ascmap(1)                AfterStep X11 window manager                ascmap(1)



NAME
       ascmap - defines main structures and function for image quantization
       libAfterImage/ascmap.h

NAMEascmap
       - 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 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 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>

NAMEASColormapEntry
       - ASColormapEntry represents single colorcell in the colormap.


SOURCE
       Source :
       typedef struct ASColormapEntry
       {
           CARD8 red, green, blue;
       }ASColormapEntry;

NAMEASColormap
       - ASColormap represents entire colormap generated for the image.


SOURCE
       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;

NAMEcolormap_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.

NAMEdestroy_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.



3rd Berkeley Distribution      AfterStep v.2.2.9                     ascmap(1)
