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Using GPUs for offline rendering to gain speed is a dangerous fallacy. Dangerous because the GPU has many restrictions which developers have to invest time in w

Using GPUs for offline rendering to gain speed is a dangerous fallacy. Dangerous because the GPU has many restrictions which developers have to invest time in working around. I feel in recent years makers of offline renderers have spent more time dealing with these restrictions to get their renderers to work well with GPUs than actually improving their renderers. I.e. reduce level of noise, better algorithms for hair, volumes, etc. The renderers produce way too much noise (or have prohibitive render times w/o except for big studios). Solution? Denoising! Which is (mostly) using GPU-powered ML! The irony of this seems lost on many freelancers who buy expensive GPU rigs to be able to deliver their jobs on the likes of OTOY or Redshift. For this money you can buy a vastly superior CPU rig. If only your renderer could use it! Most people compare apples to oranges here. When I hear people rave about GPU offline renderers, I usually ask them: - Have you made time to first pixel part of your comparison (complex scenes can take dozens of minutes before they start rendering on GPU renderers). - Are you comparing images as they come out of renderer A before denoising to images of renderer B before denoising? Because anyone can pipe their shit through Intel Open Image Denoise after the fact. It's free even. - Have you actually used a machine that has more than eight cores personally in production? If all your money goes into buying expensive graphics cards your CPU is usually shite. A notable exception is 3Delight which doesn't even ship a denoiser. On simple scenes the renderer can't beat the GPUs but on anything a bit more complex -- the stuff you deal with in actual production -- it smokes the competition CPU & GPU alike. Not least in the amount of noise it produces using comparable sampling settings. Without using any GPU compute. When this GPU craze took over other vendors who's renderers were traditionally CPU bound too, in the last 5-6 years, 3Delight's developers have spent all their time finessing the performance and output quality w/o relying on GPUs. It shows.

ben-schaaf 6y ago
Cycles does the same work regardless of whether it's running on the CPU or GPU. You can even run mixed where both the CPU and GPU do rendering. The amount of noise has nothing to do with it running on the GPU or not, that's simply how path-tracing works...
- wtracy 6y ago
  I think the implication is that GPU rendering ends up using lower-precision floating point numbers than are available on modern CPUs, which leads to more rounding errors.
  - gmueckl 6y ago
    No, CPU renderers also use IEEE single precision floats for performance. Unless you do something really absurd[0], single precision has more than enough range and resolution to not lead to any noticable inaccuracies in the result. [0] like trying to render an insect eye to scale while having everything shifted 1000km from the origin.
    - fxtentacle 6y ago
      While that is in general correct, most GPUs do not fully adhere to IEEE or they do not calculate at full resolution. It's a well-documented fact that the same floating point operations on a GPU will have less precision than on CPU, despite both using the same IEEE storage format. https://developer.nvidia.com/sites/default/files/akamai/cuda/files/NVIDIA-CUDA-Floating-Point.pdf https://developer.nvidia.com/sites/default/files/akamai/cuda...
      - dahart 6y ago
        That paper does not say what you claim it does, it is not showing that GPUs are computing at lower resolution, nor that CUDA is failing to adhere to the IEEE 754 standard. In at least two cases, it says the opposite of what you just claimed: FMA on the GPU was a way to achieve higher float precision than you could with CPU math, in 2011 when the paper was written. That is changing now with CPU adoption of fma, but that doesn't change the claims of the paper. The GPU's strict adherence to IEEE 754 is mentioned multiple times. "The same inputs will give the same results for individual IEEE 754 operations to a given precision on the CPU and GPU. As we have explained, there are many reasons why the same sequence of operations may not be performed on the CPU and GPU. The GPU has fused multiply-add while the CPU does not. Parallelizing algorithms may rearrange operations, yielding different numeric results. The CPU may be computing results in a precision higher than expected. Finally, many common mathematical functions are not required by the IEEE 754 standard to be correctly rounded so should not be expected to yield identical results between implementations."
wtracy 6y ago
As far as I can tell, Pixar only started using GPUs for previsualization around 2013 or 2014. It looks like Renderman didn't have real GPU super until 2018. They've never actually confirmed moving from CPUs to GPUs rendering for their final production renders. So, yeah, I think you're on to something.
dahart 6y ago
The trend in renderers is extremely clear, nearly everybody is building a GPU version, and the makers and their users all report pretty big speedups with GPUs... > Because anyone can pipe their shit through Intel Open Image Denoise after the fact. It’s free even. The same is true of the OptiX denoiser. I’m not sure what point you’re making? > Not least in the amount of noise it produces using comparable sampling settings. Without using any GPU compute. The amount of noise has nothing to do with GPU vs CPU.
- virtualritz 6y ago
  > The trend in renderers is extremely clear, nearly everybody is building a GPU version, and the makers and their users all report pretty big speedups with GPUs... The 'trend' of the US government under president Trump is also extremely clear. Sorry, I couldn't resist. :) TLDR; This 'trend' is not economically viable except for two parties. Makers of GPUs and companies renting out GPU rigs in the cloud. Aka: it's just a trend. It's not that anyone sat down and really looked a the numbers. Because if they did this trend wouldn't exist. It's also history repeating itself for those that do not learn from it. It will not go anywhere. Mark my words. I've been there, in 2005/2006, when NVIDIA tried to convince everyone that we should buy their Gelato GPU renderer. I can elaborate why that went nowhere and why it will go nowhere again. But it's a tad off topic.
  - dahart 6y ago
    Feel free to elaborate, I have no idea what your point is here or what you mean with your non-sequitur about the government or how that relates to developers of 3d renderers in any way. I don't know what you mean by "it's just a trend." The fact is that there's evidence for my argument, and you're attempting to dismiss it without any evidence. Comparing Gelato to RTX seems bizarre, they're not related, other than that they're both Nvidia products. Are you trying to say you distrust Nvidia? RTX already is a commercial success, and there are already dozens of games & rendering softwares using RTX on the market and hundreds more building on it. RTX already went somewhere.
    - 6y ago

