Sparkl   --  Add "sparkles" simulating diffraction spikes to an image.

This is a small utility to add those glint or sparkle-type effects you
may have seen to your own images. It doesn't add sparkles arbitrarily,
but works by painting in radial spikes or rays around what are already
the brightest pixels in an image.  The number of spokes, their
intensity and length, and overall rotation angle is adjustable through
command-line parameters. 

Also adjustable is the brightest-pixels-percentile cutoff value, ie,
what fraction of brightest pixels should be enhanced with glints? The
default value of 0.001 (ie 0.1 percentile) seems to work pretty well.
Much more than that and you'll have the entire picture looking
sparkly. This percentile value is then translated into an absolute
brightness level by taking a histogram of the image and adding up the
numbers of brightest pixels.  

IMPORTANT: Not all images are good candidates for "sparkle" treatment.
You want a fairly dark image, with a few bright points. The effect of
the program on all bright, or low-contrast images is difficult or 
impossible to see. If you aren't getting the result you want, try first
darkening the image and making sure there are a few small bright points
in it. Initial image contrast is critical for best results.

Caution: too many sparkles will probably make your image look like a
home-shopping-network advertisement for cheap jewlery. 

Note that the only image file format used by this program is the
24-bit ("type 2") uncompressed, top-down, TGA (Targa) format file.
This format is very common in the DOS/PC world, less so for Unix boxes
but you can convert other formats to it with "xv" version 3.1, or
ImageMagick, or the pbmplus tools.

UNIX: type "make" and the program should compile.
DOS:  zip file includes .exe, you'll also need DJGPP GO32.EXE
      DOS extender which should be available from same FTP site.    

Dec. 15th 1995  -- John P. Beale  <beale@jumpjibe.stanford.edu>