Gamma Correction for Images and Video

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.

Phil Lacroute