Screen Replacement with Mocha Doesn't Work Correctly

[Stefan Wills_90]

Hello,

I'm doing a phone green screen replacement project and I decided to use Mocha for the purpose. I watched most available tutorials on youtube about it and thought I did everything correctly, but the end track data export moves the replacement matte. Here's a video recording of what I've done: https://www.awesomescreenshot.com/video/27435382?key=6884872880d708ece28cc595c99742fb

I'm also describing the steps I took below:

1 - Made a base track of the phone and activated the planar surface, after which I repositioned it so that every corner can be placed on each of the tracking + of the green screen. Made small adjustments to the phone shape track by tweeking the points of the x spline. I also did a 2nd layer for the thumb that swipes the phone and placed it above the base track as an occlusion.

2 - removed the green screen with keylight. I had to increase the screen deposit white to remove the remaining + marks but that ate some pixels from the background, so I duplicated the original video and did a mask of the phone and subtracted it.

3 - I added the replacement photo of the sunset for the matte and added the corner pin effect to it. Then adjusted the pins to fit the photo in the screen. Then precomposed the photo layer so the new pre comped layer it can have the sizes of the original composition (I learnt this was important for things to work).

4 - I created tracking data from Mocha by choosing the base track layer. Chose corner pin (support motion blur) as export option and chose the layer of the replacement photo pre comp.

Then what happens is the replacement photo matte layer dislocates from where it was and is supposed to be, as you can see from the video recording I gave above. I can't understand why this is happening. Does anyone have a clue? I'll be very grateful for any insight.

[RobShultz · Accepted solution]

If the video that you're trying to have Mocha put on the screen it is a different resolution than your comp you may have to use the positioning numbers to tweak where it is on the screen.

[Stefan Wills_90]

In this case this is the only thing that worked for me, thanks. Would that mean that even if I execute the workflow correctly as the others have suggested (which after my post I later think that I did), in a situation like this, it won't work, just because the asset that I want  used in the place of the green screen is a different resolution than the one of the original clip I'm tracking?

[Warren Heaton]

It looks like you are pre-Composing the sunset image last.  That should happen earlier.

The picture being inserted should be in the pre-Comp that is the same width and height as the Comp that you're working in (the containing Comp).  It can be counter-intuitive, but if the dimensions of the picture being inserted does not already match that width and height of the Comp, it should be stretched to fit such that it looks distorted in the pre-Comp.

Try comparing what you're doing to these steps:

• Screen Replacement (Screen Tracking) in After Effects
  https://borisfx.com/blog/screen-replacement-screen-tracking-in-after-effects-borisfx/

[Rick Gerard]

Your workflow is incorrect. Put the Sunset Photo in a new comp with the same resolution, frame rate, and duration as the footage you track. The easiest way to do that would be to put the Sunset photo on a new layer, set the outpoint to the same place as the footage you are trying to track, press Alt/Option + Ctrl/Cmnd + f, or select Layer/Transform/Fit To Composition, then Pre-compose making sure that you move All Attributes and Trim the comp to the layer size. Give the pre-comp a meaningful name like "Sunset BG."

Then, choose Corner Pin instead of Corner Pin with Motion blur and select the nested Sunset BG layer. The Motion Blur option can sometimes be finicky, and there is not enough motion in your footage layer to cause any problems. 

Spend some time here:  Boris FX Mocha AE Training

. You'll learn much about Mocha AE. Be careful when searching YouTube for tutorials. Most of them are poorly explained, and many of the workflows are inefficient because a great many AE tutorials were prepared by well-meaning enthusiasts trying to share a recipe they copied from someone else.

[Stefan Wills_90]

Thank you all for jumping in the thread!

I did just what you recommended, but still something went wrong. Here's another recording: https://www.awesomescreenshot.com/video/27474818?key=3f9006ffc43fc73a5783e7504e2dc5c9

1 - I added the photo of the sunset and pre composed it

2 - I went into the pre comped layer and selected Fit To Comp to box the photo into the comp sizes.

3 - I went back into the main comp and went to the original Footage I tracked with Mocha. I exported the tracking again and selected the pre comped sunset layer. The sunset pre comp doesn't get its dimensions reduced down to the phone screen size, but instead it gets zoomed in. I also tried another way - When I had the pre comped sunset photo fit to size I went back to the main comp and added corner pin manually as an effect and made it fit the phone, after which I precomposed the pre comped layer again and exported the mocha tracking to the newest pre comped layer, but the whole picture just moves quite away from the main comp again.

I've uploaded the whole project and I'll be very thankful if someone gave it a closer look and see what was done wrong again if the second video recording doesn't help. If it's not the workflow now, maybe there's something wrong with the Mocha tracking, but I don't see what could be breaking the whole thing: https://we.tl/t-Kg1KBnUPJE

Any further help will be greatly appreaciated.

[Stefan Wills_90]

I just found a specific video from Boris FX that covers such exact cases. If someone in the future with a similar problem stumbles upon this thread, here's the best video that can help: https://www.youtube.com/watch?v=UbeosgZkWj8