Hey Guys,

We all know that Texture resources are normally swizzled in GPU memory for better cache hit rate, since normally we read texture with neighbor data. In order to see how much this would impact the performance, I write a volume rendering program to profile it.

The program is in DX12, and I modified the Microsoft MiniEngine to build this demo based on it and added GPU profiler and ImGUI. If you run it in Profile and Debug build you should see GPU perf graph on top of the window which timed each GPU tasks. The scene is simple: A dynamic volumetric cube is updated and rendered while it is rotating constantly. every frame the volumetric cube content is updated through a compute shader, and then rendered using raycasting algorithm running on pixel shader. The GUI give you the ability to switch between Typedbuffer volume, StructuredBuffer volume and Texture3D volume each effectively have 4 32bit data for every voxel. For detail information you could look through the code:

https://github.com/pengliu916/Texture3D_StructuredBuffer.git

The result I got is strange:

when the virtual camera is far enough, everything works as expected: StructuredBuffer/TypedBuffer volume have huge variance render time as the cube keeps rotating (since volume data is not swizzled, from some point of view, cache hit rate may be very bad) Texture3D volume render time is almost constant, and always faster than StructuredBuffer/TypedBuffer(which is as expected).

The weird thing is when virtual camera get close to volume, StructuredBuffer/TypedBuffer volume render time still varying, Texture3D volume render time still very stable, but it get much slower as camera get close to volume. Please see this video for the result:

I can't understand why Texture3D volume render time will be much slower than StructuredBuffer/TypedBuffer when camera get close. It will be greatly appreciated if someone could run this test to confirm this is not related to my hardware, and provide some insights why this may happen this way.

Thanks

Peng

Anyone, any ideas, guesses?

Thank Adam for run the test, and it's good to know this maybe related to specific hardware. FYI my test machine has 2 nvidia 680m (sli is disabled) with latest driver (can't check the driver version right now, but I updated the driver yesterday night right before my test run). And I believe I have the latest released windows version.

So any idea why in some hardware the Texture3D performance is strangely related to zoom level?

And one other interesting question I would like to ask is: in this volume update,render case what's the pro and con side of using Texture3D instead of linear layout buffer if built in typed format is all we need? In my machine, I noticed the updating compute shader runs a slight slower on Texture3D (but may not be noticeable in your case)

Thanks

Thanks Adam, I will try 64-bit format later today.

Just one question: from what I know (correct me if I am wrong): For stucturedbuffer/typedbuffer load, GPU will only load 'one voxel' for each PS thread from PS point of view, while Texture3D will load 4(or more) voxels even if I specificity use load function (not a sampler), right? So is that the 'fetch cost' you mentioned when camera is zoomed in? which make Texture3D slower since it fetches more data than StructuredBuffer

Also it seems if your data structure satisfied any of the built-in typed format, Texture should be preferred over buffer, right?

Thanks

Hi Adam,

I have tried the 16bit per channel version. Texture3D version runs almost twice as fast as TypedBuffer version at the default distance, but as I move all the way in, they almost run the same speed. So as you said I definitely hit some HW specific bottleneck (There is no difference between switch between Load or more expensive anisotropic sampler for Texture3D when I zoomed in). But what still confuse me is why Texture3D is more sensitive than TypedBuffer when zoom in (as you said the amount of fetch ops should be the same for both Texture3D and TypedBuffer, so why Texture3D perf drops more than TypedBuffer)?

Sorry for being annoy, I am just a grad student interested in DirectX, want to know what happened under the hood....

Thanks

Peng