After Effects Trim paths offset corner position based on rectangle width and height

Question

I think it would be easier to share an image to show what I'm trying to accomplish.

This will be a mogrt so the actual rectangle size / aspect ratio is determined by variable text size.

But, I think I should still be able to figure out offset angle so the trimmed paths will always be centered on the corners.

Edit: Getting a bit further but still not there. This should make the yellow stroke start directly in the lower right corner, but it doesn't.

# text layer determines width and height
txt=thisComp.layer("quote").sourceRectAtTime();

# padding adding to width and height
p=thisComp.layer("controls").effect("box padding")("Slider");
h=txt.height+p;
w=txt.width+p;

# angle from upper right corner to lower right corner (yay trigonometry)
x=radiansToDegrees(Math.atan(h/w))*2;
[x]

I still haven't figured out how I'm going to account for the length of the line yet (in degrees).

Edit 2: Decided to take a different approach and use a mask with 2 rectangle shapes (controlled with a second slider)

The yellow stroke shape uses the dual rectangle shape layer as a track matte and I'm using these two expressions to determine the width/height of the rectangles.

txt=thisComp.layer("quote").sourceRectAtTime();
h=txt.height;
w=txt.width;
p=thisComp.layer("controls").effect("box padding")("Slider");
sw=thisComp.layer("controls").effect("stroke length")("Slider")
x=w-sw;
y=h+p*2;
[x,y]

txt=thisComp.layer("quote").sourceRectAtTime();
h=txt.height;
w=txt.width;
p=thisComp.layer("controls").effect("box padding")("Slider");
sw=thisComp.layer("controls").effect("stroke length")("Slider")
x=w+p*2;
y=h-sw;
[x,y]

I won't be able to animate the way I had planned, but I should still be able to come up with a cool look by animating the track matte. I'm leaving this question open because I would love to know why my first solution wasn't correct.

Edit3: Thanks for the answer Clif!

Answer 1

In order to align the lines to each corner, I was able to do this:

Bottom left corner:

• No offset
• Set trim start to 50%
• Set trim end to 50%
• Set expression on trim end to be 100 minus the trim start or something like 100 - content("Rectangle 1").content("Trim Paths 1").start
• Now if you change the value of the trim start, it should stay equidistant with the trim end.

This first corner can be used as a starting point for all of the other corners and we should only have to update the offset to get them to align correctly to each corner.

Top right corner

• Set offset to 180 degrees

Bottom right corner

• Set expression for offset to be:

// If needed, change your width and height variables to whatever values you 
// are using to drive your dimensions.
width = content("Rectangle 1").content("Rectangle Path 1").size[0];
height = content("Rectangle 1").content("Rectangle Path 1").size[1]
perimeter = 2 * width + 2 * height;
perc = width / perimeter;

-perc * 360

Top left corner

• Pickwhip the offset of the bottom right corner and add 180.

Then you should be able to hook up each of the trim path's start positions to a 'slider control' effect to have them all animate together.