# RGB Curve Point Coordinates

Is there any way to view and/or set the exact numeric coordinates of a control point on a curve in the RGB Curves node?

For my present task (creating a custom luminance image generator) I can use math nodes after an RGBA splitter instead of the RGB curves node. But it would be great to view and set the exact coordinates for node curve points.

No but that’s not exactly the object of a curve anyway. You can cause any point to snap to the grid lines or to points halfway in between the grid lines if you hold shift while moving them.

Thanks for the info. I’m just following the advice given by Steve Write in his book Compositing for Film and Video (2nd Ed.). He gives a flowgraph for creating a custom luminance image. That flowgraph uses an RGB Curves node with numeric input & output axis lables, so that one can set percentages for the luminance mix proportions (R=0.30, G=0.59, B=0.11).

I could guess using the grid in Blender’s RGB Curves node. But the Math node solution is exact and repeatable.

So how do you re-create exactly the same curve in a new project or node? For example, what if I want to put exactly the same curve into the R & G channels. Can I copy from one to the other?

You have to make the curve a group (CTRL+G) then you can append that group to the working file from another .blend file. If you don’t make it a group then it won’t be available for appending or linking.

My bad, it looks like you can if you click the orange box on the curves node and enter the coordinates into the Min X and Min Y fields then allign your point to the intersection of those fields (it grays the node out and won’t let you place a point outside those values), then set the values again for your next point.

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Ahhh! That works!! I had overlooked the Clipping options, as I thought those were global limits that applied to all curves [for setting limits like NTSC StudioRGB color]. I’m glad to see that the clipping values can be changed without affecting existing points.

Thank you.

NOTE: Your Group>Append idea is actually the basis of my current project. I’m building a library of custom nodegroups for common operations (keying, luminance conversion, etc.) based on the flowgraphs in Steve Wright’s book. If I save all of the groups into one empty project file, then I will have a library of compositing tools from which to import for any Blender compositing task.

Given the power of Blender’s basic nodes, you can build just about anything you need for compositing. The ability to group and then append means you only have to build the tool once – great functionality and so flexible.

I’m not sure if the issue has been resolved yet but you used to have to ungroup any nodes if they had any preprogrammed values and then regroup them in order for those values to properly flow through. My major problem with groups is that they aren’t “Smart” and can’t currently properly connect themselves to the correct renderlayer channels for the effect that you want. That should change in v.2.50. Till then I personally find it easier to just rebuild the chains because some of them are really hard for me to understand if I’m not walking through them (something to do with my peripherral vision).

I noticed this. It’s a little weird. I’ve started including Value nodes for all of the numeric values. It would take too long to explain why…but suffice it to say it makes things cleaner with more flexibility.

My major problem with groups is that they aren’t “Smart” and can’t currently properly connect themselves to the correct renderlayer channels for the effect that you want. That should change in v.2.50. Till then I personally find it easier to just rebuild the chains because some of them are really hard for me to understand if I’m not walking through them (something to do with my peripherral vision).
I have not explored node groups enough to understand this bug. Worst case, I guess I can just ungroup and then modify things as discreet nodes. Hopefully this will at least save me some time inserting & connecting nodes. As long as the nodegroup is well labeled and I have my copy of Steve Wright’s book handy, it’s pretty easy to remember how each specific tool flows & operates.

If this whole project ends up being a waste of time until 2.50 comes out…well, 2.50 can’t be THAT far off.

Thanks again for all the help. Whenever I get this project to a final draft form, I’d like to post the .blend file somewhere. Has something like this already been done? Do you think anybody would find it useful?

Well, we have the Blender model repository and the Blender materials library but as far as I’m aware there’s no library for configured node based effects.

As far as 2.50 being not too far off only a dev can answer that one but I expect this is probably their most monumental challenge to date. I’m not counting on seeing it for a year or two…at least as advertized anyway…because it involves the entire program (global) rather than an individual section (local) such as the VSE or compositor.