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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 |