Indexed Image handling

libAfterImage/ascmap.h

• NAME

  ascmap

  - Defines main structures and function for image quantization.

• DESCRIPTION

  Image quantization is needed primarily in order to be able to export
  images into file, with colormap format, such as GIF and XPM.
  libAfterImage attempts to allocate colorcells to the most used colors,
  and then approximate remaining colors with the closest colorcell.
  
  Since quality of quantization is in reverse proportion to the number
  of colors in original image, libAfterImage allows one to set arbitrary
  level of downsampling of the color spectrum in the range of 8 bit per
  channel to 1 bit per channel. Downsampling is performed by simple
  dropping of less significant bits off of color values.
  
  In order to be able to determine closeness of colors, 3-channel RGB
  values are converted into flat 24bit (or less if downsampling is used)
  index. That is done by intermixing bits from different channels, like
  so : R8G8B8R7G7B7...R1G1B1. That flat index is used to arrange colors
  in ascending order, and later on to be able to find closest mapped
  color. Simple hashing technique is used to speed up the
  sorting/searching, as it allows one to limit linked lists traversals.
  
  

• SEE ALSO

  Structures :
           ASColormapEntry
           ASColormap
  
  Functions :
           colormap_asimage(), destroy_colormap()
  
  Other libAfterImage modules :
           ascmap.h asfont.h asimage.h asvisual.h blender.h export.h
           import.h transform.h ximage.h
  

• AUTHOR

  Sasha Vasko <sasha at aftercode dot net>
  

libAfterImage/ASColormapEntry

• NAME

  ASColormapEntry

  - ASColormapEntry represents single colorcell in the colormap.

• SOURCE

  
  typedef struct ASColormapEntry
  {
      CARD8 red, green, blue;
  }ASColormapEntry;
  

  
  

libAfterImage/ASColormap

• NAME

  ASColormap

  - ASColormap represents entire colormap generated for the image.

• SOURCE

  typedef struct ASColormap
  {
      ASColormapEntry *entries ;  /* array of colorcells */
      unsigned int count ;        /* number of used colorcells */
      ASSortedColorHash *hash ;   /* internal data */
      Bool has_opaque ;           /* If True then Image has opaque pixels */
  }ASColormap;
  

  
  

libAfterImage/colormap_asimage()

• NAME

  colormap_asimage()

• SYNOPSIS

  int *colormap_asimage( ASImage *im, ASColormap *cmap,
                         unsigned int max_colors, unsigned int dither,
                         int opaque_threshold );
  

• INPUTS

  im

  - pointer to valid ASImage structure.

  cmap

  - preallocated structure to store colormap in.

  max_colors

  - maximum size of the colormap.

  dither

  - number of bits to strip off the color data ( 0...7 )

  opaque_threshold

  - alpha channel threshold at which pixel should be treated as opaque

• RETURN VALUE

  pointer to the array of indexes representing pixel's colorcells. This
  array has size of WIDTHxHEIGHT where WIDTH and HEIGHT are size of the
  source image.
  

• DESCRIPTION

  This function is all that is needed to quantize the ASImage. In order
  to obtain colorcell of the pixel at (x,y) from result, the following
  code could be used :
  cmap->entries[res[y*width+x]]
  where res is returned pointer.
  Recommended value for dither parameter is 4 while quantizing photos to
  256 colors, and it could be less , if original has limited number of
  colors.
  
  

libAfterImage/destroy_colormap()

• NAME

  destroy_colormap()

• SYNOPSIS

  void destroy_colormap( ASColormap *cmap, Bool reusable );
  

• INPUTS

  cmap

  - pointer to valid ASColormap structure.

  reusable

  - if True, then the memory pointed to by cmap will not be deallocated, as if it was allocated on stack

• DESCRIPTION

  Destroys ASColormap object created using colormap_asimage.
  

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