| e14743d1 |
1 | .TH "SDL_SetAlpha" "3" "Tue 11 Sep 2001, 23:01" "SDL" "SDL API Reference" |
| 2 | .SH "NAME" |
| 3 | SDL_SetAlpha \- Adjust the alpha properties of a surface |
| 4 | .SH "SYNOPSIS" |
| 5 | .PP |
| 6 | \fB#include "SDL\&.h" |
| 7 | .sp |
| 8 | \fBint \fBSDL_SetAlpha\fP\fR(\fBSDL_Surface *surface, Uint32 flag, Uint8 alpha\fR); |
| 9 | .SH "DESCRIPTION" |
| 10 | .PP |
| 11 | .RS |
| 12 | \fBNote: |
| 13 | .PP |
| 14 | 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 (\fBSDL_ALPHA_TRANSPARENT\fP) is now considered transparent and 255 (\fBSDL_ALPHA_OPAQUE\fP) is now considered opaque\&. |
| 15 | .RE |
| 16 | .PP |
| 17 | \fBSDL_SetAlpha\fP is used for setting the per-surface alpha value and/or enabling and disabling alpha blending\&. |
| 18 | .PP |
| 19 | The\fBsurface\fR parameter specifies which surface whose alpha attributes you wish to adjust\&. \fBflags\fR is used to specify whether alpha blending should be used (\fBSDL_SRCALPHA\fP) and whether the surface should use RLE acceleration for blitting (\fBSDL_RLEACCEL\fP)\&. \fBflags\fR can be an OR\&'d combination of these two options, one of these options or 0\&. If \fBSDL_SRCALPHA\fP is not passed as a flag then all alpha information is ignored when blitting the surface\&. The \fBalpha\fR 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 \fBSDL_RLEACCEL\fP\&. |
| 20 | .PP |
| 21 | .RS |
| 22 | \fBNote: |
| 23 | .PP |
| 24 | 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\&. |
| 25 | .RE |
| 26 | .PP |
| 27 | Alpha effects surface blitting in the following ways: |
| 28 | .TP 20 |
| 29 | RGBA->RGB with \fBSDL_SRCALPHA\fP |
| 30 | The source is alpha-blended with the destination, using the alpha channel\&. \fBSDL_SRCCOLORKEY\fP and the per-surface alpha are ignored\&. |
| 31 | .TP 20 |
| 32 | RGBA->RGB without \fBSDL_SRCALPHA\fP |
| 33 | The RGB data is copied from the source\&. The source alpha channel and the per-surface alpha value are ignored\&. |
| 34 | .TP 20 |
| 35 | RGB->RGBA with \fBSDL_SRCALPHA\fP |
| 36 | The source is alpha-blended with the destination using the per-surface alpha value\&. If \fBSDL_SRCCOLORKEY\fP is set, only the pixels not matching the colorkey value are copied\&. The alpha channel of the copied pixels is set to opaque\&. |
| 37 | .TP 20 |
| 38 | RGB->RGBA without \fBSDL_SRCALPHA\fP |
| 39 | The RGB data is copied from the source and the alpha value of the copied pixels is set to opaque\&. If \fBSDL_SRCCOLORKEY\fP is set, only the pixels not matching the colorkey value are copied\&. |
| 40 | .TP 20 |
| 41 | RGBA->RGBA with \fBSDL_SRCALPHA\fP |
| 42 | The source is alpha-blended with the destination using the source alpha channel\&. The alpha channel in the destination surface is left untouched\&. \fBSDL_SRCCOLORKEY\fP is ignored\&. |
| 43 | .TP 20 |
| 44 | RGBA->RGBA without \fBSDL_SRCALPHA\fP |
| 45 | The RGBA data is copied to the destination surface\&. If \fBSDL_SRCCOLORKEY\fP is set, only the pixels not matching the colorkey value are copied\&. |
| 46 | .TP 20 |
| 47 | RGB->RGB with \fBSDL_SRCALPHA\fP |
| 48 | The source is alpha-blended with the destination using the per-surface alpha value\&. If \fBSDL_SRCCOLORKEY\fP is set, only the pixels not matching the colorkey value are copied\&. |
| 49 | .TP 20 |
| 50 | RGB->RGB without \fBSDL_SRCALPHA\fP |
| 51 | The RGB data is copied from the source\&. If \fBSDL_SRCCOLORKEY\fP is set, only the pixels not matching the colorkey value are copied\&. |
| 52 | .PP |
| 53 | .RS |
| 54 | \fBNote: |
| 55 | .PP |
| 56 | 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\&. |
| 57 | .PP |
| 58 | Also note that per-pixel and per-surface alpha cannot be combined; the per-pixel alpha is always used if available |
| 59 | .RE |
| 60 | .SH "RETURN VALUE" |
| 61 | .PP |
| 62 | This function returns \fB0\fR, or \fB-1\fR if there was an error\&. |
| 63 | .SH "SEE ALSO" |
| 64 | .PP |
| 65 | \fI\fBSDL_MapRGBA\fP\fR, \fI\fBSDL_GetRGBA\fP\fR, \fI\fBSDL_DisplayFormatAlpha\fP\fR, \fI\fBSDL_BlitSurface\fP\fR |
| 66 | .\" created by instant / docbook-to-man, Tue 11 Sep 2001, 23:01 |
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