GPU rendering in V-Ray has always been a progressive renderer; this allowed users to see their renderings progressively refine as a complete image and update on-the-fly when in IPR mode. While this can be very useful as an interactive tool, there are still advantages to rendering in buckets — especially when it comes to final frame rendering.
The first advantage is speed. Generally speaking, if you are using a single GPU on your system, you will not see much difference in the speed between Progressive and Bucket mode. However, it’s possible that when multiple GPUs are introduced, there could be a more significant difference in speed.
One of our beta testers, Tomasz Wyszolmirski, Studio Director at Dabarti, tested several different scenes with multiple GPUs both in Progressive and Bucket mode. He found that, when those GPUs are on the same system, bucket rendering offers only a slight advantage over progressive — if anything at all. However, when doing distributed rendering to multiple computers, bucket rendering starts to provide a real advantage. The reason for this is that progressive rendering puts a high demand on network traffic since it has to refresh the entire buffer with every pass of every node, which can slow the whole process down. With Bucket mode, each node only needs to send the result of its bucket when it’s done, making it very efficient.
But speed is not the only reason to decide to use Bucket mode instead of Progressive for GPUs. Bucket rendering also allows for the addition of some new features that were not possible in Progressive mode. While in Bucket mode, users can now add the popular Cryptomatte render element. In an upcoming update, we are planning to add deep image output with Bucket mode in V-Ray GPU.