Is that the Contrast? No, its the Gamma.

I have been really trying things to separate the difference between contrast and gamma.

CONTRAST:
So far i have noticed that contrast is effecting that range of difference between dark and light (b&w). What i see in contrast is that the lighter colors become brighter and the darker colors become darker.
Contrast doesn't seem to change the mids much other than the fact that it seems to make the range in which mids live, smaller.

GAMMA:
Gamma seems to effect the dept of the blacks the most. It really doesn't appear to make the whites any whiter. It also enhances the richest of the chroma which i would attribute to stronger blacks.
When converting QTSC files to h264 there is a gamma loss i would guess. In this converted file i notice that in the darkest of areas there is a slight haze over the blacks and darker colors. This haze is very similar to what you would see if you cleaned glass with Ajax or Comet.
In FCP gamma seems to do the same but not as much of the haze. I do see some haze but it isn't as apparent as in the h264 file. Its more that noise is gently more pronounced.

Either of these could be confused with the other. In color correction (COLOR), when adjusting whts and blks, it seem to give the illusion that the gamma is being adjusted also. I notice that colors become richer. But i know that adjusting B&W is contrast and not gamma ( absolute or perfect contrast is dependent on B&W).


So, How do you tell the difference between you need a Contrast vs Gamma adjustment?

""" What you do with what you have, is more important than what you could do, with what you don't have."

> > > Knowledge + Action = Wisdom - J. Corbett 1992 """"

Yeah, this is a very tricky subject for people who don't have a strong background in the mathematical representation of color. Which sucks, because you shouldn't have to be an electrical engineer (or whatever) to tweak your look. But without the mathematical understand of exactly what each correction does to the numerical values of your image, you're stuck doing just what you're doing: tweaking and making subjective judgments about what did or didn't change, and never really getting what each knob really does.

I wish I could take screenshots of my waveform monitor, because that's by far the easiest way to illustrate what each of these corrections actually does. Go into After Effects and create a simple linear black-to-white ramp, from the left edge to the right edge of the screen. The waveform monitor should show something close to a straight line, from bottom left to top right. (It may not be precisely a straight line, depending on your I/O board and signal path and stuff, but it'll be close.)

What this scope actually shows you is the luminance value of each "column" of samples on the screen. Left is left and right is right, and we're gonna ignore up and down because it makes the scope easier to read.

If you apply a gamma correction ? something drastic, like 2 or 3 ? you'll notice that the luminance curve bends. A gamma correction greater than 1.0 pulls the curve up, and a correction less than 1.0 pushes it down. That's because gamma is a power-law correction; the luminance value of each sample is raised to the power gamma, where gamma is any non-negative real number. Gamma leaves the white and black points alone, and bends the luminance curve.

Contrast does something entirely different. It alters the slope of the luminance curve, leaving the midpoint alone. Dark values are made darker, and light values are made lighter, and if you're working in integer space this will result in clipping at the highs and lows. This is hardly ever what you actually want; you don't want to take values that are different ? the difference between pure white and almost pure white in your highlights, for instance ? and smash them all to the same value, because that causes detail to be lost and creates areas of solid flat black or white in your image. That's why nobody ever uses a contrast adjustment.

(Brightness, on the other hand, moves the whole luminance curve up or down. It's an additive correction; some value is added to each luminance sample. Negative values cause the whole image to get darker, positive values cause the whole image to get lighter, but the slope or shape of the luminance curve doesn't change. This also results in white or black clipping, which again is why nobody ever uses it.)

Color ? the application, I mean ? is a very traditional color corrector, and as such it's based around lift, gain and gamma tools. In that environment, gamma is exactly the same as gamma in any other environment; it applies a power curve transformation to the luminance of your image. Lift and gain, on the other hand, work differently from either brightness or contrast. Lift leaves the white point where it is and moves the black point up and down; this has the effect of changing the slope of the luminance curve just like contrast, but the point of reference is the white point, not the mid point. A contrast adjustment leaves your mids alone and stretches out both highlights and shadows, while a lift adjustment leaves your white point alone and stretches out the whole curve. Gain is the same thing, only the other way around: it leaves your black point alone and shifts the white point up or down.

Lift, gain and gamma affect each other. If you make a gamma adjustment, it moves the midpoint of your curve. If you subsequently make a lift adjustment, that also moves the midpoint of your curve. So in some situations, it can look like pushing the gamma one way and pulling the lift the other way cancel each other out. They don't, really, they just manipulate the luminance curve in different ways.

Knowing what each adjustment actually does to your image is a really important step toward knowing which adjustment to make in order to get the result you want. Of course, if you've ever watched a colorist in action, you know that it's far more art than science. Knowing what each tool does helps you understand the results you get from each move you make, but you're still going to want to roll the balls by hand (or drag the mouse or whatever) and make your corrections by gut and feel rather than by math.

Quote
jeff
Contrast does something entirely different. It alters the slope of the luminance curve, leaving the midpoint alone. Dark values are made darker, and light values are made lighter, and if you're working in integer space this will result in clipping at the highs and lows.

I often see this clipping i believe. It shows up when i import clips to Color. Before i uncheck the broadcast safe i notice a lot of clipping or should i say cramming, of the trace happens at 100 and 0 in the waveform & RGB monitors.
When Broadcast Safe is off, the trace is allowed to show all of the pixels above and below, 100 and 0.

I also see it when using the luma curve to pull -blks & +whts.


Thanks that clears it up for me. I will keep experimenting with it.

""" What you do with what you have, is more important than what you could do, with what you don't have."

> > > Knowledge + Action = Wisdom - J. Corbett 1992 """"

Nice description, Jeff. I was able to visualize what you were talking about without seeing your scopes, but it occurs to me that this discussion would make a great tutorial with the relevant illustrations. Got time?

Scott

Oh, I could probably scare up the time; it's the end of our fiscal year, and I have some vacation days to use up before the end of the month. But it'd be redundant. I'm absolutely positive I saw just what you're talking about on the Internet someplace a while back. It was a detailed description of what lift, gain and gamma really are, mathematically. Pictures and everything. I just couldn't find it in a quick Googling, so I wrote that post instead.