ICOP(1)

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.