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Bug 4761 - Alpha blended blitting behaves inconsistently with and without MMX
Summary: Alpha blended blitting behaves inconsistently with and without MMX
Status: NEW
Alias: None
Product: SDL
Classification: Unclassified
Component: video (show other bugs)
Version: 2.0.10
Hardware: x86 All
: P2 normal
Assignee: Sam Lantinga
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2019-08-12 05:50 UTC by Kichikuou
Modified: 2019-08-12 05:50 UTC (History)
0 users

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Description Kichikuou 2019-08-12 05:50:10 UTC 
main.c:
----
#include <SDL.h>

int main(void) {
  SDL_Surface *src, *dst;
  src = SDL_CreateRGBSurfaceWithFormat(0, 16, 16, 32, SDL_PIXELFORMAT_RGBA32);
  dst = SDL_CreateRGBSurfaceWithFormat(0, 16, 16, 32, SDL_PIXELFORMAT_RGBA32);

  SDL_FillRect(src, NULL, SDL_MapRGBA(src->format, 255, 255, 0, 128));
  SDL_FillRect(dst, NULL, SDL_MapRGBA(dst->format, 255, 255, 255, 0));
  SDL_BlitSurface(src, NULL, dst, NULL);

  Uint8 r, g, b;
  SDL_GetRGB(*(Uint32*)dst->pixels, dst->format, &r, &g, &b);
  printf("(%d, %d, %d)\n", r, g, b);
}
----

This program prints (253, 253, 126) on my MacOS where BlitRGBtoRGBPixelAlphaMMX3DNOW() is used for blitting, but prints (255, 255, 127) on a Raspberry Pi where non-accelerated BlitRGBtoRGBPixelAlpha() is used.

In BlitRGBtoRGBPixelAlphaMMX3DNOW(), the resulting pixel value is calculated as:

 (s*a >> 8) + (d*(255-a) >> 8)

where s is the source's RGB component value, a is the source's alpha value, and d is the destination's RGB component value. In BlitRGBtoRGBPixelAlpha(), a different calculation is used:

 d + ((s-d)*a >> 8)

IMO BlitRGBtoRGBPixelAlpha's behavior is more desirable because it preserves the total signal level.