SDL_SetAlpha(3) SDL API Reference 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 blend-
ing have changed since version 1.1.4. Up until ver-
sion 1.1.5, an alpha value of 0 was considered
opaque and a value of 255 was considered transpar-
ent. This has now been inverted: 0
(SDL_ALPHA_TRANSPARENT) is now considered transpar-
ent 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 blit-
ting (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 infor-
mation 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 pix-
els is set to opaque. If SDL_SRCCOL-
ORKEY 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 des-
tination surface is left untouched.
SDL_SRCCOLORKEY is ignored.
RGBA->RGBA without SDL_SRCALPHA
The RGBA data is copied to the desti-
nation 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 col-
orkey 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 can-
not 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
SDL Tue 11 Sep 2001, 23:01 SDL_SetAlpha(3)
