, dB, dA);
ASSEMBLE_RGBA(dst, dstbpp, dstfmt, dR, dG, dB, dA);
src++;
dst += dstbpp;
@@ -475,7 +476,7 @@ static void Blit1toNAlphaKey(SDL_BlitInfo *info)
Uint32 ckey = info->colorkey;
int dstbpp;
Uint32 pixel;
- unsigned sR, sG, sB;
+ unsigned sR, sG, sB, sA;
unsigned dR, dG, dB, dA;
const unsigned A = info->a;
@@ -490,8 +491,9 @@ static void Blit1toNAlphaKey(SDL_BlitInfo *info)
sR = srcpal[*src].r;
sG = srcpal[*src].g;
sB = srcpal[*src].b;
+ sA = (srcpal[*src].a * A) / 255;
DISEMBLE_RGBA(dst, dstbpp, dstfmt, pixel, dR, dG, dB, dA);
- ALPHA_BLEND_RGBA(sR, sG, sB, A, dR, dG, dB, dA);
+ ALPHA_BLEND_RGBA(sR, sG, sB, sA, dR, dG, dB, dA);
ASSEMBLE_RGBA(dst, dstbpp, dstfmt, dR, dG, dB, dA);
}
src++;
@@ -531,8 +533,10 @@ SDL_BlitFunc SDL_CalculateBlit1(SDL_Surface *surface)
return one_blitkey[which];
case SDL_COPY_COLORKEY | SDL_COPY_BLEND: /* this is not super-robust but handles a specific case we found sdl12-compat. */
- return (surface->map->info.a == 255) ? one_blitkey[which] : (SDL_BlitFunc)NULL;
+ return (surface->map->info.a == 255) ? one_blitkey[which] :
+ which >= 2 ? Blit1toNAlphaKey : (SDL_BlitFunc)NULL;
+ case SDL_COPY_BLEND:
case SDL_COPY_MODULATE_ALPHA | SDL_COPY_BLEND:
/* Supporting 8bpp->8bpp alpha is doable but requires lots of
tables which consume space and takes time to precompute,