SDL_SetAlpha(3) SDL API Reference SDL_SetAlpha(3)NAMESDL_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 consid‐
ered 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 accelera‐
tion 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 sur‐
face need not have an alpha channel to use per-surface alpha and blit‐
ting 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_SRCCOL‐
ORKEY is set, only the pixels not matching the col‐
orkey 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_SRCCOL‐
ORKEY is set, only the pixels not matching the col‐
orkey value are copied.
RGB->RGB without SDL_SRCALPHA
The RGB data is copied from the source. If SDL_SRC‐
COLORKEY 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 com‐
bined; 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
SDL Tue 11 Sep 2001, 23:01 SDL_SetAlpha(3)
