SDL_SetAlpha - Online Linux Manual PageSection : 3
Updated : Tue 11 Sep 2001, 23:01
Source : SDL
Note : SDL API Reference

NAMESDL_SetAlpha − Adjust the alpha properties of a surface

SYNOPSIS#include "SDL​.h" int SDL_SetAlpha(SDL_Surface *surface, Uint32 flag, Uint8 alpha);

DESCRIPTIONNote: 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 VALUEThis function returns 0, or -1 if there was an error​.

SEE ALSOSDL_MapRGBA, SDL_GetRGBA, SDL_DisplayFormatAlpha, SDL_BlitSurface
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