SDL_SetAlpha(3)
NAME
SDL_SetAlpha - Adjust the alpha properties of a surface
SYNOPSIS
#include "SDL.h" int SDL_SetAlpha(SDL_Surface *surface, Uint32 flag, Uint8 alpha);
DESCRIPTION
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
RETURN VALUE
This function returns 0, or -1 if there was an error.
SEE ALSO
- SDL_MapRGBA, SDL_GetRGBA, SDL_DisplayFormatAlpha, SDL_BlitSurface