Choose a Random Null Expression

[George Goodman]

This might help:

https://www.videocopilot.net/forum/viewtopic.php?f=5&t=59219

“|_ (°_0) _|”

Sincerely,

George

[David Bland]

Awesome thanks!

One other quick thing, is it possible to put a null in a random position in a comp but only within a certain colour area?

for instance, i have a map which is black and white and i want the nulls to be in a random position but only on the land area in black, is this possible?

[George Goodman]

You miiiight be able to do something with sample image, but honestly, that would be above my skill level. Someone else will have to chime in for a better approach.

Could you just create an additional layer and use the black as a track matte? It wouldn’t place the nulls there specifically, but it would only show anything within the black part of the image.

“|_ (°_0) _|”

Sincerely,

George

[Darby Edelen]

You’d have to keep trying random positions in a loop until one gets a hit with sampleImage(). Something like this:

l = thisComp.layer("Control Layer");
p = l.anchorPoint;
min = [0,0];
max = [l.width, l.height];
black = false;
maxIterations = 1000;
for(x = 0; (!black && x < maxIterations); x++){
seedRandom(x, true);
p = random(min, max);
if(l.sampleImage(p)[0] < 0.01) black = true;
}
l.toComp(p);

This would look at random points on the target “Control Layer” and whenever the red component is (almost) black would return that point translated into composition space. The last line as well as some of the details in the expression might need to be changed depending on your setup.

The “maxIterations” property is there to prevent an infinite loop in case there are no black values in the red channel on the entire layer. If the black areas are very small you may need to increase that value (and wait longer for the expression to evaluate).

Darby Edelen

[Darby Edelen]

Just a quick addendum to my last post. The expression there will re-evaluate the control layer on every frame, so if the black/white sections change over time (fractal noise with evolution animated for example) the Nulls will choose a new random position whenever their position is no longer over black.

Darby Edelen