CIE L*a*b* extraction and recombo question

[aworonow]

I have a pair of linear RGB images that I want that I want to swap L (lightness) channels from the first to the second. As a test, I simply extracted the CIE L*a*b* from one of the images then tried to recombine them. I did not get the original image back: Red cast (due in part to nebula) became teal, for example. Why? How do I get a proper RGB image back? (see attached screen shot)

Thanks, Alex

[msmythers]

Alex

Are you applying STF to both images. Use unlinked AutoSTF and see if you have the same image.

Be careful about swapping L channels while linear. You will probably need to do a linear fit to the L image.


Mike

[aworonow]

Tried your suggestion, with same (wrong) outcome. Basically, there is zero red in the background, and most of the image. The reason for this is that both the a* and b* channels are flat gray (both with the uniform value of 0.54286). Why would that be? The L* shows star brightnesses. There is no mask in over any image, and the input image is linear. (I retested it on another image that I composed from indivdual RGB stacks.)

I tried extracting CIE L*c*h* and recombining them, that worked perfectly. Maybe my version CIE L*a*b* extraction or combination is corrupt? Is there a way to reinstall those two modules without reinstalling all of PI?

thanks again, Alex

[aworonow]

I found that doing the extraction with the Sample Format set to less than 32-bit integer fails in the manner I described above. Is  this a bug? If not, when would the 8-bit and 16-bit options be appropriate?

Alex