This page provides a tutorial on using V-Ray's Render Mask feature.
Here's our base render:
In the next steps, we render just a few different objects and not the whole scene again.
Let's dull the chrome material and match the hubcaps to the car body. In the case of the hubcaps, we change the diffuse color from pink to blue to match the car.
Next, select the objects to re-render: all the chrome parts, and the hubcaps.
Then we go to theV-Ray tab > Image sampler (Antialiasing) rollout and choose the dropdown for Render mask.
From the dropdown we have several choices:
Texture – Will use a black and white image to control the rendered areas.
Selected – Will render only the currently selected objects.
Include/Exclude list – Will render only the objects added to the list.
Layers – Will render only the objects from the selected layers.
In our case, we choose Selected.
Render the scene, and you'll notice only the selected objects are re-rendered. The rest of the image remains unchanged.
To make it a bit more noticeable, clear the VFB by selecting Clear Image from the VFB toolbar.
In the VFB Confirm dialog that appears, choose Yes to confirm.
Now that we've cleared the VFB, we render again.
This time it's easier to see that only the selected objects were re-rendered.
If we look at the Alpha Channel, it is updated as well.