LRGB combination

[astropixel]

Let me know if you try this method.

As follows - Combined individual Grey R, G, B using ChannelCombination and uniform RGBWorkingSpace. This was followed by STF and BackgroundNeutralisation and ColorCalibration. SCNR Green Average Neutral was applied before recombining the L* component with the RGB, to prevent green rings forming around the brighter stars. A good result.

It didn't seem to matter that on the first try I forgot to redefine the default RGBWorkingSpace for the RGB image.

Plus a bit more HWT, LRGBCombination and CurvesTransformation. This is a much better result.

http://pixinsight.com/forum/index.php?action=dlattach;topic=2497.0;attach=1263

[mmirot]

Fabio,

Did you do it on linear images?

Max

[astropixel]

Combining the RGB images was linear. My understanding is that after that it is necessary to stretch the L image and the combined RGB to an approximately acceptable amount and extract L*, linear fit with L as the reference and channel combine with the RGB - as per...

- Apply the initial nonlinear histogram transformations to RGB and L, as we have described above. Adjust the L image first, to the desired brightness and contrast. Then try to match the overall illumination of L when you transform RGB. Do it roughly by eye using the CIE L* display mode (Shift+Ctrl+L, Shift+Cmd+L on the Mac). Don't try to do a particularly accurate work here; we'll do much better in the next steps.

[jmtanous]

Nice thread!

@astropixel: I can't access the url you pasted! (http://pixinsight.com/forum/index.php?action=dlattach;topic=2497.0;attach=1263)

Cheers,

Jose

[astropixel]

Try this one - http://pixinsight.com/forum/index.php?topic=2497.msg16833#msg16833

Incidentally, I have scheduled a run for shorter luminance exposures to manage the core brightness more effectively.

[astroedo]

On a side note, combining linear L and RGB is possible in PixInsight. However you must perform a YRGB combination instead of LRGB.

In this case, instead of the CIE L*a*b* space, the CIE XYZ space must be used. CIE XYZ is a linear color space; however, the underlying RGB working space must also be linear in order to preserve linearity in the combined image. The basic procedure would be as follows:

- Define a linear RGB working space. With the RGBWorkingSpace tool, set gamma = 1.0 (disable sRGB gamma function). In addition, I would define a uniform RGB working space in the same operation. So the steps with RGBWorkingSpace are:

* Set the three luminance coefficients = 1.
* Define a uniform energy illuminant by setting the six chromaticity coordinates = 0.333333.
* Disable the sRGB gamma function and set gamma = 1.
* Apply to both the RGB and L images.

- Use the ChannelCombination tool to perform a YRGB combination. Select the CIE XYZ space, disable the X and Z components, and select the L image for the Y slot. Apply to the RGB image and you're done.

- To process the linear luminance of the combined YRGB image, you must select "Linear luminance" in the applied tools, when applicable (for example, Deconvolution and ATrousWaveletTransform allow you to select it).

- After the initial nonlinear stretch with HistogramTransformation, you may want to apply the default RGBWorkingSpace parameters, as a linear uniform space no longer will describe the image appropriately.

This should work

Hi Juan,
I've tried this method just for curiosity, but when I try to use RGBWorkingSpace with all six chromaticity set to 0.333333 I get the error:

Instance Validation Failed
Reason: Singular Matrix in RGB working color space inizialization

any Idea?

[Geoff]

Hi Juan,
I've tried this method just for curiosity, but when I try to use RGBWorkingSpace with all six chromaticity set to 0.333333 I get the error:

Instance Validation Failed
Reason: Singular Matrix in RGB working color space inizialization

any Idea?

I also tried it a while ago with the same result.
Geoff

[jerryyyyy]

Very interesting thread.  In my workflow I go from crop and DBE of the stacked images to linear fit of all RGB and L images before I go to the LRGB Procedures to make the RGB image.  I then stretch with the script for masked stretch to get to 0.25 median.  The histograms of the RGB and L images (of Ha) seem to overlap.  This seems reasonable, right?  If I do not do the linear fit, colors are definitely screwed up. 

Is there any reason to use the AIP script for the combination of L or Ha with the RGB?  I seem to like if better than just using the LRGB Procedure.  I believe they said they do the process in several stages, masking.