Camera projection is pretty straight forward when it comes to mapping one projection onto one object. It becomes less straight forward when you want to map multiple projections onto one object.
That’s where the MergeMat node comes in handy. It allows you to composite your Project3D nodes together before you apply them to an object. It acts exactly like the regular Merge node so you’ll need to have an alpha channel in the projection going into the Foreground Input (A) on the MergeMat.
If you do want to do multiple projection on multiple objects which share a similar 3d space, you may find that Nuke has trouble figuring out which object is supposed to be in front and which is at the back. This happens due to a lack of precision in the Z-Buffer. Nuke creates the depth pass in the ScanlineRenderer by remapping the near and far clipping plane values on the rendering camera to zero and one (respectively). If the near and far clipping planes are too far apart then this can result in a “chattering” effect where the two objects intersect.
This lack of precision is very similar to colour-banding you see 8-bit images. Even through Nuke is storing the depth as a floating point value which has lot’s of precision to begin with it can still result in banding if the near and far clipping planes are too far apart.
The way to fix this inside Nuke is to adjust the clipping planes so that they tightly bound the 3D scene as much as possible – taking into account any animation on the cameras or geometry. You can animate the clipping planes, but it’s best to leave them static, animating them results in an animated depth pass, which can cause troubles if your using the depth pass to do depth effects such as defocusing or atmospherics.
Another option which can also help is to adjust the Z-Blend Mode and Z-Blend Range within the ScanlineRenderer. This works at a pixel level by taking the depth values of each object in the scene, if the depth values of any object are within the Z-Blend Range of each other it’ll render the objects blended together.