Scanning footage for changes in the scene

[Darby Edelen]

After Effects isn’t particularly well suited for this sort of thing, but you could use the Time > Time Difference effect to get the difference between the current frame and the next (or previous) frame and then adjust those results to create something that could work for your purposes.

I’d apply this expression to the Time Difference’s Time Offset property:

source.frameDuration
So that it will find the difference between the current and next frames. I would also enable ‘Absolute Difference’ to get a more meaningful result for your purposes. After that I’d apply a Gaussian Blur to reduce the affect that noise and small changes might have on your result. I’d then apply a Levels effect and increase the Input Black to remove those small differences and decrease the Input White to boost the more apparent differences.

There are a couple of other things I’d probably do but let’s see if the above is at all helpful before we dive any farther in 🙂

Darby Edelen

[Dave Rogers]

Hi Darby,

Thank you for the reply, I wasn’t aware of “time difference” but I have now watched a couple of tutorials to understand what it does.

Unfortunately, it doesn’t do what I need. I am hoping for a solution something similar to the way “magnum edit detector” works where it scans the footage and can add markers to where there are changes in the scene. Magnum does this when there is a complete scene change but I need it for when there is movement within a security camera static cameras footage.

Thanks for any help

[Darby Edelen]

Just because a technique doesn’t do exactly what you want doesn’t mean it can’t be useful if modified to fit your needs. You obviously know what you need better than I do which is why I wasn’t initially prescribing an exact solution. I don’t know exactly what your requirements are but the outline for a solution that I described may still be useful to you if you’re flexible in the way you think about it.

For example: once you have Time Difference setup in a meaningful way you could use a sampleImage() expression to sample the average difference between frames. You could then convert that sampleImage() expression to keyframes and then view the value graph in the Graph Editor to see a representation of differences across time through the entire clip. In fact, that may be more useful than markers as it can provide information about the amount of motion across time instead of a binary ‘motion/no motion’ marker.

Darby Edelen

[Dave Rogers]

Hi Darby,

Thank you so much, that looks like it will work for me. I will try and get it working, I really appreciate your time and effort in responding.

Dave