Make properties of video footage affect other layers

[Fredrik Åkerlund]

Interesting. I have no idea if this works but here’s a spontaneous idea:

Bring up an exposure plugin and open up an expression
ConvertRBGtoHSL in the flickering image, so you get the light value.
sampleImage() a part of the layer with flicker yiu see clearly.

now you should have a value that says something about the lightness. Convert this value to something in the range of whats workable with the exposure.
Return the new value and you might have a flicker that corresponds with your green screen footage..

Like I said, I have no idea if this works but it would be interesting if it did. You should try. I know I will, if I have the time. 🙂

[Fredrik Åkerlund]

So now I am home from the supermarket so I tried this. I made a test with a scrolling black and white gradiant ramp and a photo and it seems to work. Try this:

On the layer you are using to create the fake flicker, bring up the Exposure plugin.
Alt click on the stopwatch so you can write an expression.
Paste this expression:

sample = thisComp.layer("yourGreenScreenLayer").sampleImage([10,10]);
hsl = rgbToHsl(sample);
value = hsl[2] *10;

change “yourGreenScreenLayer” to your layer.
change the sampleImage point to some pixel that works in your footage.
adujst “hsl[2] * 10” to something that works for you.

What this does is that it samples one pixel at the coordinates 10, 10. “sample” gets an rgbArray and I don´t want that. So I convert it to HSL (hue, saturation and lightness). Lastly, I return the lightness value (that is, hsl[2]) and the exposure plugin reacts to the point I sampled. hsl[2] is apparently a value between 0-1 so I multiplied it with 10 just so I could see the effect clearly.

Let me know if it works for you. This is of course a proof of concept, so you may want to tweak it. Probably a lot. But it´s hopefully a start. 🙂

[Wilhelm Hagberg]

Thanks a lot! Great idea, I will try it out when I’m back at work Monday. I see many other possibilities with this approach – why not let one layers darkness properties affect particles on another layer. Or make a sound layer affect color of another layer and so on. Unfortunately I’m not that great with scripting! 🙂

Best-
Wilhelm