trevlac
17th January 2004, 20:48
I have a farily good understand of gamma, but there is 1 thing I don't get. How do you change the intensity/brightness/contrast/etc. of a gamma corrected source? Just by eye? How do people use a number like 128? A gamma corrected 128 is not 50% as bright as 255. It's more like 22% of 255. 186 is about 50% of 255. (This is all using a 2.2 gamma).
Here's my take. Maybe I'm confused.
Gamma- The exponential effect when a crt converts a signal voltage to an intensity. Due to this effect, mid range numbers are lower than they would be if there was no effect. The standard gamma numbers for all CRTs (TV & PC) are 2.2, 2.8, 2.5 The NTSC standard assumes the 1st. The PAL the middle (maybe), and for computers, the last.
Gamma Correction - By applying the inverse gamma to the voltages the camera 'captures', the effect is pre-adjusted so TVs don't have to worry. 0.45 is a standard gamma correction. BTW: the camera is assumed to capture intensity in a linear fashion. DV cams probably do not do this as well as film.
Stuff you change on a PC - If this is done by eye, you are gamma correcting because you see the results thru your monitor which applies gamma. The confusing part is that some software may further gamma correct or un correct, so the values you use/save may not be fully gamma corrected.
Filtering gamma corrected source Most video source would be pre gamma corrected. There seem to be ways to get linear rgb source (un-gamma corrected), but it seems you would have to go out of your way. But since gamma corrected source is non-linear, how can you just increase say the brightness by 25% without 1st making it linear 1st and then reversing the process after wards.
Maybe the 25% number is 'gamma corrected' .25^1/2.2 = .536, and then used. That way the complexity is hidden.
Can anyone set me straight ?
Here is a possible mis understanding. 75% color bars are 75% of the gamma corrected number. AKA they are 75% of the signal amplitude. Any composite signal above 75% saturation, may be clipped. It happens that 75% of the gamma corrected signal is the same as 75% of a linear intensity measure.
Here's my take. Maybe I'm confused.
Gamma- The exponential effect when a crt converts a signal voltage to an intensity. Due to this effect, mid range numbers are lower than they would be if there was no effect. The standard gamma numbers for all CRTs (TV & PC) are 2.2, 2.8, 2.5 The NTSC standard assumes the 1st. The PAL the middle (maybe), and for computers, the last.
Gamma Correction - By applying the inverse gamma to the voltages the camera 'captures', the effect is pre-adjusted so TVs don't have to worry. 0.45 is a standard gamma correction. BTW: the camera is assumed to capture intensity in a linear fashion. DV cams probably do not do this as well as film.
Stuff you change on a PC - If this is done by eye, you are gamma correcting because you see the results thru your monitor which applies gamma. The confusing part is that some software may further gamma correct or un correct, so the values you use/save may not be fully gamma corrected.
Filtering gamma corrected source Most video source would be pre gamma corrected. There seem to be ways to get linear rgb source (un-gamma corrected), but it seems you would have to go out of your way. But since gamma corrected source is non-linear, how can you just increase say the brightness by 25% without 1st making it linear 1st and then reversing the process after wards.
Maybe the 25% number is 'gamma corrected' .25^1/2.2 = .536, and then used. That way the complexity is hidden.
Can anyone set me straight ?
Here is a possible mis understanding. 75% color bars are 75% of the gamma corrected number. AKA they are 75% of the signal amplitude. Any composite signal above 75% saturation, may be clipped. It happens that 75% of the gamma corrected signal is the same as 75% of a linear intensity measure.