Mercurial > sdl-ios-xcode
annotate docs/man3/SDL_PixelFormat.3 @ 3162:dc1eb82ffdaa
Von: Thomas Zimmermann
Betreff: [SDL] [PATCH] Make static variables const
Datum: Tue, 19 May 2009 19:45:37 +0200
Hi,
this is a set of simple changes which make some of SDL's internal static
arrays constant. The purpose is to shrink the number of write-able
static bytes and thus increase the number of memory pages shared between
SDL applications.
The patch set is against trunk@4513. Each of the attached patch files is
specific to a sub-system. The set is completed by a second mail, because
of the list's 40 KiB limit.
The files readelf-r4513.txt and readelf-const-patch.txt where made by
calling 'readelf -S libSDL.so'. They show the difference in ELF sections
without and with the patch. Some numbers measured on my x86-64:
Before
[13] .rodata PROGBITS 00000000000eaaa0 000eaaa0
0000000000008170 0000000000000000 A 0 0 32
[19] .data.rel.ro PROGBITS 00000000003045e0 001045e0
00000000000023d0 0000000000000000 WA 0 0 32
[23] .data PROGBITS 00000000003076e0 001076e0
0000000000004988 0000000000000000 WA 0 0 32
After
[13] .rodata PROGBITS 00000000000eaaa0 000eaaa0
0000000000009a50 0000000000000000 A 0 0 32
[19] .data.rel.ro PROGBITS 0000000000306040 00106040
0000000000002608 0000000000000000 WA 0 0 32
[23] .data PROGBITS 0000000000309360 00109360
0000000000002e88 0000000000000000 WA 0 0 32
The size of the write-able data section decreased considerably. Some
entries became const-after-relocation, while most of its content went
straight into the read-only data section.
Best regards, Thomas
| author | Sam Lantinga <slouken@libsdl.org> |
|---|---|
| date | Wed, 03 Jun 2009 04:37:27 +0000 |
| parents | 546f7c1eb755 |
| children | 1238da4a7112 |
| rev | line source |
|---|---|
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1 .TH "SDL_PixelFormat" "3" "Tue 11 Sep 2001, 23:01" "SDL" "SDL API Reference" |
| 0 | 2 .SH "NAME" |
|
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3 SDL_PixelFormat \- Stores surface format information |
| 0 | 4 .SH "STRUCTURE DEFINITION" |
| 5 .PP | |
| 6 .nf | |
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7 \f(CWtypedef struct SDL_PixelFormat { |
| 0 | 8 SDL_Palette *palette; |
| 9 Uint8 BitsPerPixel; | |
| 10 Uint8 BytesPerPixel; | |
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11 Uint8 Rloss, Gloss, Bloss, Aloss; |
| 0 | 12 Uint8 Rshift, Gshift, Bshift, Ashift; |
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13 Uint32 Rmask, Gmask, Bmask, Amask; |
| 0 | 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 | |
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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\&. 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 the number of bits to the right of each component in the pixel value and the loss fields tell us the number of bits lost from each component when packing 8-bit color component in a pixel\&. |
| 0 | 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); | |
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103 pixel=*((Uint32*)surface->pixels); |
| 0 | 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 | |
|
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140 ...\" created by instant / docbook-to-man, Tue 11 Sep 2001, 23:01 |