Re: Random layer position constrained by another layer's alpha

The way I would approach this is by looping through random points on the layer, sampling its alpha and stopping the loop when the sample point is over a threshold of alpha value.

I should note that I haven't error checked this code at all, I don't have AE on this computer so I'm writing this from memory (and the online documentation) =)


l = thisComp.layer("Island.psd"); //The 'island' layer
w = l.width; //The island layer's width
h = l.height; //The island layer's height
thresh = 0.9; //Threshold for alpha value on the island layer

seedRandom(index, true); //Seed our random numbers and don't update them based on time (timeless)

p = random([0,0], [w,h]); //'p' is a random point in the layer space of the island layer
alpha = l.sampleImage(p)[4]; //The alpha value at the point 'p' on the island layer (values range from 0-1)

//While the alpha value is below our threshold...
while(alpha < thresh){
p = random([0,0], [w,h]); //Pick a new random point on the island layer
alpha = l.sampleImage(p)[4]; //Update the alpha value at the point
}

l.toWorld(p); //Transforms the point 'p' from layer space to world space



This will return a point in 3D space that should correspond to a random point on the island layer that has an alpha value greater than or equal to 0.9 (0 is fully transparent, 1 is fully opaque), so make sure you have your anchor point on your tree layer set to the base of the trunk (where it will meet the island) and apply this expression to each tree's position property.

Let me know if there are problems with this code, I can check it tomorrow for errors.

Darby Edelen
Designer
Left Coast Digital
Santa Cruz, CA