Video in AE - Dimension/Proportion match animation

This question and @Mylenium 's reply intrigued me and I wondered if it is as simple as the big M says.

As soon as I saw Shape Layers, I started thinking that Trim Paths was a better option.  This allows you to control the width without having to manage the anchor point.  But then height is still an issue.

So I tried out shape layers and with a couple of simple expressions, you can get 4 boxes to respond... but when each box grows or shrinks the video underneath would be unaffected. And that made me think a better way would be to use the layer's scale to control the box size.

So... I did the following:

• Made a red solid the size of the comp
• Set it's anchor point to -960 by -540 (I was using a 1920 x 1080 comp)
• Set it's position to 0,0
• This puts the anchor point in the top left corner of the solid and the comp.  Any scale changes will grow from the left and top.
• To get the rounded corners, I added a Simple Choker and set that to about 40.  I might end up putting that on an adjustment layer if needed.
• For the blue solid, I wanted the anchor point in the top right corner, so it was 960,540 and its position 1920,0
• Then I linked the blue's scale to the red's using this expression:

100-thisComp.layer("Box 1").transform.scale;​

• For the green, it's anchor was -960,540 and position 0,1080 and its scale expression:
  

  X = value[0];
  Y = 100-thisComp.layer("Box 1").transform.scale[1];
  [X,Y]​

  This allows the X / width scale to be keyframed while the Y scale is driven by the red solid.

  
  
  
• For the final purple box, the anchor is 960,540, position: 1920x1080 and the scale:
  

  X = 100-thisComp.layer("Box 3").transform.scale[0];
  Y = 100-thisComp.layer("Box 1").transform.scale[1];
  [X,Y]​

This gives me 4 boxes, all on their own layer for matting and any videos I choose to add can be set to be parented to the corresponding layer to scale as needed.

Here's the project with all 3 options (comp 3 is the one above) : https://www.dropbox.com/scl/fi/wwt4anhl85niwlvawrwjv/Matte-boxes.zip?rlkey=y1szjh9yohmrxmcb607avoase&st=ys5fj637&dl=1

You don't use masks, you use shape layers as track mattes. That takes care of half the work. you know, there is a parametric rectangle with rounded corners... Multiple boxes can then be linked with some expressions so they push and pull each other out of the way as one resizes. the only thing you may need to do fully manually is re-centering and scaling the underlying images for a nice zoom-in effect. It's really not that complicated.

Mylenium