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Initial revision
author Sam Lantinga <slouken@lokigames.com>
date Thu, 26 Apr 2001 16:45:43 +0000
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1 .TH "SDL_PixelFormat" "3" "Mon 12 Mar 2001, 01:04" "SDL" "SDL API Reference"
2 .SH "NAME"
3 SDL_PixelFormat\- Stores surface format information
4 .SH "STRUCTURE DEFINITION"
5 .PP
6 .nf
7 \f(CWtypedef struct{
8 SDL_Palette *palette;
9 Uint8 BitsPerPixel;
10 Uint8 BytesPerPixel;
11 Uint32 Rmask, Gmask, Bmask, Amask;
12 Uint8 Rshift, Gshift, Bshift, Ashift;
13 Uint8 Rloss, Gloss, Bloss, Aloss;
14 Uint32 colorkey;
15 Uint8 alpha;
16 } SDL_PixelFormat;\fR
17 .fi
18 .PP
19 .SH "STRUCTURE DATA"
20 .TP 20
21 \fBpalette\fR
22 Pointer to the \fIpalette\fR, or \fBNULL\fP if the \fBBitsPerPixel\fR>8
23 .TP 20
24 \fBBitsPerPixel\fR
25 The number of bits used to represent each pixel in a surface\&. Usually 8, 16, 24 or 32\&.
26 .TP 20
27 \fBBytesPerPixel\fR
28 The number of bytes used to represent each pixel in a surface\&. Usually one to four\&.
29 .TP 20
30 \fB[RGBA]mask\fR
31 Binary mask used to retrieve individual color values
32 .TP 20
33 \fB[RGBA]loss\fR
34 Precision loss of each color component (2^[RGBA]loss)
35 .TP 20
36 \fB[RGBA]shift\fR
37 Binary left shift of each color component in the pixel value
38 .TP 20
39 \fBcolorkey\fR
40 Pixel value of transparent pixels
41 .TP 20
42 \fBalpha\fR
43 Overall surface alpha value
44 .SH "DESCRIPTION"
45 .PP
46 A \fBSDL_PixelFormat\fR describes the format of the pixel data stored at the \fBpixels\fR field of a \fI\fBSDL_Surface\fR\fR\&. Every surface stores a \fBSDL_PixelFormat\fR in the \fBformat\fR field\&.
47 .PP
48 If you wish to do pixel level modifications on a surface, then understanding how SDL stores its color information is essential\&.
49 .PP
50 8-bit pixel formats are the easiest to understand\&. Since its an 8-bit format, we have 8 \fBBitsPerPixel\fR and 1 \fBBytesPerPixel\fR\&. Since \fBBytesPerPixel\fR is 1, all pixels are represented by a Uint8 which contains an index into \fBpalette\fR->\fBcolors\fR\&. So, to determine the color of a pixel in a 8-bit surface: we read the color index from \fBsurface\fR->\fBpixels\fR and we use that index to read the \fI\fBSDL_Color\fR\fR structure from \fBsurface\fR->\fBformat\fR->\fBpalette\fR->\fBcolors\fR\&. Like so:
51 .PP
52 .nf
53 \f(CWSDL_Surface *surface;
54 SDL_PixelFormat *fmt;
55 SDL_Color *color;
56 Uint8 index;
57
58 \&.
59 \&.
60
61 /* Create surface */
62 \&.
63 \&.
64 fmt=surface->format;
65
66 /* Check the bitdepth of the surface */
67 if(fmt->BitsPerPixel!=8){
68 fprintf(stderr, "Not an 8-bit surface\&.
69 ");
70 return(-1);
71 }
72
73 /* Lock the surface */
74 SDL_LockSurface(surface);
75
76 /* Get the topleft pixel */
77 index=*(Uint8 *)surface->pixels;
78 color=fmt->palette->colors[index];
79
80 /* Unlock the surface */
81 SDL_UnlockSurface(surface);
82 printf("Pixel Color-> Red: %d, Green: %d, Blue: %d\&. Index: %d
83 ",
84 color->r, color->g, color->b, index);
85 \&.
86 \&.\fR
87 .fi
88 .PP
89 .PP
90 Pixel formats above 8-bit are an entirely different experience\&. They are considered to be "TrueColor" formats and the color information is stored in the pixels themselves, not in a palette (packed-pixel)\&. The mask, shift and loss fields tell us how the color information is encoded\&. The mask fields allow us to isolate each color component, the shift fields tell us how far left we have to shift the masked value and the loss fields tell us for far right we have to shift the final value to convert it to a full 8-bit color component\&.
91 .PP
92 .nf
93 \f(CW/* Extracting color components from a 32-bit color value */
94 SDL_PixelFormat *fmt;
95 SDL_Surface *surface;
96 Uint32 temp, pixel;
97 Uint8 red, green, blue, alpha;
98 \&.
99 \&.
100 \&.
101 fmt=surface->format;
102 SDL_LockSurface(surface);
103 pixel=(Uint32*)surface->pixels;
104 SDL_UnlockSurface(surface);
105
106 /* Get Red component */
107 temp=pixel&fmt->Rmask; /* Isolate red component */
108 temp=temp>>fmt->Rshift;/* Shift it down to 8-bit */
109 temp=temp<<fmt->Rloss; /* Expand to a full 8-bit number */
110 red=(Uint8)temp;
111
112 /* Get Green component */
113 temp=pixel&fmt->Gmask; /* Isolate green component */
114 temp=temp>>fmt->Gshift;/* Shift it down to 8-bit */
115 temp=temp<<fmt->Gloss; /* Expand to a full 8-bit number */
116 green=(Uint8)temp;
117
118 /* Get Blue component */
119 temp=pixel&fmt->Bmask; /* Isolate blue component */
120 temp=temp>>fmt->Bshift;/* Shift it down to 8-bit */
121 temp=temp<<fmt->Bloss; /* Expand to a full 8-bit number */
122 blue=(Uint8)temp;
123
124 /* Get Alpha component */
125 temp=pixel&fmt->Amask; /* Isolate alpha component */
126 temp=temp>>fmt->Ashift;/* Shift it down to 8-bit */
127 temp=temp<<fmt->Aloss; /* Expand to a full 8-bit number */
128 alpha=(Uint8)temp;
129
130 printf("Pixel Color -> R: %d, G: %d, B: %d, A: %d
131 ", red, green, blue, alpha);
132 \&.
133 \&.
134 \&.\fR
135 .fi
136 .PP
137 .SH "SEE ALSO"
138 .PP
139 \fI\fBSDL_Surface\fR\fR, \fI\fBSDL_MapRGB\fP\fR
140 ...\" created by instant / docbook-to-man, Mon 12 Mar 2001, 01:04