Why is gpu and cpu rendering different at all?

[tmcthree]

I have a programmer who refuses to believe that GPU and CPU rendering produces different results. He insists that anything that can be done on the cpu should be exactly reproducable on the gpu.
As this clearly isn't the case in Vray. Can anyone give me something fundamental to tell him to get him off my back please?

[a7az0th]

In theory everything can be done 1:1, but in practice there are 2 different architectures, that sometimes handle computations differently.This is one source of possible differences.
The main and more adequate reason however is that the GPU being a piece of hardware following a completely different design, requires you to write code in a different way.
Some algorithms that are very efficient fast and reliable on CPU show terrible results when implemented on GPU in terms of performance.

This is why, in order to both be efficient and fast, V-Ray and V-Ray GPU use different sets of algorithms, that are optimally tuned to work on the specific hardware the engine runs on.
Sometimes this means the results won't match.

[tmcthree]

Copied and pasted. Thank you.

[francomanko]

But for example, things like certain render elements, like glossiness, not being gpu supported, is this for the same reason?

[oglu]

This should be a stickyTthread. And moved to Render Theory!