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corner pin expression for linear tracking with depth change
Hello,
I have a shot of a train rolling out of frame to the left with an angled perspective. I’d like to map shots onto the side of the train and have them roll out with it. Manually animating the corner pin to match that change is impossible without some unwanted warping.
My idea was to tie each screen to a horizontal slider that controls the corner pin effect with an expression. The line for the top points would be the bottom of the train windows, and the line for the bottom would be the bottom of the train. Because these are different angles, I can’t figure out how to tie all four points together so I can use the slider to animate the screen going out with the train.
I’m really new to expressions. Posted the shot so hopefully someone can understand what I mean.
Thank you!
Clay
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4 Replies
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I don’t think there’s enough information to solve it completely. You’re not going to get the foreshortening right unless you have the information that would let you calculate the top and bottom perspective lines in 3D Space. I think you’d have to know the height and position of the camera, and a couple of points on each 3D line.
I’m sure there are algorithms for working it out, but I think it would be a stretch to try it with an expression.
Dan
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Thanks for your response, Dan.
Is there another approach that I’m missing that would be simpler?
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I can’t think of a simple way to do it. You have a couple of decent horizontal lines (the edge of the platform and the top of the cars) that will let you figure out the left vanishing point, but not the one on the right right, so this may not help much (but I found it very interesting):
https://www.fxphd.com/tips/finding-the-cameras-position-tutorial/
Dan
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Thanks again Dan!
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