# Capturing a reflective screen at every angle. Xpresso question

#1

I’m working on a product viz shot, with a lot of fast moving animation, and need to capture a reflective card in every frame. Currently I’m constraining 2 cards and having to manually offset the rotation to capture it, but that involves way too many key frames and a lot of glitches so far.

Trying to figure out what I would need to do in Xpresso to make sure that the reflective card is always at the correct angle for full reflection every time, where I can capture the perfect screen card falloff with just an animated gradient.

For the life of me though, I can’t figure out what math would be needed to make that work.

#2

I feel your pain! Im in the same boat right now. About half my setup time for each shot in my current project is moving lights/refl cards around as the camera moves to keep highlights looking like the client wants. Would love to see a clever approach to this.

#3

What is a reflective card, and what object are you rotating in relation to what?

When working with reflections, the main rule is that the angle of incidence is the same as the angle of emergence. The emerging light hits the camera, so the reflected object or light needs to be at the inverse angle, calculated by the normal at the point where the reflection is supposed to appear.

The interesting thing starts when you need to art direct the point of reflection. Since this point influences the formula for the position on the reflected object/light, you need to tell the formula where this point is. If the camera moves, it is fairly likely that the point of reflection also needs to move. Now imagine the surface of the reflecting object being curved - this will cause the reflected object to hop around quickly while the normal changes. Maybe it is necessary to explicitly set the point of reflection through a helper object?