Author Topic: Linear "L" RGB Combination  (Read 634 times)

Offline astroedo

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Linear "L" RGB Combination
« on: 2017 March 05 10:27:54 »
Hi All,

it is a bit of time that I'm trying to find a way to make an "L" RGB combination at linear stage.
We all know that the real LRGB combination must be done on NON linear images, matching the luminosity profile of the L image with the RGB one and combining in with the LRGBCombination Process (or Channel Combination); this because the CIE Lab color space is nonlinear, so we cannot use a linear L.

Googling on the internet I haven't found any solution, except an old post here on the forum

https://pixinsight.com/forum/index.php?topic=2485.0

where Juan suggests a method to combine a linear L with RGB.
Unfortunately I haven't managed to make it work: on my images this method creates horrible Green and magenta artifacts (don't know if I'm doing something wrong).

So I've tried to find a solution by myself, and, maybe, I've found it.

This is my workflow:

I have 4 images: R G B and "L"

  • RGB combination with ChannelCombination
  • BackgroundNeutralization
  • ColorCalibration
  • Extract I Channel from HSI color space with ChannelExtraction
  • Use LinerFit on "L" using  I  as reference
  • Insert fitted "L" in RGB image with ChannelCombination (Use HSI color space deselect H and S checkboxes and use "L" as I )
Thats all folks!  :D

Processing the image the final result is practically Indistinguishable from a normal LRGB.
The I component of the HSI color space is a linear combination of RGB so it is linear and can be combined at linear stage.

We can call it IRGB Combination instead LRGB.  ;)

What do you think about this workflow?
Am I doing somthing wrong?

Bye

Edoardo