7 ms·

Writing a Portable CPU/GPU Ray Tracer in C#

116 points by mellinoe 8y ago

dahart 8y ago
Great write-up! Your speedups look like they line up as expected. Would you happen to know why Aras's look a bit weird, is it the devices he's using? I'm referring to his Metal timings being more than 10x slower than D3D, and being almost comparable to C++. > One of my goals was “use as much of the same code as possible on the CPU and GPU”. Great idea, it's hard to debug GPU code, and super convenient to have something that you can run & debug on the CPU, then just flip a switch and get GPU speedups! > Another obvious limitation is that GPU code cannot recurse. Yep. I bet this changes relatively soon, since you can sometimes use recursion in OpenCL & CUDA. It's a fun exercise in ShaderToy to write a recursive ray tracer. Here's mine: https://www.shadertoy.com/view/XllBRf https://www.shadertoy.com/view/XllBRf And here's a much better one: https://www.shadertoy.com/view/4scfz4 https://www.shadertoy.com/view/4scfz4
- mellinoe 8y ago
  > Great write-up! Your speedups look like they line up as expected. Would you happen to know why Aras's look a bit weird, is it the devices he's using? I'm referring to his Metal timings being more than 10x slower than D3D, and being almost comparable to C++. Thanks for reading! Since Aras is using a GTX 1080 TI on Windows, and an Intel Iris Pro on Mac, I think the numbers make sense. In my case, my GTX 770 is pretty old, and my 2017 MacBook is pretty new, so the numbers line up better. > It's a fun exercise in ShaderToy to write a recursive ray tracer. Here's mine: https://www.shadertoy.com/view/XllBRf https://www.shadertoy.com/view/XllBRf > And here's a much better one: https://www.shadertoy.com/view/4scfz4 https://www.shadertoy.com/view/4scfz4 Nice! I should add accumulation to mine -- it'd look much better for barely any effort.
- ohf 8y ago
  > Yep. I bet this changes relatively soon, since you can sometimes use recursion in OpenCL & CUDA. Am I correct in assuming you can only recurse when the recursion can be "unrolled" into a simple loop? In other words, no program flow structures.
  - dahart 8y ago
    You can do real recursion sometimes, and real program flow. It does depend on CUDA version & chipset, but most modern ones support flow control. (https://stackoverflow.com/questions/3644809/does-cuda-support-recursion https://stackoverflow.com/questions/3644809/does-cuda-suppor...) I think there are still cases where it won’t work right due to other libraries that might be involved, I’ve had recursion fail even on a new GPU with cuda 9. Even if you can use recursion, it’s not usually a good idea since you’ll run into thread divergence problems.
- aras_p 8y ago
  > I'm referring to his Metal timings being more than 10x slower than D3D In my case it's the difference in hardware. DX11 results are on GTX 1080 Ti, whereas Mac are on Iris Pro on 2013 MacBookPro. > 10x performance difference between these GPUs is entirely expected.
  - dahart 8y ago
    Totally makes sense; thanks for the explanation!
wolfspider 8y ago
Always love your work! Going to check this one out right away. Yeah GLES and readpixel aren’t friends like for instance GPUImage uses the hardware backbuffer to store it as a workaround: https://stackoverflow.com/questions/12813442/cvopenglestexturecache-vs-gltexsubimage2d-on-ios https://stackoverflow.com/questions/12813442/cvopenglestextu... So it’s device specific to implement the same thing I believe GPUImage uses a stub for this to apply the same shaders you have in mind.
IvanK_net 8y ago
I made a real-time ray-traced game a few years ago, that works in a browser :) You can increase the resolution, while FPS drops. http://powerstones.ivank.net http://powerstones.ivank.net
Const-me 8y ago
> You give it regular C# structures and methods, and it automatically converts them to HLSL, GLSL SPIR-V, and Metal shaders. Code introspection is one of the huge strengths of .NET ecosystem. You can even do it multiple levels of abstraction: bytecode (using e.g. Mono.Cecil), with built-in reflection API, with built-in expressions API since .NET 3.5, and apparently this project uses even higher-level approach, Roslyn. I have never transformed C# code into shaders, when I’m working on something similar I usually prefer traditional GPU languages like HLSL or CUDA, because tooling (e.g. visual studio can debug both). I transformed expressions into NoSQL database queries, worked well. Here’s the main parts: https://github.com/Const-me/EsentSerialize/blob/master/Core/EsentSerialize81/Linq/FilterQuery.cs https://github.com/Const-me/EsentSerialize/blob/master/Core/...
- pjmlp 8y ago
  While it is great feature that I enjoy a lot, it isn't the only eco-system with it. :)
- seanmcdirmid 8y ago
  You can also roll your own expression language using operator overloading and then plug that into the DLR or HSL, or whatever as needed (I did this in https://archive.codeplex.com/?p=bling https://archive.codeplex.com/?p=bling). It doesn’t allow for statements, however, which introspection gives you. It is much harder in the new UWP, however, where dynamic features (reflection, introspection, native JIT via the DLR) are no longer guaranteed to be present.
  - zamalek 8y ago
    Don't forget that Linq operates over duck-typed extension methods, so you can also define your own wizardry there.
    - seanmcdirmid 8y ago
      Extension methods are important to doing an EDSL in C#. Link syntax is mostly a dud, however.
pjmlp 8y ago
Very interesting reading, we need more article about alternative approaches.
ionafgnio 8y ago
Is there a legitimate reason for D3D12 to exist? It seems like just a locked-down version of Vulkan to me intended to kill cross-platform gaming (textbook Microsoft Embrace, Extend, Extinguish). I haven't heard of any advantages in performance or ease of development. But I'm certainly no expert. Unrelated note: there's something really funky going with the comparison between the desktop and the laptop. The desktop is pretty high-end and should just obliterate that Macbook. It's seriously weird that they're so close. I'm not sure I trust the results.
- mellinoe 8y ago
  > The desktop is pretty high-end and should just obliterate that Macbook I was also a little surprised at how well the MacBook performed, but it's quite reproducible for me. To be fair, this desktop is actually about 5 years old, and it wasn't really "top of the line" when I built it.
- mattnewport 8y ago
  There are several good reasons. D3D12 was finalized some time before Vulkan and was available in early / preview versions for quite a while before that. Stable drivers were available much sooner than for Vulkan. It was an easier transition for code bases that already had D3D11 support and Microsoft offered additional code to help with the transition (the D3D11 on 12 stuff). You could bring your existing HLSL shaders over easily. There were better debugging and profiling tools available sooner. The Xbox One got some additional extensions to better utilize its hardware.
- pjmlp 8y ago
  OpenGL always lagged behind proprietary 3D APIs. Microsoft is not alone, people keep forgetting OpenGL never mattered on game consoles. Even Vulkan is supported on the Swift, the main 3D API is NVN. Some of the benefits of DirectX, proper 3D API, with a toolkit that supports math, fonts, textures, sprites instead of playing "go fish" libraries, OOP API, OOP shading language, nice debugging integrated into visual studio. Same applies to the other 3D APIs. Game developers care about their IP, how it gets into each platform is secondary to that.
- zamalek 8y ago

