Setting up a procedural effect

[Dotcommer]

::EDIT::
I just realized I put this into the nuke tutorials forum for some reason. Not sure why. Hugh, if you want to move this over to the actual nuke forum, thats cool with me.
::EDIT::



Hey all, how's everyone doing?

Alright, here's whats up right now:

I'm getting ready to set up an effect where there is a bright pulsing light behind a group of subjects (green screen). I want to take the luminance key of the green screen elements, and take their highlights, and have them pulse with intensity based on the pulsing light behind them. Any thoughts for setting this up?

I'm thinking maybe using something that can measure the intensity in a specific area, then, once its recorded, apply that intensity data to the luma key of the green screen subjects so their luminance pulses procedurally to the background light. But i'm not sure how I should go about getting the data.

I've heard a little about a curve tool or something like that which can measure the brightest and darkest parts of a sequence and use it to animate other properties, but is that what I'm looking for? or maybe something more direct... Not sure, and would love to get some ideas on this.

Thanks guys.
~Bry

[ygorcwb]

Hey,

I remember to have done something like this inside Shake usign PixelAnalizer to drive the Keylight RGB values. The same could be accomplished with Nuke's Keylight and the Sampler node. Sometimes can be easier to just keyframe some values in keylight, as things could turn to be kinda messy and problematic with ease when it comes to tweak your expressions. From Sampler node you will get RGB values, and not luminance, so you can consider to use a keyer node before the sampler to get the luminance values of you clip, and if needed, create some user knobs to keyframe, tweak, or limit min and max values multiplying and/or adding numbers to it.
It is difficult to build a setup without having a plate, but here are the basics on such thing.
For the question on how to do on a specific area, I would consider doing a crop where you need to sample the values, in Shake, Pixel Analizer lets you to specify a region to sample from.

I have attached a very basic example, no Reads, just showing the connection between Sampler and Keylight.

Good luck!

[Dotcommer]

Hey, thanks for your feedback.

Currently I think I've found what I'm looking for. I looked into the curves node in nuke, and found out there are multiple items it analyzes. Max and Min values, but also average intensities. And apparently it uses a bounding box, so I could narrow in where I wanted the curve node to analyze pixels. I let it run, it gathered the data, then i applied it to a simple constant node with a grade node, and put the intensity data in with the multiply slider in the grade node, and it worked! The constant "pulsed" in sync with the background light.

I did not know about this sampler node, so i'll look into it as well. Thanks again for your feedback.

If anyone else has a different technique they can think of for tackling this situation, I'd love to hear it. Thanks!

[Akiva]

You can use the CurveTool to drive an expression in your grade node. The curve tool will analyse each frame and gather a few types of data, depending on what you specify within the dropdown.

You can analyse the exposure changes the the sequence using exposure difference, or find the average pixel values in a sequence using avg intensities.

The manual has a more in depth breakdown, and i often find i need to multiply the result to get a usable number that gives me the effect im looking for.

[Dotcommer]

Right on, thats exactly what I ended up doing. Good to see I'm not the only one who thought about solving it like this.

[Akiva]

Aww, we're in harmony