dave@imax.com (Dave Martindale) (06/06/91)
In article <14070@exodus.Eng.Sun.COM> srnelson@nelsun.Eng.Sun.COM (Scott R. Nelson) writes: > >The broadcast television industry has settled on a standard gamma value of >2.222222 (1.0/0.45). This has been build into television sets for >decades. This value happens to look correct on all properly adjusted >monitors that I have seen. Well, actually, NTSC specifies a monitor gamma of 2.2 exactly, giving a camera (or lookup table, since your frame buffer mimics a camera) of 0.4545. For practical purposes, 0.45, but it is the 2.2 number that is defined, not 0.45. This was done with full knowledge that the "average" receiver has a gamma of about 2.8, because they wanted the image on screen to have a gamma higher than is "realistic" because people prefer that when looking at images in a room with dimmed lighting. The result of gamma correction for 2.2 followed by display on a monitor with a real gamma of 2.8 is an image with a gamma of 2.8/2.2 = 1.27 relative to the original scene. PAL specifies a monitor gamma of 2.8 instead of 2.2. This should give more accurate tone reproduction on sets with a real gamma of 2.8. However I have heard at least one report that European broadcasters adjust cameras for a gamma of 0.45 anyway, not 0.36, because they want the same contrast increase that the NTSC standard gives. (Is there anyone out there who works in a European television studio? Are cameras set up for a gamma of 0.45 or 0.36, or something else entirely?) The HDTV production standard specifies a more complex "opto-electronic transfer characteristic" that is basically gamma correction with a factor of 0.45 in bright areas, spliced to a linear function at low brightnesses. They did this to have a characteristic that camera gamma-correctors could adhere tightly to; the NTSC and PAL gamma-correction functions are not physically realizable for dark areas of the picture. So, the "gamma correction" function you should use is standardized, but the standard you should use depends on the television system you are encoding your picture for. On the other hand, if you just want to display your picture on a monitor, you should gamma-correct for that particular monitor. If you want to accurately reproduce tones, you should fully correct for the monitor gamma, to give an on-screen picture with a gamma of 1.0 relative to the calculated intensities. If you want to see what the image will look like on TV, though, you might want to use a lookup table that gives the on-screen picture a gamma of about 1.25 relative to the calculated intensities, since NTSC encoding will do that. And if you want to see what it will look like when recorded on film and projected in a theatre, you might want to adjust the on-screen picture for a gamma of 1.5, since that's what film does. Of course, as Scott points out, not doing any gamma correction at all will get you an extremely dark picture that bears little relationship to the intensities you calculated. It's better to gamma-correct using almost any value of gamma than to ignore the problem. Picking the *precise* value of gamma that is appropriate to your circumstances is less important. Dave Martindale