Link effect parameter to luminosity

[Todd Kopriva]

It might be rather slow to process, but you could use the sampleImage expression method to sample the color data from the layers.

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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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[Anthony Diehl]

Thanks for the reply! Ya, I have another similar comp that is using this method to recolour a bunch of layers based on another layer that is sampled. It works great, but I just copy and pasted some script I found online.

How exactly would I use sampleImage to get usable data that I can then link to an effect parameter?
I guess I want to look at luminosity or alpha somehow, yes?

I’m really green when it comes to expressions like this. ANy examples of expression code that would work?

thanks!

[Darby Edelen]

The sampleImage() function returns a color as a 4-component vector in this form: [red, green, blue, alpha]. The ranges of the values are 0-1. You can convert these to Hue/Saturation/Lightness values and then sample just the lightness like this (I’m including some variables that stand in for things like the “point” and “radius” for the sampleImage function):


c = sampleImage(point, radius);
lum = rgbToHsl(c)[2];


The [2] component of the HSL version of the color is the lightness. Also, it’s important to note that the “point” variable is not a composition or world point, but one in the layer’s local space.

Once you have that value you can use it with a linear() or ease() function to remap the 0-1 range to another range, here’s an example for scale:


linear(lum, 0, 1, [50,50], [150,150]);


Which would vary scale from 50% to 150% on X and Y axes for a 2D layer as the brightness of the layer being sampled goes from 0 (black) to 1 (white).

I’m not covering all the details of implementation, so if you’re new to expressions/JavaScript you’ll likely have lots of additional learning to do in order to make this happen.

Also, if you are going to use an easily defined shape like a circle, ellipse, box or rectangle then I wouldn’t use sampleImage() but rather use code to drive the expression. It would evaluate much faster.

Darby Edelen

[Anthony Diehl]

Sweet, thanks. I played around with this method. Could;t get it working, but thats probably because I’m just super new to building out longer expressions.

I did manage to find a method that worked by piecing together a few other examples I found.


target = thisComp.layer("Expand");
alpha = target.sampleImage(transform.position, [width, height]/2, true, time)[3];
ease(alpha, 0, 1, 0, 100)

I have a pre-comped layer (“Expand”) that has a variety of masks, ramps, etc. I am sampling the alpha value on that layer and putting this expression in to control the scale value of the Transform effect on about 150 small layers which are arranged in a big grid. Its kind of slow to render but otherwise works great. I can get some cool effects and really complicated patterns going by just tweaking my “Expand” layer.

Thanks for the feedback.