"Image" color overrides alpha?

Hi, I’m learning the Compositor and I completely hit a wall.

I’m trying to add a white outline to a rendered object. I’m using the Dilate/Erode filter to expand the alpha. It works fine when the image color is black. But when I change the color to white, the whole comp is flooded white, even though I haven’t changed anything about the alpha.

Here is an example scene: https://www.dropbox.com/s/b58u5gfgangvtri/compositor%20white%20outline.blend?dl=0

So the comp only respects my alpha when I’m adding a black color. Why?

Is the Image channel overriding the alpha channel somehow? Feel like I’m misunderstanding something fundamental about the Compositor.

Thanks for any advice,


I should be able to adjust the color without messing up the alpha, right? Aren’t they separate?

Am I missing a step somewhere?

You have to use alpha on both steps, separate the border and the image, then alpha over.

Awesome, that totally did the trick. Thank you.

I don’t quite understand why, but maybe I don’t have to. I also don’t understand why the image that gets alpha over-ed goes in the lower channel. Seems like some of this stuff could have a little lower barrier to entry.

Thanks again!