Gamma Correction for Images and Video
Images that look good on your workstation screen will probably appear
too dark if recorded onto video tape or on a hardcopy output device
such as a dye-sublimation printer or a slide recorder. To solve this
problem you must gamma correct the images before recording them. Each
output device has a different intrinsic gamma and therefore requires a
I use the following procedure to gamma correct images:
- Set the hardware gamma correction on your workstation using
the gamma program and one of the following values. This step
changes the gamma of the monitor to roughly match the target
output device. Smaller arguments make your screen
darker. Important: these values are
arguments for "gamma," not "gammawarp"!!!
- NTSC video (via Abekas): 1.3
- Color prints on hues: 1.1
- Color transparencies on hues: 1.0
- 35mm slides at Visual Arts: 0.8
- SGI monitor factory default: 1.7
Note: hues is a Tektronix Phaser IISDX dye-sub printer.
Note: The "gamma" program must be run as root unless
you change the modes on the program to be setuid root:
chmod ug+s /usr/sbin/gamma.
Example for 35mm slides: gamma 0.8
- Use gammawarp to correct your image until it looks good
on the monitor. Gammawarp with an argument less than 1.0
lightens the image. The smaller the "gamma" argument in the
list above, the more you need to lighten your image.
Example: gammawarp input.rgb image.rgb .7; xv image.rgb
[Changed 11/2/98 TMM - lighten with args < 1, not > 1!]
- Convert the corrected image to the format for the output
device (YUV for the Abekas, Postscript for hues, TIFF or PICT
for Visual Arts), and record it or print it.
- When you are done, set your monitor gamma back to the original
value. The factory setting is 1.7.
Example: gamma 1.7
This procedure won't work for recording live demos with the Lyon-Lamb
(since you can't run gammawarp on a video stream!).
For this situation, just set your monitor gamma to 2.2. You
may also want to change the RGB output pedestal (the video signal
level for zero) on the Lyon Lamb. To do this using the menus on the
front panel of the Lyon-Lamb: choose the "Output" menu, hit the "Next"
button to go to the next page of options, and choose "RGB Ped". The
default is 0 IRE. Try choosing 7.5 IRE instead.
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Last modified: Mon Nov 2 15:34:55 PST 1998