116 points by mellinoe 8y ago

Great write-up! Your speedups look like they line up as expected. Would you happen to know why Aras's look a bit weird, is it the devices he's using? I'm referring to his Metal timings being more than 10x slower than D3D, and being almost comparable to C++. > One of my goals was “use as much of the same code as possible on the CPU and GPU”. Great idea, it's hard to debug GPU code, and super convenient to have something that you can run & debug on the CPU, then just flip a switch and get GPU speedups! > Another obvious limitation is that GPU code cannot recurse. Yep. I bet this changes relatively soon, since you can sometimes use recursion in OpenCL & CUDA. It's a fun exercise in ShaderToy to write a recursive ray tracer. Here's mine: https://www.shadertoy.com/view/XllBRf https://www.shadertoy.com/view/XllBRf And here's a much better one: https://www.shadertoy.com/view/4scfz4 https://www.shadertoy.com/view/4scfz4

mellinoe 8y ago
> Great write-up! Your speedups look like they line up as expected. Would you happen to know why Aras's look a bit weird, is it the devices he's using? I'm referring to his Metal timings being more than 10x slower than D3D, and being almost comparable to C++. Thanks for reading! Since Aras is using a GTX 1080 TI on Windows, and an Intel Iris Pro on Mac, I think the numbers make sense. In my case, my GTX 770 is pretty old, and my 2017 MacBook is pretty new, so the numbers line up better. > It's a fun exercise in ShaderToy to write a recursive ray tracer. Here's mine: https://www.shadertoy.com/view/XllBRf https://www.shadertoy.com/view/XllBRf > And here's a much better one: https://www.shadertoy.com/view/4scfz4 https://www.shadertoy.com/view/4scfz4 Nice! I should add accumulation to mine -- it'd look much better for barely any effort.
ohf 8y ago
> Yep. I bet this changes relatively soon, since you can sometimes use recursion in OpenCL & CUDA. Am I correct in assuming you can only recurse when the recursion can be "unrolled" into a simple loop? In other words, no program flow structures.
- dahart 8y ago
  You can do real recursion sometimes, and real program flow. It does depend on CUDA version & chipset, but most modern ones support flow control. (https://stackoverflow.com/questions/3644809/does-cuda-support-recursion https://stackoverflow.com/questions/3644809/does-cuda-suppor...) I think there are still cases where it won’t work right due to other libraries that might be involved, I’ve had recursion fail even on a new GPU with cuda 9. Even if you can use recursion, it’s not usually a good idea since you’ll run into thread divergence problems.
aras_p 8y ago
> I'm referring to his Metal timings being more than 10x slower than D3D In my case it's the difference in hardware. DX11 results are on GTX 1080 Ti, whereas Mac are on Iris Pro on 2013 MacBookPro. > 10x performance difference between these GPUs is entirely expected.
- dahart 8y ago
  Totally makes sense; thanks for the explanation!

