today I discovered this method at Vray, but I think it will work with other render engines too, if screen mapping is supported. If screen mapping isn’t supported, than a way must be found to place a plane in front of the camera that macht the view. Here my post from the VfR forum:
Since a long time I dream to free define regions with the mouse at the frame buffer, free shape, multiple areas. Today I needed to rerender some areas where I like to show different kind of wood on different seats in an interior - I found a solution:
- create a black and white mask based on the first rendering, where the render regions are white on a black background
- create a small sphere around your camera
- create a material with an emissive layer only (grey or black emissive color) and set your region map at the transparency, set UVW type at “environment” -> “screen”
- assign the material to the sphere around your camera
- ready for rendering like looking through a keyhole
The region mask can be created per free drawing white areas or per prerendered material ID mask.