Defines implemented methods for  ASScanline  combining, that could
be passed to merge_layers() via  ASImageLayer  structure.
Also includes functions for  colorspace  conversion RGB<->HSV and
RGB<->HLS.

Functions :
   Colorspace conversion :
         rgb2value(), rgb2saturation(), rgb2hue(), rgb2luminance(),
         rgb2hsv(), rgb2hls(), hsv2rgb(), hls2rgb().

   merge_scanline methods :
         alphablend_scanlines(), allanon_scanlines(),
         tint_scanlines(), add_scanlines(), sub_scanlines(),
         diff_scanlines(), darken_scanlines(), lighten_scanlines(),
         screen_scanlines(), overlay_scanlines(), hue_scanlines(),
         saturate_scanlines(), value_scanlines(),
         colorize_scanlines(), dissipate_scanlines().

   usefull merging function name to function translator :
         blend_scanlines_name2func()

Other  libAfterImage  modules :
          ascmap .h  asfont .h  asimage .h  asvisual .h  blender .h  export .h
          import .h  transform .h  ximage .h

Sasha Vasko <sasha at aftercode dot net>

RGB  colorspace : each  color  is represented as a combination of
red, green and blue values. Each  value  can be in 2 formats :
8 bit and 24.8 bit. 24.8 bit makes for 32bit  value  with lower 8 bits
used as a fraction for better calculation precision.

HSV  colorspace : each  color  is represented as a combination of
 hue , saturation and  value . Hue is generally colorizing component where
 value  represents brightness.

HLS  colorspace : each  color  is represented as a combination of
 hue , luminance and saturation. It is analogous to HSV with  value 
substituted by luminance, except that luminance could be both
negative and positive.

alpha channel could be added to any of the above colorspaces. alpha
channel is generally used to define transparentness of the  color .
 libAfterImage  is using ARGB  colorspace  as a base  colorspace , and
represents most colors as  ARGB32  values or  ASScanline  scanlines of
pixels.

CARD32 rgb2value( CARD32 red, CARD32 green, CARD32 blue );
CARD32 rgb2saturation( CARD32 red, CARD32 green, CARD32 blue );
CARD32 rgb2hue( CARD32 red, CARD32 green, CARD32 blue );
CARD32  rgb2luminance  (CARD32 red, CARD32 green, CARD32 blue );

red

- 32 bit  value , 16 lower bits of which represent red channel

green

- 32 bit  value , 16 lower bits of which represent green channel

blue

- 32 bit  value , 16 lower bits of which represent blue channel

32 bit  value , 16 lower bits of which represent  value , saturation,  hue ,
or luminance respectively.

This functions translate RGB  color  into respective coordinates of
HSV and HLS colorspaces.
Returned  hue  values are in 16bit format. To translate it to and from
conventional 0-360 degree range, please use :
degrees2hue16() - converts conventional  hue  in 0-360 range into hue16
hue162degree()  - converts 16bit  hue   value  into conventional degrees.

CARD32 rgb2hsv( CARD32 red, CARD32 green, CARD32 blue,
                CARD32 *saturation, CARD32 * value  );
CARD32 rgb2hls( CARD32 red, CARD32 green, CARD32 blue,
                CARD32 *luminance, CARD32 *saturation );

red

- 32 bit  value , 16 lower bits of which represent red channel

green

- 32 bit  value , 16 lower bits of which represent green channel

blue

- 32 bit  value , 16 lower bits of which represent blue channel

32 bit  value , 16 lower bits of which represent  hue .
32bit  value  pointed to by luminance,  value  and saturation will be  set 
respectively to  color  luminance,  value  and saturation.

This functions translate RGB  color  into full  set  of HSV and HLS
coordinates at once. These functions work faster then separate
translation into each channel.

void hsv2rgb( CARD32  hue , CARD32 saturation, CARD32  value ,
              CARD32 *red, CARD32 *green, CARD32 *blue);
void hls2rgb( CARD32  hue , CARD32 luminance, CARD32 saturation,
              CARD32 *red, CARD32 *green, CARD32 *blue);

hue

- 32 bit  value , 16 lower bits of which represent  hue .

saturation

- 32 bit  value , 16 lower bits of which represent saturation.

value

- 32 bit  value , 16 lower bits of which represent  value .

luminance

- 32 bit  value , 16 lower bits of which represent luminance.

