SDL_SetAlpha(3) | SDL API Reference | SDL_SetAlpha(3) |
SDL_SetAlpha - Adjust the alpha properties of a surface
#include "SDL.h"
int SDL_SetAlpha(SDL_Surface *surface, Uint32 flag, Uint8 alpha);
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.
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:
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
This function returns 0, or -1 if there was an error.
SDL_MapRGBA, SDL_GetRGBA, SDL_DisplayFormatAlpha, SDL_BlitSurface
Tue 11 Sep 2001, 23:01 | SDL |