2

If I have a locked-off camera filming an athlete moving through the frame, how could I stack the frames so that every new frame of movement was layered on top of the next? Assuming the background remains static throughout, is there a clever way to do it without tonnes of rotoscoping?

For example, here's a (famous) sequence of a horse running:

enter image description here

And here's the kind of effect I'd like to be able to achieve, without keying/rotoscoping etc:

enter image description here

Note that I made this with a simple luma key, because the background is mostly white.

  • Interesting :) The example is nice to understand your question, but I don't know if this is a good example for what do you want to achieve (foreground and background are easy to seperate in this specific case). If possible, please add a frame of your final footage to think about a solution. – p2or Jul 24 '15 at 12:07
2

Think about what you're asking. When you did the horse you used a luma key to isolate just the horse. If the horse was running through trees or down a racetrack what would you want to happen to the trees and other stuff in the background? Would they go over the horse from the layer underneath? If so, how are you going to see the layer underneath?

There's no way of overlaying frames without the front layer being partially transparent - either transparent in part of the frame (masks or roto), transparent depending on their pixel values (transfer mode, or key) or with an overall opacity of less than 100%. If it's not transparent then you can't see the background, it's that simple.

If you can, shoot against a green screen and key out your foreground. This will let you layer it up to create a visual echo as you show in your example. That's the best way to do this, even though it's "cheating".

If chroma keying is not possible, and if only part of your frame is moving, you might be able to use a difference matte. Get an empty shot of your background and use that to matte out only the parts of the frame that have changed. This will work best with locked-off shots.

Failing that, have a play with various transfer modes. They're basically pixel level keys that work depending on the value of the foreground and background colour. I worked with a visual artist who produced some lovely dance videos using time-delayed layering kind of technique. She filmed the dancers against black, and rather than pulling a key we just used the lighten transfer mode - which is basically a way of keying parts of your image that are darker than the layers below (we were using hundred of layers in each comp, I did some scripting work to make the whole process a lot easier).

Otherwise, if transfer modes, chroma / luma keys or difference keys don't work, put on a pot of tea, turn up the music and get stuck in to the roto.

1

I'm not sure I get exactly what you're asking, but if you're looking for a sort of stroboscopic motion effect, you can set up multiple layers, say 4 or 5, containing the same clip but offset in time by one or more frames. Then set the transparency of each (level of dissolve) in ratio to the number of layers (4 layers, each 25%, 5 layers each 20% etc). Experiment with the transparency and offset to get the look you want.

I don't do AE, but there may already be an effect or plugin that does this for you.

  • Thanks, but I'm not looking for a visual echo effect - I don't want the earlier frames to be less opaque than the later ones, I want every new frame to appear overlaid on top of the previous older frames. I'll add some examples to the question to make it clearer. – tomh Jul 23 '15 at 18:48
  • Depending on the makeup of the video, perhaps you can use a technique called "non-additive mix" which is a sort-of key, sort-of dissolve. NAM used to be common in electronic mixers, but I don't recall seeing it in lists of digital effects, at least not under that name. It looks quite a bit like what you posted. – Jim Mack Jul 23 '15 at 21:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.