Trigger an action by luminance

[Filip Vandueren]

By automated, do you mean that running auto-trace on the image and then using the resulting mask is not an option ?

It would need to automatically fit on a shape if something new is dropped in the comp ?

[Kashif Nawaz]

I may have overcomplicated things here. Auto-trace is not an option.

I want the center of the red circle to be placed at the edge of the black shape. I believe this can be achieved by ‘sampleImage’ to determine the luminosity of the black layer and applying to the position of red circle. Hope this makes sense.

[Kashif Nawaz]

The red circle is not supposed to be moving along the edges or along any path. It will simply sit there or on any other side (right/left/top/bottom etc.).

[Filip Vandueren]

And where along the shape should it be ? Random ? At a certain angle ? At a certain X-position ?

[Kashif Nawaz]

It will be at a random point. No particular angle.

[Filip Vandueren]

Hi Kashif,

here’s a slight adaptation of an expression I posted a few days ago which will do what you need:

// posterizeTime(0); // uncomment for still images
includeEffects = false;
l=thisComp.layer("Blob"); // your layer
w=l.width;
h=l.height;
threshold = 0;
width_bits = Math.ceil(Math.log(w)/Math.log(2));
height_bits = Math.ceil(Math.log(h)/Math.log(2));
left = 0; right = 0;
seedRandom(index,true);
for (i=0; i<1000; i++) {
	// do a limited loop, because Blob might be empty and we would be stuck in an infinte loop.
	randomPos = random([0,0],[w,h]);
	if (l.sampleImage(randomPos, [1,1], includeEffects)[3]>0) break;
}
for (b=width_bits-1; b>=0; b--) {
    slice_width = 2**b;
    if (l.sampleImage([left + slice_width/2, randomPos[1]], [slice_width/2, 1],includeEffects)[3]<=threshold) {
        left+=slice_width;
    }
    if (l.sampleImage([w - (right + slice_width/2), randomPos[1]], [slice_width/2, 1],includeEffects)[3]<=threshold) {
        right+=slice_width;
    }
}
right= w-right;
top = 0; bottom = 0;
for (b=height_bits-1; b>=0; b--) {
    slice_height = 2**b;
    if (l.sampleImage([randomPos[0], top + slice_height/2], [1, slice_height/2],includeEffects)[3]<=threshold) {
        top+=slice_height;
    }
    if (l.sampleImage([randomPos[0], h - (bottom + slice_height/2)], [1, slice_height/2],includeEffects)[3]<=threshold) {
        bottom+=slice_height;
    }
}
bottom = h-bottom;
// choose a random direction
switch (Math.floor(random(4))) {
	case 0:
		[randomPos[0], top];
		break;
	case 1:
		[randomPos[0], bottom];
		break;
	case 2:
		[left, randomPos[1]];
		break;
	default:
		[right, randomPos[1]];
}