32bit  value  pointed to by red, green and blue will be  set 
respectively to RGB  color  channels.

This functions performs reverse translation from HSV and HSL to
RGB  color 

- combines top and bottom RGB components based on alpha channel  value : bottom = bottom*(255-top_alpha)+top*top_alpha;

- averages each pixel between two scanlines. This method has been first implemented by Ethan Fisher aka  allanon as mode 130: bottom = (bottom+top)/2;

- tints bottom scanline with top scanline( with saturation to prevent overflow) : bottom = (bottom*(top/2))/32768;

- adds top scanline to bottom scanline with saturation to prevent overflow: bottom = bottom+top;

- substrates top scanline from bottom scanline with saturation to prevent overflow: bottom = bottom-top;

- for each pixel calculates absolute difference between bottom and top  color  value : bottom = (bottom>top)?bottom-top:top-bottom;

- substitutes each pixel with minimum  color  value of top and bottom : bottom = (bottom>top)?top:bottom;

- substitutes each pixel with maximum  color  value of top and bottom : bottom = (bottom>top)?bottom:top;

- some wierd merging algorithm taken from GIMP;

- some wierd merging algorithm taken from GIMP;

- substitute  hue of bottom scanline with  hue of top scanline;

- substitute saturation of bottom scanline with the saturation of top scanline;

- substitute  value of bottom scanline with the  value of top scanline;

- combine luminance of bottom scanline with  hue and saturation of top scanline;

randomly alpha- blend bottom and top scanlines, using alpha  value of top scanline as a threshold for random values.

void alphablend_scanlines(  ASScanline  *bottom,  ASScanline  *top, int );
void allanon_scanlines   (  ASScanline  *bottom,  ASScanline  *top, int );
void tint_scanlines      (  ASScanline  *bottom,  ASScanline  *top, int );
void add_scanlines       (  ASScanline  *bottom,  ASScanline  *top, int );
void sub_scanlines       (  ASScanline  *bottom,  ASScanline  *top, int );
void diff_scanlines      (  ASScanline  *bottom,  ASScanline  *top, int );
void darken_scanlines    (  ASScanline  *bottom,  ASScanline  *top, int );
void lighten_scanlines   (  ASScanline  *bottom,  ASScanline  *top, int );
void screen_scanlines    (  ASScanline  *bottom,  ASScanline  *top, int );
void overlay_scanlines   (  ASScanline  *bottom,  ASScanline  *top, int );
void hue_scanlines       (  ASScanline  *bottom,  ASScanline  *top, int );
void saturate_scanlines  (  ASScanline  *bottom,  ASScanline  *top, int );
void value_scanlines     (  ASScanline  *bottom,  ASScanline  *top, int );
void colorize_scanlines  (  ASScanline  *bottom,  ASScanline  *top, int );
void dissipate_scanlines (  ASScanline  *bottom,  ASScanline  *top, int );

bottom

- pointer to the  ASScanline that will be overalayed

top

- pointer to  ASScanline that will  overlay bottom.

This functions accept 2 scanlines as an arguments stored in
 ASScanline  structures with data in 24.8 format. Merging operation is
performed on these scanlines and result is stored in bottom
 ASScanline .
The following are merging methods used in each function :

merge_scanlines_func blend_scanlines_name2func( const char *name );
void list_scanline_merging(FILE* stream, const char *format);

name

- string, identifying scanline merging function.

returns pointer to the scanline merging function on succes.
NULL on failure.

blend_scanlines_name2func() will strip leading whitespaces off of
the supplied name, and then will attempt to match it against the  list 
of names of merging functions. It will then return pointer to the
function with matching name.
list_scanline_merging() simply prints out description of implemented
blending/merging methods onto the supplied stream, in supplied format.
Format must include 2 string specs, like so : "%s - %s" where first
one will be substituted to short method name, and second - description