L-RGB combine

[mwilliams]

Hello, what a fabulous piece of software!  I am really getting into
the functionality with the help of the tutorials.  I have a quick question
regarding combining the Luminance with the "already created" RGB
image (as opposed to combining all 4 images separately) from
Juans excellent tutorial.

Juan states that  "On the Channels / Source Images group box, the R, G and B slots must be disabled, since
 we are going to combine the luminance image with the already combined and stretched RGB image. When a channel slot is disabled, the LRGBCombination process simply obtains the corresponding channel data from the target image. This is just what we want."

When I try to do this, I just get a copy of my Lum image in RGB, however, there is no color data,
which is confirmed when opening the image with HistogramTransform.  Also, I assume the
"Target Image" is the RGB image that is currently open, as per the tutorial.
Thanks for any help!

Matt

[David Serrano]

I think I can help but I don't know what tutorial you are referring to...

[Juan Conejero]

Hi Matt,

Thank you, and welcome to PixInsight Forum.

The tutorial Matt refers to is here:

http://pixinsight.com/tutorials/STD/LRGB/en.html

This is an old document and a lot of things have changed since the time I wrote it. We now have many new tools, and have learned/developed new processing techniques that obsolete some parts of this tutorial. However the LRGBCombination tool remains the same.

I have prepared a small example that reproduces what you want to do (M45 image courtesy of Vicent Peris):

http://forum-images.pixinsight.com/legacy/LRGB-example/LRGB-1.jpg

Here you can see the luminance and RGB images, along with the LRGBCombination interface ready to work. Note the luminance image selected in the L slot, and the disabled R, G and B slots. The target image is not used (it is seldom used; we probably will drop this parameter in future versions of this tool).

http://forum-images.pixinsight.com/legacy/LRGB-example/LRGB-2.jpg

About to apply the LRGBCombination instance to the RGB image. Note that LRGBCombination can be applied to previews (as most processes in PixInsight), so you can make tests on small regions of interest before applying it to the main image. In this case, I have defined a preview covering the entire image.

To apply a process you can clic the New Instance icon at the bottom left corner of a processing interface (the blue triangular icon) and drag it to a view, as the figure above shows.

http://forum-images.pixinsight.com/legacy/LRGB-example/LRGB-3.jpg

The final result. Note that I have applied slight luminance and saturation increases (0.4 and 0.45 transfer functions, respectively) to improve the corresponding components in the combined image. Using previews it's very easy to fine tune these parameters.

Hope this helps. As stated in the tutorial, our LRGB combination tool has several advantages over similar tools in most applications. The most important one is that it works using colorimetrically-defined RGB and CIE Lab spaces, which means a virtually perfect isolation between luminance and chrominance (no noise transferences, no hue changes). Another key advantage of our implementation is that it allows you to adapt luminance and chrominance very accurately.

[Jack Harvey]

To take this a step further, can you then put Ha in the Lum slot and leave the R slot enabled but adjust the channel contribution to then add a Ha:Red (80%:20%) to the RGB frame?

[mwilliams]

Ahhh...thank you for the help!    Matt

[mmirot]

Is better to have your images streched the same or can you do linear LRGB combines?

I have trouble low saturation if I stay linear.

 I suppect this has to do with intensity mismatches between the L and color channels.

Max

[Juan Conejero]

Hi Max,

Yes, of course you can make LRGB combinations of linear data. The main problem, as you have pointed out,  is insufficient chrominance to support the luminance, or vice-versa. This problem is less severe with stretched data because you can make a previous adaptation of RGB and L data with histogram transformations, or using the integrated midtones transfer functions in LRGBCombination. Of course, these are nonlinear functions, so they make no sense with linear data.