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