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stefslon  
#1 Posted : Tuesday, January 8, 2019 12:27:22 AM(UTC)
stefslon

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Hello, I am trying to figure how exactly vMix applies Lift/Gamma/Gain/Hue/Saturation Color Correction settings to the input RGB or YCbCr data. I found something elsewhere that says the equation is output = (gain * (input + lift * (1-input)))^(1/gamma) that gets applied to each channel in RGB independetly. This does not make sense though, because default gamma value in vMix is zero. How can you have a divide by zero in the equation?

Can someone please provide some details on how exactly vMix does color correction, preferrably with an equation?

Thanks!

DWAM  
#2 Posted : Tuesday, January 8, 2019 5:55:57 AM(UTC)
DWAM

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Color correction is a matter of histogram and vector scopes, not a matter of scientific formulas.
The point here is to correct several cams inputs so that they match all together even if they are not exactly faithful to reality. That's what vMix does...

I think you're trying to use vMix module to calibrate your monitor or make sure your cams generate exact colors. That's a different thing.

Guillaume
stefslon  
#3 Posted : Tuesday, January 8, 2019 6:17:34 AM(UTC)
stefslon

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DWAM,

maybe I was not clear on my question. When I calibrate a given input by changing its Lift/Gamma/Gain values, what does vMix do to the actual RGB values for that input? In other words, how does vMix translate or apply Lift/Gamma/Gain to the input to come up with a corrected output? Hope my question makes more sense now.

Thanks.

DWAM  
#4 Posted : Friday, January 11, 2019 6:13:45 AM(UTC)
DWAM

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I'm afraid I still don't really understand what you mean...

First I'd like to say that "color calibration" is not an appropriate term for what vMix does. You should say "Color Correction" or "Color Grading" here. It's important because these are totally different concepts and better keywords if you want to look for documentation on the matter...

In color grading we deal with Lift Gamma and Gain values + Hue and Saturation. These are "levels" or curves for dark, medium and highlights. Video grading is a matter of "light" management.
Color grading is designed to work with vectorscopes and histograms which help to check whether blacks are black, whites are white and so on. But not in the same way as in Print or Graphic Design where we define RGB values (which is non sense for video as it's motion with ever changing conditions)

There are several tutorials on color grading and how to use vectorscopes and histograms on the internet and Martin from vMix also did a video for learning the basics of color correction in vMix. Check their YT channel.

I'm sorry I'm not sure I answer your question but I really don't understand why you focus on RGB values for video which is not the way to go.

Guillaume
stefslon  
#5 Posted : Friday, January 11, 2019 7:06:52 AM(UTC)
stefslon

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DWAM,

thanks for taking the time to answer my question. I guess I was confusing terms, but yes, I was referring to color correction, that can be found under input settings. I understand that color correction is done by looking at vectorscopes, histograms, RGB parade plots and balancing out various color contributions. I've seen tutorial for vMix and various other tutorials for other software packages, for example DaVinci Resolve. So I think I understand all of that. What I was going after are the details of how Lift/Gamma/Gain/Hue/Saturation settings transform input video into the output video so I can play around with these outside vMix; say in Python or in MatLab to model these transforms and possibly solve for values that will give me the desired results.

Anyway, I figured out roughly what vMix does at least for Lift/Gamma/Gain application:

output = ((input*gain)+lift)^(1/(1-gamma))

This looks very similar to the definition provided by American Society of Cinematographers Color Decision List (ASC CDL). So I have my answer.

Thanks.


thanks 1 user thanked stefslon for this useful post.
sinc747 on 1/12/2019(UTC)
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