Linear Fit Question

[javajunkie2121]

Hi:

I've seen some tutorials suggest using linear fit on the (stretched) L channel of the Master RGB to match it to the Master Luminance before LRGB combination.

I've also seen a tutorial suggest that linear fit should just be used to match the linear (nonstretched) Master-Lum and Master-R and Master-G and Master-B before linear RGB combination, but not to repeat it at the LRBCombination step as you want the brightness to reflect primarily the Master-Lum

I'm wondering what most folks do?

jeff

[rdryfoos]

To me it does not make sense to linear fit the lum to each of the RGB because the lum contains RGB combined--so linear fitting the lum to 1/3 of the signal seems incorrect.  I have always done it the first way--linear fit the extracted lum of the non linear  RGB to the non linear master lum--reinsert the extracted lum using channel combination in Clab mode, then insert the Master lum using LRGB combination.  Seems to work.  I think it is important to have the histograms of the master lum and the RGB as close as possible when you stretch.  The sTF makes this easy, but I do it manually now.
Rodd

[javajunkie2121]

 thanks for the reply Rodd. I've had success, i think, with matching luminance and extracted luminance from RGB with linear fit as well.

But I'm just wondering how people feel about the argument that you are diminishing the final lightness contribution from the master luminance by using linear fit to equalize lum and RGB in the final LRGB when actually you want to highlight/rely on the master luminance lightness more heavily in the final LRGB?
jeff

[LarryC]

No expert here, but when I LRGBCombine a Lum to a NB HOS "RBG" I find I get a better, to my eye, result when I do not LinearFit, but rather adjust lightness and saturation through the LRGBCombine panel.  In fact, I just tested this last night on an image I'm working on and did not like the result when I linear fitted the RBG to the Lum.  I'm using a separately processed Ha image for my Lum process. 

[troypiggo]

My basic workflow is as follows, and it's repeatable to the point that I have process icons set up that I can just apply in sequence for the most part.

Calibrate/integrate subs to get master L, R, G, B
LRGBCombination just the RGB channels
BackgroundNeut and ColorCalibration the RGB image
Stretch the L and RGB images (masked stretch or use the STF function)
Extract the L channel from the RGB image in the Lab color space
Linearfit the extracted L from Lab to the stretched L image
Channel combine the linearfit L back into the RGB image
Then LRGBCombine the L into the RGB image.