How can I find the lightest and darkest frame of a video?

[.Ava.]

I've been trying to create a script with ChatGPT and it's very buggy. What I've tried to do is render a video in black and white, at 1/4 resolution, cropped so that it's just the center, and only rendering 1 frame every 300 frames, in order for me to get a rapid render so I can roughly know which part of the video is the darkest.

Not only have I not been able to get that script to work the way I want it to, but I have no clue how to get it to scan the video for the lightest and darkest frame after render.

Any ideas?

[Dan Ebberts]

Scripting doesn't have the tools to evaluate luminance like that. Your script would have to apply a sampleImage() expression and harvest the result. And if you're going to do that, there's no need to render anything out--you could just put your comp in another comp and apply sampleImage() to the comp layer and harvest the result at whatever interval you want.

[.Ava.]

I'm new to After Effects. How would I go about doing that?

[Dan Ebberts · Accepted solution]

Something like this, basically:

function getLightestAndDarkest(){
	var fO = 300; // frame offset
	var sR = 25; // sample radius
	var comp = app.project.activeItem;
	if (comp && comp instanceof CompItem){
		var layer = comp.layer(1);
		var pt = [layer.width/2,layer.height/2];
		var lightExpr = "s = sampleImage([" + pt[0] + "," + pt[1] + "],[" + sR + "," + sR + "],false,time);\rrgbToHsl(s)[2];";
		var slider = layer.property("Effects").addProperty("ADBE Slider Control");
		slider.property("ADBE Slider Control-0001").expression = lightExpr;
		var sliderMin = 1;
		var sliderMax = 0;
		var frameMin = 0;
		var frameMax = 0;
		var f = 0;
		var t = 0;
		var curVal;
		while (t < comp.duration){
			curVal = slider.property("ADBE Slider Control-0001").valueAtTime(t,false);
			if (curVal > sliderMax){
				sliderMax = curVal;
				frameMax = f;
			}
			if (curVal < sliderMin){
				sliderMin = curVal;
				frameMin = f;
			}
			f += fO;
			t = f*comp.frameDuration;
		}
		slider.remove();
		alert ("Lightest frame = " + frameMax + "\nDarkest frame = " + frameMin);
	}else{
		alert("No comp active.");	
	}
}
getLightestAndDarkest();

[.Ava.]

Oh wow. Did you write that? If so, thanks so much!

So how would I use it? I just ran the script and it instantly printed "Lightest Frame = 0, Darkest Frame = 0".

[.Ava.]

Oh, I got it working. This is amazing, and so much better than I could have expected, seriously, thank you. I have been working on this for days!

Really really appreciate that you did this, especially when you didn't have to.

[Dan Ebberts]

It's just a demo of how I'd approach it. It could be fleshed out to make it a lot more user friendly, possibly with a UI to let you enter the various parameters, rather than having to hard code them into the script. But UIs are another story...

[.Ava.]

It's okay. This is just created for a base template for future videos. I've used ChatGPT to add some additional functions to the script. But do you know if there's any way of making it only sample from a set radius, if it could instead sample the brightness from a mask?

I want it to scan the brightness of my strangely shaped mask.

[Ines292392935tto]

Hi!

sorry, where do i paste this expression? in "sampled color output"?

thank you so much!

Ines

[Dan Ebberts]

If you're talking about the code I provided, it's a script, not an expression. The script applies an expression to a slider that it creates, but I'm not sure if that's what you're referring to.

[Ines292392935tto]

Hello Dan,

thank you so much for the super fast reply. Yes if got confused, sorry.  I guess my question is, how can i use your script. I understand i need to use a program like " Adobe ExtendScript Toolkit (ESTK) or Visual Studio Code" to paste your script and save it in order to save it on the "scripts" folder. Is that correct?

thank you!

[Dan Ebberts]

You can also just copy and paste it into a simple text editor (one that doesn't modify the text--like changing quotes to curly quotes) and save it as a .jsx file (with UTF-8 encoding). Then just navigate to it from File > Scripts > Run Script File...

[Ines292392935tto]

ok great thank you!!