Set an additional alpha value used in blit operations.
int SDL_SetSurfaceAlphaMod(SDL_Surface * surface, Uint8 alpha);
|surface||the SDL_Surface structure to update|
|alpha||the alpha value multiplied into blit operations|
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
When this surface is blitted, during the blit operation the source alpha value is modulated by this alpha value according to the following formula:
srcA = srcA * (alpha / 255)
This function is available since SDL 2.0.0.