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.

SEE ALSO

       SDL_MapRGBA, SDL_GetRGBA, SDL_DisplayFormatAlpha, SDL_BlitSurface