ColorYUV: difference between gain_y and gamma_y?

[stevevid]

Last week I experimented with ColorYUV to adjust levels for a VHS capture. In my simple script I enabled Histogram("levels") and used AvsPmod with video preview on so I could see the effect of different settings in ColorYUV.

I noticed that adjusting gain_y and gamma_y had the same effect on the picture and histogram. To confuse things, the AviSynth wiki said that the gain_y control is the same as the contrast control in the Tweak filter (then what does the ColorYUV contrast do?). The gamma_y didn't change how the image shadows appear. It didn't seem to do what the AviSynth wiki says it does. Am I missing something about the actual effect of these two controls?

Thanks,
Steve

(General forum question: Should I put several questions in each thread to reduce the number of threads or do you prefer I limit the number of questions per thread as I have done here?)

[lordsmurf]

Quote:

Originally Posted by stevevid

(General forum question: Should I put several questions in each thread to reduce the number of threads or do you prefer I limit the number of questions per thread as I have done here?)

No. Keep separate topics separate.

When the topic is the same, then group the items. For example, running two Avisynth scripts on a single video is one topic, not two (regardless of 2 filters being used).

When the topic is preexisting and the same, and will add to, or questions the content of, that thread, then bump it. Tick the box to allow bumping. If it's the same general item, but you have different questions on it, then you make a new thread. I merged your new thread with the old one yesterday, for that very reason.

I'd prefer more posts, not less.
Multi-topic threads can get bogged down, sidetracked, threadjacked, etc.

And if you refer to other threads, link it.

[sanlyn]

Quote:

Originally Posted by stevevid

I noticed that adjusting gain_y and gamma_y had the same effect on the picture and histogram.

No it didn't. Gain stretches luma values from the darkest values outward, or contrast them in the oppsite direction. Gamma works on the lower midtones through the lower brights, having its greatest effect on the midtones, eirther positive or negative.

Quote:

Originally Posted by stevevid

To confuse things, the AviSynth wiki said that the gain_y control is the same as the contrast control in the Tweak filter (then what does the ColorYUV contrast do?).

They are not the same, and the Avisynth doc doesn't say so -- if it does, it's not correct. Tweak's Contrast pushes or shrinks brights friom the midtones outward or inward, and has some effect on blacks but works mainly on brights, then on upper mids. ColorYUV's luma contrast expands values from the middle in both directions, or contracts toward the middle from both ends. I.e., ColorYUV's contrast control in effect adjusts saturation when applied to chroma.

Quote:

Originally Posted by stevevid

The gamma_y didn't change how the image shadows appear. It didn't seem to do what the AviSynth wiki says it does. Am I missing something about the actual effect of these two controls?

Gamma is a midrange adjustment. It never adjusts deep shadows. It isn't supposed to. if Avisynth says gamma adjusts darkest shadows, then you're either misreading or it's plain wrong. Gamma doesn't go below values in the upper shadow areas around RGB 40 or so. If you don't know what values lie in RGB 40 or above, start learning to use pixel value readers. I believe AVSPmod has one in its status bar. It's probably the only thing that actually works in AVSpmod without causing serious psychological complications, as far as I can tell.

If you're trying to adjust specific color or luminance ranges without affecting others, you're driving yourself bananas trying to do it in YUV and Avisynth. To target and isolate more specific elements or ranges, use the greater variety of RGB controls in VirtualDub or in Premiere Pro, After effects, or DaVinci, or the specilized YUV control arsenals in the last 3 apps mentioned. Avisynth's YUV and RGB controls are purposely rudimentary, even if they are essential.

BTW, one of the last things a colorist or graphics pro will tell you to use is a brightness or contrast control. The experts use three-way levels and curve controls.
How to Use the RGB Curves in Adobe Premiere Pro
Premiere Pro tutorial: The Levels effect
Premiere Pro CS6 Techniques: 63 Color 16 Levels Effect