ben-schaaf 6y ago

Cycles does the same work regardless of whether it's running on the CPU or GPU. You can even run mixed where both the CPU and GPU do rendering. The amount of noise has nothing to do with it running on the GPU or not, that's simply how path-tracing works...

wtracy 6y ago
I think the implication is that GPU rendering ends up using lower-precision floating point numbers than are available on modern CPUs, which leads to more rounding errors.
- gmueckl 6y ago
  No, CPU renderers also use IEEE single precision floats for performance. Unless you do something really absurd[0], single precision has more than enough range and resolution to not lead to any noticable inaccuracies in the result. [0] like trying to render an insect eye to scale while having everything shifted 1000km from the origin.
  - fxtentacle 6y ago
    While that is in general correct, most GPUs do not fully adhere to IEEE or they do not calculate at full resolution. It's a well-documented fact that the same floating point operations on a GPU will have less precision than on CPU, despite both using the same IEEE storage format. https://developer.nvidia.com/sites/default/files/akamai/cuda/files/NVIDIA-CUDA-Floating-Point.pdf https://developer.nvidia.com/sites/default/files/akamai/cuda...
    - dahart 6y ago
      That paper does not say what you claim it does, it is not showing that GPUs are computing at lower resolution, nor that CUDA is failing to adhere to the IEEE 754 standard. In at least two cases, it says the opposite of what you just claimed: FMA on the GPU was a way to achieve higher float precision than you could with CPU math, in 2011 when the paper was written. That is changing now with CPU adoption of fma, but that doesn't change the claims of the paper. The GPU's strict adherence to IEEE 754 is mentioned multiple times. "The same inputs will give the same results for individual IEEE 754 operations to a given precision on the CPU and GPU. As we have explained, there are many reasons why the same sequence of operations may not be performed on the CPU and GPU. The GPU has fused multiply-add while the CPU does not. Parallelizing algorithms may rearrange operations, yielding different numeric results. The CPU may be computing results in a precision higher than expected. Finally, many common mathematical functions are not required by the IEEE 754 standard to be correctly rounded so should not be expected to yield identical results between implementations."

wtracy 6y ago

As far as I can tell, Pixar only started using GPUs for previsualization around 2013 or 2014. It looks like Renderman didn't have real GPU super until 2018. They've never actually confirmed moving from CPUs to GPUs rendering for their final production renders. So, yeah, I think you're on to something.

dahart 6y ago

The trend in renderers is extremely clear, nearly everybody is building a GPU version, and the makers and their users all report pretty big speedups with GPUs... > Because anyone can pipe their shit through Intel Open Image Denoise after the fact. It’s free even. The same is true of the OptiX denoiser. I’m not sure what point you’re making? > Not least in the amount of noise it produces using comparable sampling settings. Without using any GPU compute. The amount of noise has nothing to do with GPU vs CPU.

virtualritz 6y ago
> The trend in renderers is extremely clear, nearly everybody is building a GPU version, and the makers and their users all report pretty big speedups with GPUs... The 'trend' of the US government under president Trump is also extremely clear. Sorry, I couldn't resist. :) TLDR; This 'trend' is not economically viable except for two parties. Makers of GPUs and companies renting out GPU rigs in the cloud. Aka: it's just a trend. It's not that anyone sat down and really looked a the numbers. Because if they did this trend wouldn't exist. It's also history repeating itself for those that do not learn from it. It will not go anywhere. Mark my words. I've been there, in 2005/2006, when NVIDIA tried to convince everyone that we should buy their Gelato GPU renderer. I can elaborate why that went nowhere and why it will go nowhere again. But it's a tad off topic.
- dahart 6y ago
  Feel free to elaborate, I have no idea what your point is here or what you mean with your non-sequitur about the government or how that relates to developers of 3d renderers in any way. I don't know what you mean by "it's just a trend." The fact is that there's evidence for my argument, and you're attempting to dismiss it without any evidence. Comparing Gelato to RTX seems bizarre, they're not related, other than that they're both Nvidia products. Are you trying to say you distrust Nvidia? RTX already is a commercial success, and there are already dozens of games & rendering softwares using RTX on the market and hundreds more building on it. RTX already went somewhere.
  - 6y ago