revLum v1.0
Revert luma and chroma changes
Mar 29, 2020
Well hello, fellow Nuke compositor. Do you use this slider in the Grade node a lot?
The Grade node's mix luminance slider
…or wish it was easier to grade only colors but keep the luminance? Me too!
So I made this tool, which attempts to do exactly that. I've found it to be especially useful when doing complicated color changes (like despills) or using several grade nodes. The idea is, that you do all your grades and modifications as usual and then use the revLum node to revert to the original luminance.
A simple grade setup with revLum
I've included Hue and Saturation as well sliders because they usually also plays a part in the perceived luminance of an image - they can produce some funky results though so use at your own discretion ?.
revLum v1.0 node control panel
Behind the scenes it converts the input image to a color space that splits the lightness and color into channels (HSL, HSV, L*a*b* or CIE-Yxy), and adds the difference of the selected components on top of the changed input.

Usage example

Photo by Swapnil Bhagwat on Unsplash
Let's say you graded an image super green by modifying the gamma with a Grade node. Now you'd like to get back the luminance from the original image, so you crank up the mix luminance slider. It does a pretty good job - but if you look at the red pants, they are actually too dark.
Turning off the grade node's mix luminance and instead using revLum keeps the pants' luminance nicely. Taking it a step further, you could also retain the saturation, which in this case especially affects the bright white areas.
I hope this tool will be of help in your future grading endeavors.
Available on Nukepedia here.
revLum v1.0 Node Graph