Note: This function and the semantics of SDL alpha blending have changed since version 1.1.4. Up until version 1.1.5, an alpha value of 0 was considered opaque and a value of 255 was considered transparent. This has now been inverted: 0 (SDL_ALPHA_TRANSPARENT) is now considered transparent and 255 (SDL_ALPHA_OPAQUE) is now considered opaque.
SDL_SetAlpha is used for setting the per-surface alpha and/or enabling and disabling per-pixel alpha blending.
The surface parameter specifies which surface whose alpha attributes you wish to adjust. flags is used to specify whether alpha blending should be used (SDL_SRCALPHA) and whether the surface should use RLE acceleration for blitting (SDL_RLEACCEL). flags can be an OR'd combination of these two options, one of these options or 0. If SDL_SRCALPHA is not passed as a flag then all alpha information is ignored when blitting the surface. The alpha parameter is the per-surface alpha value, a surface need not have an alpha channel to use per-surface alpha and blitting can still be accelerated with SDL_RLEACCEL. Setting the per-surface alpha value to 0 disables per-surface alpha blending.
Note: The per-surface alpha value of 128 is considered a special case and is optimised, so it's much faster than other per-surface values.
Alpha effects surface blitting in the following ways:
| RGBA->RGB with SDL_SRCALPHA | The source is alpha-blended with the destination, using the alpha channel. SDL_SRCCOLORKEY and the per-surface alpha are ignored. |
| RGBA->RGB without SDL_SRCALPHA | The RGB data is copied from the source. The source alpha channel and the per-surface alpha value are ignored. |
| RGB->RGBA with SDL_SRCALPHA | The source is alpha-blended with the destination using the per-surface alpha value. If SDL_SRCCOLORKEYliteral> is set, only the pixels not matching the colorkey value are copied. |
| RGB->RGBA without SDL_SRCALPHA | The RGB data is copied from the source and the destination alpha is zero to opaque. If SDL_SRCCOLORKEY is set, only the pixels not matching the colorkey value are copied. |
| RGBA->RGBA with SDL_SRCALPHA | The source is alpha-blended with the destination using the source alpha channel. The alpha channel in the destination surface is left untouched. SDL_SRCCOLORKEY is ignored. |
| RGBA->RGBA without SDL_SRCALPHA | The RGBA data is copied to the destination surface. If SDL_SRCCOLORKEY is set, only the pixels not matching the colorkey value are copied. |
| RGB->RGB with SDL_SRCALPHA | The source is alpha-blended with the destination using the per-surface alpha value. If SDL_SRCCOLORKEY is set, only the pixels not matching the colorkey value are copied. |
| RGB->RGB without SDL_SRCALPHA | The RGB data is copied from the source. If SDL_SRCCOLORKEY is set, only the pixels not matching the colorkey value are copied. |