wolfspider 8y ago

Always love your work! Going to check this one out right away. Yeah GLES and readpixel aren’t friends like for instance GPUImage uses the hardware backbuffer to store it as a workaround: https://stackoverflow.com/questions/12813442/cvopenglestexturecache-vs-gltexsubimage2d-on-ios https://stackoverflow.com/questions/12813442/cvopenglestextu... So it’s device specific to implement the same thing I believe GPUImage uses a stub for this to apply the same shaders you have in mind.

IvanK_net 8y ago

I made a real-time ray-traced game a few years ago, that works in a browser :) You can increase the resolution, while FPS drops. http://powerstones.ivank.net http://powerstones.ivank.net

Const-me 8y ago

> You give it regular C# structures and methods, and it automatically converts them to HLSL, GLSL SPIR-V, and Metal shaders. Code introspection is one of the huge strengths of .NET ecosystem. You can even do it multiple levels of abstraction: bytecode (using e.g. Mono.Cecil), with built-in reflection API, with built-in expressions API since .NET 3.5, and apparently this project uses even higher-level approach, Roslyn. I have never transformed C# code into shaders, when I’m working on something similar I usually prefer traditional GPU languages like HLSL or CUDA, because tooling (e.g. visual studio can debug both). I transformed expressions into NoSQL database queries, worked well. Here’s the main parts: https://github.com/Const-me/EsentSerialize/blob/master/Core/EsentSerialize81/Linq/FilterQuery.cs https://github.com/Const-me/EsentSerialize/blob/master/Core/...

pjmlp 8y ago
While it is great feature that I enjoy a lot, it isn't the only eco-system with it. :)
seanmcdirmid 8y ago
You can also roll your own expression language using operator overloading and then plug that into the DLR or HSL, or whatever as needed (I did this in https://archive.codeplex.com/?p=bling https://archive.codeplex.com/?p=bling). It doesn’t allow for statements, however, which introspection gives you. It is much harder in the new UWP, however, where dynamic features (reflection, introspection, native JIT via the DLR) are no longer guaranteed to be present.
- zamalek 8y ago
  Don't forget that Linq operates over duck-typed extension methods, so you can also define your own wizardry there.
  - seanmcdirmid 8y ago
    Extension methods are important to doing an EDSL in C#. Link syntax is mostly a dud, however.

pjmlp 8y ago

Very interesting reading, we need more article about alternative approaches.

ionafgnio 8y ago

Is there a legitimate reason for D3D12 to exist? It seems like just a locked-down version of Vulkan to me intended to kill cross-platform gaming (textbook Microsoft Embrace, Extend, Extinguish). I haven't heard of any advantages in performance or ease of development. But I'm certainly no expert. Unrelated note: there's something really funky going with the comparison between the desktop and the laptop. The desktop is pretty high-end and should just obliterate that Macbook. It's seriously weird that they're so close. I'm not sure I trust the results.

mellinoe 8y ago
> The desktop is pretty high-end and should just obliterate that Macbook I was also a little surprised at how well the MacBook performed, but it's quite reproducible for me. To be fair, this desktop is actually about 5 years old, and it wasn't really "top of the line" when I built it.
mattnewport 8y ago
There are several good reasons. D3D12 was finalized some time before Vulkan and was available in early / preview versions for quite a while before that. Stable drivers were available much sooner than for Vulkan. It was an easier transition for code bases that already had D3D11 support and Microsoft offered additional code to help with the transition (the D3D11 on 12 stuff). You could bring your existing HLSL shaders over easily. There were better debugging and profiling tools available sooner. The Xbox One got some additional extensions to better utilize its hardware.
pjmlp 8y ago
OpenGL always lagged behind proprietary 3D APIs. Microsoft is not alone, people keep forgetting OpenGL never mattered on game consoles. Even Vulkan is supported on the Swift, the main 3D API is NVN. Some of the benefits of DirectX, proper 3D API, with a toolkit that supports math, fonts, textures, sprites instead of playing "go fish" libraries, OOP API, OOP shading language, nice debugging integrated into visual studio. Same applies to the other 3D APIs. Game developers care about their IP, how it gets into each platform is secondary to that.
zamalek 8y ago