Updated: Dec 28, 2019
This Gizmo converts your image to the HSL colorspace and uses an RolloffContrast just on the luminance channel.
You can reduce or increase the contrast of your image or areas of you image without changing the hue or saturation.
Some use cases and advices:
- You can use it for adding shadows to a plate or cg render.
- Or for generell contrast adjustments in specific areas of your cg render. For example for reducing the contrast on the diffuse color of a creature or anything else.
- This is like all the other nodes not a one-click-wonder, so it might not work well in some situations.
- You can get some weird looking color / over saturation when you cranck the contrast down to 0 or almost 0. Most of the time that can be fixed by a counter color correction.
"contrast", "center" and "soft clip" are picked from the RolloffContrast inside the gizmo.
- When the "In Log Space" checkbox is checked, all calculations are done in Log.
This checkbox is turned on by default to prevent clipping, caused by the RolloffContrast.
If you use images with a value range from 0 - 1 you can technically turn of the In Log Space“ checkbox, but most of the time it looks better when its turned on.
- You can choose between the 3 different merge methods "copy", "min" and "max".
Use min or max to not affect all values below or above the center value.
- The "output" drop down give you the possibility to look at the luminance after the conversion to log space (or without log space if the specific checbox is unchecked).
You should look at the Luminance to figure out which value you need in the „center“ knob.
- With the „Turn of contrast“ checkbox you can disable the effect of the „contrast“ slider, to see the luminance values unchanged by the gizmo when you are in the „Luminance“ view.
An example usage would be the creation of shadows on a plate.
1. To start creating your shadows you need a shadow matte which you connect to the mask input.
2. After that set the output to luminance and sample your target shadow you want to match your new shadow to.
Use this value in the "center" knob.
3. Then change the contrast knob to adjust the luminance. Changing the contrast to 0 will match the luminance completly to the center value, but could introduce some wierd saturation or hue issues. You can try to use the "soft clamp" to work against these problems. Or do a counter correction afterwards.
4. Use the merge method "min" to keep all areas that are darker then your center shadow value.
This method of creating shadows is not a replacement for normaly grading shadows in with a grade or colorcorrect node, but it could be another method for your toolbox that works really good in combination with other methods of grading.
Get it on nukepedia:
Copy script from Pastebin:
Place the 'RolloffLuminance.gizmo' in your .nuke folder or any other user-defined plugin path.
Reload your plugins inside of nuke.
Tested with Nuke 10 and newer versions, but it could also work in older ones.
Feel free to comment for any questions and suggestions.