NAME

     icop - The Image Cop

SYNOPSIS

     icop

WHAT IS ICOP?

     Icop is an	interactive  program  that  can	 be  used  to  make  computer-
     generated colors legal in the NTSC	(or PAL) color systems.

     Often, images generated on	the computer are made for use in movies	 which
     ultimately	 end  up  on  video  tape.   However,  the range of colors (as
     specified by their	RGB values) on a computer does not match the range  of
     colors  that  can	be represented using the NTSC (or PAL) systems.	 If an
     image with	"illegal" colors is sent directly to an	NTSC  (or  PAL)	 video
     system  for  recording,  the  "illegal" colors will be clipped.  This may
     result in an undesirable looking picture.

     This utility will test each pixel in an image to see if it	 falls	within
     the  legal	 NTSC  (or  PAL)  range.   If  not,  the pixel's luminance and
     saturation	is reduced so that it does fall	within legal limits.

How are colors corrected?

     To	understand how colors are corrected, the YS plot should	be  displayed.
     On	 this  plot, The vertical axis is Y or luminance.  The horizontal axis
     is	saturation.  Saturation	is computed as follows:

     NTSC: sat = sqrt(I**2 + Q**2) PAL:	sat = sqrt(U**2	+ V**2)

     Each pixel	in the image is	converted to Y and S and then placed  on  this
     plot.   The legal color gamut for NTSC (or	PAL) is	the triangular area on
     the left.	Any pixels that	extend to the right of this area are illegal.

     To	correct	these illegal colors, both the	luminance  and	saturation  of
     each   of	 these	illegal	 colors	 is  reduced  (the  pixels  are	 moved
     perpendicularly toward the	line representing the boundary	between	 legal
     and illegal colors) until they fall within	legal limits.  This is done by
     first adjusting both Y and	S by equal proportions then  reducing  both  I
     and  Q (or	U and V) by equal proportions.	To understand this it may help
     to	think of I and Q as legs of a right triangle  and  saturation  as  the
     hypotenuse.

File Menu

     Icop is capable of	reading	a number  of  image  formats.	These  formats
     include:

      .rgb Silicon Graphics RGB	format
      .sgi Silicon Graphics RGB	format
      .rle Utah	Raster Toolkit format
      .rla Wavefront image format
      .gif Graphics Interchange	Format
      .tiff Tag-based File Format
      .jpg Joint Photographic Experts Group
      .*       ASCII or	binary color map files

Plot Menu

     Icop is able to show the colors used in an image in a variety of systems.
     These systems include:

     YS	  Luminance vs Saturation (sat = sqrt(I**2 + Q**2))
     RGB  3D RGB cube
     YIQ  3D YIQ cube
     YUV  3D YUV cube

Image Menu

     This menu can be used to display any of the following:

     Legal pixels only
     Illegal pixels only
     Corrected pixels only
     Corrected image
     Original image

Preference Menu

     Icop can be used to test colors for either	the NTSC or PAL	color systems.

LIMITATIONS

     The following tools must  already	be  in	your  path  for	 Icop  to  run
     properly:

     fromjpeg  (available from
		    explorer.dgp.utoronto.ca:pub/sgi/sgijpeg)
     fromutah  (available from sgi.com)
     fromgif   (/usr/sbin)
     fromtiff  (available from sgi.com)
     fromrla   (/usr/sbin)

SEE ALSO

     rlelegal(1)

AUTHOR

     Wesley C. Barris (wes@msc.edu)
     Army High Performance Computing Research Center (AHPCRC)
     Minnesota Supercomputer Center, Inc.