After Effects Expression baking

posted on Jan 07 by Floris in the Blog, Resources & Tips category

I’m working on a project where I have a video file in After Effects, and I need the lightness value for a certain set of pixels in that video file, on every frame. The expression took some time to write, but wasn’t very complicated and quite easy to figure out. After I had added a few of those expression tracks though, it all became very slow. At first I thought that was because each expression was animating an instance of Particular, meaning thousands of particles had to be rendered on each frame.

For quite some time, I didn’t really consider the waiting times. But suddenly it hit me: this is stupid, what exactly am I waiting on? I have a pretty fast computer, and these particles should be rendering on the GPU. And what I realized, is that the expression itself was taking up all the processor cycles! On every frame, After Effects scrubs through the entire video, finding the right pixel values and converting them to a value. It does that for all the expression animated tracks. And then the next frame starts, and it goes through again…

Luckily there is a solution. After effects allows you the option to ‘bake’ an expression, in the form of keyframes. Did that, and rendering is now incredibly smooth. No waiting at all!