I’m trying to composite a refractive material over a pre-rendered background. For some reason, the transparency of the material ignores any IOR and renders as though the IOR is 1, ie totally see-through.
I’ve tried using the background in the environment slot in the material and using the reflection overrides in the render dialog, and its still the same.
I am using a vrayphysical camera if that has anything to do with this.
Has anyone else had this problem? is there a workaround?
Cheers
Well, after some digging around the forum, it seems to be a standard problem with any form of screen-mapped background image.
So, does this mean it is technically impossible to use a pre-rendered sequence as a background which then has transparent objects rendered over the top that refract the background? I can’t use a plane, as my camera is animated, and matching the plane to the camera will be a nightmare to get right.
Is the only workaround to map the background onto a plan, then parent the plane to the camera so it always sits in frame. Is there any macroscripts that automate the process of placing a bg-plane in a scene so it renders out asthough its a screen-mapped background?
My added problem is that I’m using the physical camera which would double-distort the plane depending on its distance from the camera! 
There must be a simple work-around I’m overlooking 
…
Is < Animation - Constraints - Orientation Constraint > assigned to the plane (BG) good enough for that ?
Best regards,
nikki Candelero
That would work with a standard camera, but as the vray camera changes its zoom factor depending on the distance the target is from the camera, the plane would change scale. I know how to avoid this in future, but with the plate I already have rendered, I didn’t lock off the focus distance.
a spherical env. won’t work?
Spherical Env… won’t that mean the image sequence is stretched around the scene and not anchored to the camera.?
I’m using a pre-rendered sequence as a background plate, so I need it to match the edges of the rendered frame exactly, pixel for pixel. Vrayphysicalcam has a variable level of zoom depending on the distance the target is from the camera, so as the camera moves towards a target, I’d have to scale the plane to match the zoom of the camera… ie too much hassle.
Its ok… I think its an unavoidable limitation of all rendering (raytracing) packages. As I said, I can lock off the focus distance and re-render the sequence… but I was hoping to avoid that.