Frame Blending acting strangely

[lastala]

In After Effects, on a 25fps time line and 25fps clip, I’m slowing down the clip to 50% (time stretch 200%), so every frame is repeated exactly once. While Frame Blending on, I would expect that every second frame would look identical with frame blending set to “off”, “Frame Mix” or “Pixel Motion”. But it doesn’t!

Even if the clip is set to 100% speed, “Frame Mix” creates a small “halo” – mixing some amount of previous or next frame with it.

Why does this happen?

I’m curious because I’ve had bad results with “Time Stretch 200% / Frame Blending / Frame Mix” (resembling some sort of a compression pattern), and I guess this happens due to reading “wrong” frames in the Frame Mix process – every second frame should stay disaltered, I would presume.

What am I doing wrong? Is there a setting that I’m missing?

Thanks in advance!

[Mylenium]

You are not considering thát the actual sampling is based on genuine time vs. virtual shutter speed and phase. Blending two frames may not at all occur in the middle of two frames based on these calculations when using the advanced methods. This stuff is difficult to predict, so you can only play with values or employ other techniques like changing the footage interpretation and comp frame rates and even "mixing" with two layers and the Strobe effect.

Mylenium

[lastala]

Thanks Mylenium for your reply!

Ok, understand. If I remember correctly, this was not the case with CS6, but I suppose the calculations have changed drastically from that time.

You mention playing with values (I suppose you refer to shutter speed and phase, right?). But how would I do that? In the comp settings there are advanced values only for motion blur, and there are no value adjustments for (virtual) shutter speed or phase.

And the other route of footage interpretation/comp frame rate. Changing the frame rate interpretation to 12,5 does not change anything. Could you kindly elaborate how to "employ other techniques like changing the footage interpretation and comp frame rates and even "mixing" with two layers and the Strobe effect"?

Many thanks in advance!

[Rick Gerard]

If you want duplicate frames by far the easiest thing and fastest thing to do is to open up the file interpretation and set the frame rate to half of the comp frame rate. If your comp is 25 fps and you interpret the footage at 12.5 fps then you will get duplicate frames in the timeline, fast renders and no interpretation at all.

If you use frame blending then the frames will be Blended so some kind of calculation is going to happen. It's always been that way. If you need two copies of the original footage in the project with one playing in real time and one playing at half speed you can duplicate the footage in the composition panel and adjust the frame rate of the duplicate there.

If you are getting a little frame blending with the footage and the comp at the same frame rate then this happened in camera or is being caused by compression. MPEG, MP4 footage always samples multiple frames when it compresses color and this can cause a little halo around edges you are describing. Open the footage in the Footage Panel and take a close look. If you see the problem there then it is baked in and there's nothing you can do about it but apply some kind of sharpening or edge finding filter, and those don't usually work very well.

[lastala]

Thanks Rick for your reply!

Yes, I've tried interpreting footage 12,5fps, and yes, that's a convenient way. Although this does not solve the blending issue I described.

As I wrote initially, the halo does not come visible in the clip – it only comes visible when applying "frame blending / frame mix". I had the impression that "frame mix"mixes two adjacent frames. And if the adjacent frame is the same (repeated) frame, then it would result into the same frame. But this does not happen, it seems to mix with more than two adjacent frames.

And yes, I did go to see the file in Footage Panel, and the halo does not come visible there.

So, would there be a more "accurate" way of applying "Pixel Motion" blending?

Thanks!

[Rick Gerard · Accepted solution]

The most effective blending I have ever used comes by using Twixtor - RE:Vision Effects. AE is better than it was but Twixtor can work magic. Without seeing the shot and the difference between frames it's really difficult to give you much help because each type of pixel motion and detail level respond differently to frame blending algorithms.

[lastala]

Thanks Rick for your reply!

I know of Twixtor, but haven't used it before – might give it a try. AE "Pixel Motion" seems to work well, except for low contrast details.