Combining Lum with RGB

[llpastro]

What is preparation is suggested to an RGB and a luminance image before adding the luminance to the RGB image so that the lumdata does not overwhelm the RGB?  Equal stretching of the images using HistogramTransformation?

Larry

[Rob Friefeld]

The bare bones approach is to:
1. stretch each by the automatically calculated Screen Transfer Function solution (apply STF, drag instance to bottom of HT window, reset STF, apply HT)
2. apply HDMRT to both,
3. do an LRGB combine, usually with a saturation boost in the 0.2-0.3 range.

This is from an old V. Perris video tutorial.

[llpastro]

Rob, thanks for the feedback.

Larry

[lucchett]

Cie Lab combine gives you more control vs lrgb tool.
You can later increase the saturation or reduce noise with specific techniques and settings.

[llpastro]

Would you mind giving a few more steps in how to do that?  What do I use for a Cie Lab combine?

Larry

[Dimitris Platis]

Would you mind giving a few more steps in how to do that?  What do I use for a Cie Lab combine?

Larry

1. Use Channel Extraction (Lab mode) on RGB image
2. Use Linear fit to equalize luminance between Lum and L (from RGB)
3. Put RGB back together with Channel combination (Lab mode)
4. Use LRGBCombination

[lucchett]

The approach I was suggesting avoids the lrgb tool.
Once you have adapted your stretched L and RGB, open the channel combination tool, select cielab, select yor L as luminance, disable the other two flags.
Then drag the instance to your RGB.

If you decide for linear fit to adapt the image there is no need to actually decompose the RGB, just extract the luminance to get the reference.

Using Hdrwt on the L is normally a good thing, On color data I don't know ( never tried)
Andrea

[llpastro]

Thanks to both of you for your input.

Larry

[jkmorse]

Larry,

Just for fun, you might also try stretching all four images without combining the RGB, then run them all through the SHO-AIP script.  Even though it is designed for narrowband images it works equally well with RGBs and it does a great job with color balance.

Something else to consider.  I was always shooting LRGB but, in part due to the saturation issue you mention and this great discussion thread, I switched to shooting only RGBs (binned 1x1) and build a synlum from the RGB stacks:

 http://pixinsight.com/forum/index.php?topic=6042.msg41076#msg41076

I have been thrilled with the result and likely will never shoot a lum image again.

Best,

Jim

[llpastro]

Thanks for the SHO-AIP suggestion.  I will have to give it a try. 

There have been many times when all I shot was RGB but I have wavered back and forth.  I will give the link you provided some real thought and perhaps finally standardize on RGB only.  What got me started on this subject was a basic workflow I am working on to share with others whom I am trying to convince to try PI and I wanted to include in it what to do with lum if they had it.

Larry