GPU versus CPU rendering

[Jim Andrada]

I was always my understanding that the GPU is used for real time preview and the CPU for final rendering/codec processing. The GPU cores are great for running the same small program simultaneously on a lot of data points, but applying a codec is a more complex process.

[Jeff Harper]

Vegas does offer GPU assistance for preview, as well as rendering of Sony AVC codec. And tests conducted by Kim O and a few others of us have demonstrated that GPU assistance for rendering can reduce times somewhat. In my case I've only used it a few times for rendering. I generally turn off all GPU "help" when using Vegas.

[Danny Fye]

I have used both CPU and GPU for rendering and preview. GPU on my system makes everything much faster especially with the 580 card that I have.

One big difference maker is the driver. Driver 296.10 is not only the fastest but also provides the best quality. I can get some strange looking previews with the later drivers as well as not so great quality renders and also be slower. With the 296.10 driver, I get the same great quality as I do with CPU only.

Problem is, NVidia apparently does not want to play nice in the sandbox. They also want to focus on their money making game support over video.

So what we end up with is a sort of dead-end support for video with NVidia unless we want to pay too much for their inferior premium cards (I cannot remember the names) and get inferior results.

Maybe I should take a chance and sell the NVidia cards I have and go to AMD? I do not have the time and/or money to mess with that.

Besides, what I have at the moment is mostly not broken so I will not fix it. I say mostly, because certain projects with NBTP3 and large jpg graphics can cause Vegas to crash with the 296.10 driver but not crash with the later driver. But I do not use those often enough to justify the later drivers so I use a work-around for those projects. The later drivers gives poor quality results so I do not want them anyway.

[Norman Black]

Originally Posted by Mervin Langley

Is CPU render quality inherently better? I thought I read this somewhere.

A better quality encoder will give better results, regardless if it runs on CPU or GPU. All current GPU encoders are not as good as the more mature CPU encoders. Also, they probably never will be as good because people want speed when they think GPU use and one must make compromises in algorithm to increase speed.

Note that in Vegas, MainConcept AVC CPU, OpenCL/CUDA are all completely different encoders using different algorithms. Enabling OpenCL/CUDA does not just "accelerate" the CPU encoder in some way.

Sony AVC only seems to use GPU for motion estimation with the rest being the CPU encoder. It does not have a big speed difference CPU to GPU.

Quicksync is a single function hardware based encoder available as an option in Sony AVC.

Encoding a video is not a very parallel sequence, and GPU gets performance only from massively parallel algorithms. GPUs are slower than CPUs by a good margin, but they have hundreds or thousands of simple execution units.

In contrast most graphics/video effects can be massively parallel and match well to the GPU. Compositing is massively parallel. With playback and encoding you always get GPU benefit here.