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 value and/or enabling and disabling alpha blending.
Thesurface 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.
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_SRCCOLORKEY is set, only the pixels not matching the colorkey value are copied. The alpha channel of the copied pixels is set to opaque. |
| RGB->RGBA without SDL_SRCALPHA | The RGB data is copied from the source and the alpha value of the copied pixels is set 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. |
Note: Note that RGBA->RGBA blits (with SDL_SRCALPHA set) keep the alpha of the destination surface. This means that you cannot compose two arbitrary RGBA surfaces this way and get the result you would expect from "overlaying" them; the destination alpha will work as a mask.
Also note that per-pixel and per-surface alpha cannot be combined; the per-pixel alpha is always used if available