docs/man3/SDL_PixelFormat.3
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     1 .TH "SDL_PixelFormat" "3" "Tue 11 Sep 2001, 23:01" "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 SDL_PixelFormat {
       
     8   SDL_Palette *palette;
       
     9   Uint8  BitsPerPixel;
       
    10   Uint8  BytesPerPixel;
       
    11   Uint8  Rloss, Gloss, Bloss, Aloss;
       
    12   Uint8  Rshift, Gshift, Bshift, Ashift;
       
    13   Uint32 Rmask, Gmask, Bmask, Amask;
       
    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\&. 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\&. 
       
    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, Tue 11 Sep 2001, 23:01