Tribblix: manual page: SDL

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
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 VALUE

This function returns 0, or -1 if there was an error.

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

SEE ALSO