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 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_AL-
PHA_TRANSPARENT) is now considered transparent and 255 (SDL_AL-
PHA_OPAQUE) is now considered opaque.
SDL_SetAlpha is used for setting the per-surface alpha value and/or en-
abling 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 al-
pha value of the copied pixels is set to opaque. If
SDL_SRCCOLORKEY is set, only the pixels not match-
ing 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 al-
pha of the destination surface. This means that you cannot com-
pose 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)
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