This is how to setup a render mask, or render patch, or whatever you want to call it, in Houdini using Arnold.
Render patches are generally used when a high cost render needs a fix that only affects to a small portion of the frame, or when most of the frame is going to be covered by a foreground plate.
In these scenarios there is no need to waste render time and render the whole frame, but just what is needed to finalize the shot.
This is the scene that I’m going to use for this example. Let’s pretend that we have already render 4k full range of this shot. All of the sudden we need to make some changes on the rubber toy screen left.
The best way to create a render mask is using Nuke. You can use an old render as template to make sure everything you need in the frame is covered by the mask. Rotopaint nodes are very useful specially if you need to animate your mask.
Create a camera shader and connect the render mask to its filter map.
Connect the shader to the camera shader input of the camera, in the Arnold tab.
If you render now, only the mask area will be rendered, saving us a lot of render time.
Huge limitation, that I don’t know how to fix and I’m hoping for someone to throw some light on this topic. If you are rendering with overscan, this won’t work nicely, let me show you why.
I’m rendering with a 120 pixels overscan, I know is generally speaking a lot, but I just want to illustrate this example very clearly.
Now if you render the same overscan with the render mask applied, you will get a black border around the render. Below is the render patch comped over the full frame render.
I’m pretty sure the issue is related to the wrap options of the render mask. By changing the wrapping mode you will get away of this issue in some shots, but in an example like the one on this post, there is no fix playing with the wrapping modes.
You can definitely use the camera crop options and it will work perfectly fine, no issues at all. It is not as flexible as using your own textures, but it will do in most cases.