Manual Pages for UNIX Operating System command usage for man SDL_SetAlpha

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  con-
          sidered  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  chan-
     nel  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 spe-
          cial  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-



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SDL API Reference                                 SDL_SetAlpha(3)



                         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 chan-
                         nel. The alpha channel in  the  destina-
                         tion    surface   is   left   untouched.
                         SDL_SRCCOLORKEY is ignored.

     RGBA->RGBA without SDL_SRCALPHA
                         The RGBA data is copied to the  destina-
                         tion surface. If SDL_SRCCOLORKEY is set,
                         only the pixels not matching the  color-
                         key 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 pix-
                         els 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.




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SDL API Reference                                 SDL_SetAlpha(3)



          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













































SDL            Last change: Tue 11 Sep 2001, 23:01              3