GPU vs CPU.
Right so this is part of one of my main topics I set up for this course. I wanted to see if I can create a movie quality assets and use them in games and GPU based production engines.

In this simple test I have focused on IBL lighting between Vray and iRay in GPU and CPU mode.

The test I pick is very simple I used one of my buildings that I have moddeled for my project and applied a grey material so that I can diagnose the quality of lighting, shadows, noise and so on. This is very simple test but it will tell us a lot about how well does an engine precess lighting and sample image.

I have used for this purpose a

2x xeon E5530 @ 3.7ghz

GTX 480 - 1.5gb

GRS 8800 - 640mb

Each test render was set up to render only for 1 minute. In right upper there is a statistic for Vray saying how many patch has render achieved in 1 minute time. Unfortunately iRay dont have such features so we have to compare them visually.

I tested Vray only on CPU and GTX 480 because the GTS 8800 did not work. The result you can see below.

As you can see the GPU render is almost 4x quicker than CPU render, also the IBL lighting is very close. However there is a difference in shadow details in small areas, it looks like a ambient occlusion pass is being added or the shadows are simply more/less accurate...]

The second tests were made using iRay. I have tested them using, CPU only, GPU gtx480only, CPU + GTX 480, CPU + GTX480 + GTS 8800 in case of the last test I had to lower the resolution of HDRI from 4k to 2k in order to fit in memory.

As you can see the tests are quite simmilar to Vray, with the difference in shadows. It apears that iRay GPU match vray CPU and iRay CPU render in shadow details. Quite interesting that Vray GPU produce different result which now sems to be inaccurate. However the issue with iRay is that it produces a checker box alike noise which looks unnatural. The noise can be best observed on iRay CPU example.
Both of these tests worked quite well and did show the bennefit over CPU rendering, however this example is still to simple to be conclusive for using GPU rendering in real production. There are a lot of features missing such as, render elements, hair and fur, volume effects and so on. Non the less they do get them time by time. One of recent additions to Vray RT render is displacement. Unfortunately its still unsuported on GPU mode where as iRay can do it.

Another quite interesting point to note is how well is render utilizing GPU. I have run a lot more tests in past and I realised that in quick renders if I were to use GTX 480 and GTS 8800 the render took longer than using single GTX 480. Since the difference was about 18 seconds to 16 seconds, I believe that this could be an issue of loading the secene to another GPU, or how well the software manges the Cuda cores on GPU cards.