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ascmap
Section: AfterStep X11 window manager (1)
Updated: AfterStep v.2.2.2
Index NAME
ascmap - defines main structures and function for image quantizationlibAfterImage/ascmap.h
NAMEascmap
- Defines main structures and function for image quantization.
DESCRIPTION
Image quantization is needed primarily in order to be able to exportimages 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 numberof colors in original image, libAfterImage allows to set arbitrarylevel of downsampling of the color spectrum in the range of 8 bit perchannel to 1 bit per channel. Downsampling is performed by simpledropping of less significant bits off of color values.In order to be able to determine closeness of colors, 3-channel RGBvalues are converted into flat 24bit (or less if downsampling is used)index. That is done by intermixing bits from different channels, likeso : R8G8B8R7G7B7...R1G1B1. That flat index is used to arrange colorsin ascending order, and later on to be able to find closest mappedcolor. Simple hashing technique is used to speed up thesorting/searching, as it allows to limit linked lists traversals.
SEE ALSO
Structures : ASColormapEntry ASColormapFunctions : 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. Thisarray has size of WIDTHxHEIGHT where WIDTH and HEIGHT are size of thesource image.
DESCRIPTION
This function is all that is needed to quantize the ASImage. In orderto obtain colorcell of the pixel at (x,y) from result, the followingcode could be used :cmap->entries[res[y*width+x]]where res is returned pointer.Recommended value for dither parameter is 4 while quantizing photos to256 colors, and it could be less , if original has limited number ofcolors.
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.
Index
- NAME
- NAMEascmap
- DESCRIPTION
- SEE ALSO
- AUTHOR
- NAMEASColormapEntry
- SOURCE
- NAMEASColormap
- SOURCE
- NAMEcolormap_asimage()
- SYNOPSIS
- INPUTS
- RETURN VALUE
- DESCRIPTION
- NAMEdestroy_colormap()
- SYNOPSIS
- INPUTS
- DESCRIPTION
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