Microsoft KB Archive/157152

= Microsoft Knowledge Base =

How Gamma Affects Your Picture
Last reviewed: November 27, 1996

Article ID: Q157152

The information in this article applies to:


 * Softimage 3D for IRIX versions 3.0, 3.5, 3.51
 * Softimage 3D for Windows NT versions 3.0, 3.01, 3.51

SUMMARY
Gamma is a curve that affects several parameters such as brightness, contrast, and color. The same color image may differ from monitor to monitor depending on the physical characteristics of the monitor. R, G, B intensity signals will, in general, produce slightly different colors on different monitors (electron beam and output photons). Unfortunately the relationship between the RGB values input to a monitor and the intensity produced on the screen is non-linear.

Hardware lookup tables on SGI compensate for the non-linear response of the monitor. This process is called the gamma correction. Adjusting the 'gamma correction' corrects the color difference. As a result, you see the real image that was computed by the software more accurately. However, when this image is moved to different mediums, or other monitors, you need to use a filter 'gamma correction' for that specific medium.

Modifying Gamma Defaults
When you modify gamma, there is a danger that it may affect your picture quality. Color shifts may occur, and the image contrast may be influenced. If you modify the gamma value when you use a film recorder, problems may also occur. When you use video the effect is less critical since you can change more with less apparent disturbance to the resulting image. You can have amazing results with film, but the process is time consuming and labor intensive. It is generally better if you preview the scene you want to modify on a balanced video monitor. This way you can avoid additional modifications later.

It is often necessary to change the gamma value of an image when you transfer from digital to analog media like television. An NTSC signal, for example, does not support high values in red or bright white. When you transfer full red (rgb = 255,0,0) to video, the image does not display properly since the signal is too high. In this case, it is necessary to modify the gamma value. Ideally, you should test any color scheme or scene before rendering the entire sequence.

Tools For Modifying Gamma
 If you are running IRIX, use gamma to change the SGI system gamma, gammawarp to change the gamma of an image, and gamcal to calibrate your screen. If you do change the SGI system gamma, the gamma display for your entire system, not just for one image, changes.  If you are running Softimage 3D version 3.51 for Windows NT, you can change the gamma factor in the standalone image converter, 'imgconv', or at render time you can modify the default gamma value (only available in the mental ray Options dialog box).  If you are running Softimage 3D version 3.01 for Windows NT, brightness and gamma can not be changed. This poses a problem if you import your image into another software which uses a different gamma level. In this case, it is necessary to adjust the brightness levels to match the required gamma level. In Eddie you can change the gamma of an image with the colmap tool. To change the gamma of an image with Adobe Photoshop use the Image>Adjust Curves tool. Output devices also have options to change the gamma (for example, printer, film, Solitaire, video).