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SDL_PixelFormat

A structure that contains pixel format information.

Data Fields

Uint32

format

one of the SDL_PixelFormatEnum values

SDL_Palette*

palette

an SDL_Palette structure associated with this pixel format, or NULL if the format doesn't have a palette

Uint8

BitsPerPixel

the number of significant bits in a pixel value, eg: 8, 15, 16, 24, 32

Uint8

BytesPerPixel

the number of bytes required to hold a pixel value, eg: 1, 2, 3, 4; see Remarks for related data type

Uint32

Rmask

a mask representing the location of the red component of the pixel

Uint32

Gmask

a mask representing the location of the green component of the pixel

Uint32

Bmask

a mask representing the location of the blue component of the pixel

Uint32

Amask

a mask representing the location of the alpha component of the pixel or 0 if the pixel format doesn't have any alpha information

Uint8

Rloss

(internal use)

Uint8

Gloss

(internal use)

Uint8

Bloss

(internal use)

Uint8

Aloss

(internal use)

Uint8

Rshift

(internal use)

Uint8

Gshift

(internal use)

Uint8

Bshift

(internal use)

Uint8

Ashift

(internal use)

int

refcount

(internal use)

SDL_PixelFormat*

next

(internal use)

Code Examples

8-bit pixel formats are the easiest to understand. Since its an 8-bit format, we have 8 BitsPerPixel and 1 BytesPerPixel. Since BytesPerPixel is 1, all pixels are represented by a Uint8 which contains an index into palette->colors. So, to determine the color of a pixel in an 8-bit surface: we read the color index from surface->pixels and we use that index to read the SDL_Color structure from surface->format->palette->colors. Like so:

SDL_Surface *surface;
SDL_PixelFormat *fmt;
SDL_Color *color;
Uint8 index;

.
.

/* Create surface */
.
.
fmt=surface->format;

/* Check the bitdepth of the surface */
if(fmt->BitsPerPixel!=8){
  fprintf(stderr, "Not an 8-bit surface.\n");
  return(-1);
}

/* Lock the surface */
SDL_LockSurface(surface);

/* Get the topleft pixel */
index=*(Uint8 *)surface->pixels;
color=&fmt->palette->colors[index];

/* Unlock the surface */
SDL_UnlockSurface(surface);
printf("Pixel Color-> Red: %d, Green: %d, Blue: %d. Index: %d\n",
          color->r, color->g, color->b, index);
.
.

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.

/* Extracting color components from a 32-bit color value */
SDL_PixelFormat *fmt;
SDL_Surface *surface;
Uint32 temp, pixel;
Uint8 red, green, blue, alpha;
.
.
.
fmt = surface->format;
SDL_LockSurface(surface);
pixel = *((Uint32*)surface->pixels);
SDL_UnlockSurface(surface);

/* Get Red component */
temp = pixel & fmt->Rmask;  /* Isolate red component */
temp = temp >> fmt->Rshift; /* Shift it down to 8-bit */
temp = temp << fmt->Rloss;  /* Expand to a full 8-bit number */
red = (Uint8)temp;

/* Get Green component */
temp = pixel & fmt->Gmask;  /* Isolate green component */
temp = temp >> fmt->Gshift; /* Shift it down to 8-bit */
temp = temp << fmt->Gloss;  /* Expand to a full 8-bit number */
green = (Uint8)temp;

/* Get Blue component */
temp = pixel & fmt->Bmask;  /* Isolate blue component */
temp = temp >> fmt->Bshift; /* Shift it down to 8-bit */
temp = temp << fmt->Bloss;  /* Expand to a full 8-bit number */
blue = (Uint8)temp;

/* Get Alpha component */
temp = pixel & fmt->Amask;  /* Isolate alpha component */
temp = temp >> fmt->Ashift; /* Shift it down to 8-bit */
temp = temp << fmt->Aloss;  /* Expand to a full 8-bit number */
alpha = (Uint8)temp;

printf("Pixel Color -> R: %d,  G: %d,  B: %d,  A: %d\n", red, green, blue, alpha);
.
.
.

Remarks

Everything in the pixel format structure is read-only.

A pixel format has either a palette or masks. If a palette is used Rmask, Gmask, Bmask, and Amask will be 0.

The data types used to represent pixels are as follows:

Bytes Per Pixel

Related Data Types

1

Uint8

2

Uint16

3

tuple of Uint8 RGB values

4

Uint32

An SDL_PixelFormat describes the format of the pixel data stored at the pixels field of an SDL_Surface. Every surface stores an SDL_PixelFormat in the format field.

If you wish to do pixel level modifications on a surface, then understanding how SDL stores its color information is essential. See Code Examples above for more information.

For information on modern pixel color spaces, see the following Wikipedia article: http://en.wikipedia.org/wiki/RGBA_color_space


CategoryStruct, CategoryPixels

None: SDL_PixelFormat (last edited 2014-09-22 17:40:47 by PhilippWiesemann)

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