DXR Descriptor Heap management for raytracing

Question

After watching videos and reading the documentation on DXR and DX12, I'm still not sure how to manage resources for DX12 raytracing (DXR).

There is quite a difference between rasterizing and raytracing in terms of resource management, the main difference being that rasterizing has a lot of temporal resources that can be bound on the fly, and raytracing being in need of all resources being ready to go at the time of casting rays. The reason is obvious, a ray can hit anything in the whole scene, so we need to have every shader, every texture, every heap ready and filled with data before we cast a single ray.

So far so good.

My first test was adding all resources to a single heap - based on some DXR tutorials. The problem with this approach arises with objects having the same shaders but different textures. I defined 1 shader root signature for my single hit group, which I had to prepare before raytracing. But when creating a root signature, we have to exactly tell which position in the heap corresponds to the SRV where the texture is located. Since there are many textures with different positions in the heap, I would need to create 1 root signature per object with different textures. This of course is not preferred, since based on documentation and common sense, we should keep the root signature amount as small as possible. Therefore, I discarded this test.

My second approach was creating a descriptor heap per object, which contained all local descriptors for this particular object (Textures, Constants etc..). The global resources = TLAS (Top Level Acceleration Structure), and the output and camera constant buffer were kept global in a separate heap. In this approach, I think I misunderstood the documentation by thinking I can add multiple heaps to a root signature. As I'm writing this post, I could not find a way of adding 2 separate heaps to a single root signature. If this is possible, I would love to know how, so any help is appreciated.

Here the code I'm usign for my root signature (using dx12 helpers):

bool PipelineState::CreateHitSignature(Microsoft::WRL::ComPtr<ID3D12RootSignature>& signature)
{
    const auto device = RaytracingModule::GetInstance()->GetDevice();
    if (device == nullptr)
    {
        return false;
    }

    nv_helpers_dx12::RootSignatureGenerator rsc;

    rsc.AddRootParameter(D3D12_ROOT_PARAMETER_TYPE_SRV,0);  // "t0" vertices and colors

    // Add a single range pointing to the TLAS in the heap
    rsc.AddHeapRangesParameter({
        {2 /*t2*/, 1, 0, D3D12_DESCRIPTOR_RANGE_TYPE_SRV, 1},   /* 2nd slot of the first heap */
        {3 /*t3*/, 1, 0, D3D12_DESCRIPTOR_RANGE_TYPE_SRV, 3},   /* 4nd slot of the first heap. Per-instance data */
    });

    signature = rsc.Generate(device, true);

    return signature.Get() != nullptr;
}

Now my last approach would be to create a heap containing all necessary resources -> TLAS, CBVs, SRVs (Textures) etc per object = 1x heap per object effectively. Again, as I was reading documentation, this was not advised, and documentation was stating that we should group resources to global heaps. At this point, I have a feeling I'm mixing DX12 and DXR documentation and best practices, by using proposals from DX12 in the DXR domain, which is probably wrong.

I also read partly through Nvidia Falcor source code and they seem to have 1 resource heap per descriptor type effectively limiting the number of descriptor heaps to a minimum (makes total sense) but I did not jet find how a root signature is created with multiple separate heaps.

I feel like I'm missing one last puzzle part to this mystery before it all falls into place and creates a beautiful image. So if anyone could explain how the resource management (heaps, descriptors etc.. ) should be handled in DXR if we want to have many objects which different resources, it would help me a lot.

So thanks in advance! Jakub

v.oddou · Accepted Answer · 2021-04-21T05:12:37.837

With DXR you need to start at shader model 6.2 where dynamic indexing started to have a much more official support than just "the last descriptor is free to leak in seemingly-looking overrun indices" that was the "secret" approach in 5.1

Now you have full "bindless" using a type var[] : register(t4, 1); declarative syntax and you can index freely var[1] will access register (t5,1) etc.
You can setup register ranges in the descriptor table, so if you have 100 textures you can span 100.
You can even declare other resources after the array variable as long as you remember to jump all the registers. But it's easier to use different virtual spaces:

float4 ambiance : register(b0, 0);
Texture2D all_albedos[] : register(t0, 1);
matrix4x4 world : register(b1, 0);

Now you can go to t100 with no disturbance on the following space0 declarations.
The limit on the the register value is lifted in SM6. It's

up to max supported heap allocation

So all_albedos[3400].Sample(..) is a perfectly acceptable call (provided your heap has bound the views).

Unfortunatly in DX12 they give you the feeling you can bind multiple heaps with the CommandList::SetDescriptorHeaps function, but if you try you'll get runtime errors:

D3D12 ERROR: ID3D12CommandList::SetDescriptorHeaps: pDescriptorHeaps[1] sets a descriptor heap type that appears earlier in the pDescriptorHeaps array.
Only one of any given descriptor heap type can be set at a time. [ EXECUTION ERROR #554: SET_DESCRIPTOR_HEAP_INVALID]

It's misleading so don't trust that plural s in the method name.
Really if we have multiple heaps, that would only be because of triple buffering circular update/usage case, or upload/shader-visible I suppose. Just put everything in your one heap, and let the descriptor table index in it as demanded.

A descriptor table is a very lightweight element, it's just 3 ints. A descriptor start, a span and a virtual space. Just use that, you can span for 1000 textures if you have 1000 textures in your scene. You can get the material ID if you embed it into an indirection texture that would have unique UVs like a lightmap. Or in the vertex data, or just the whole hitgroup (if you setup for 1 hitgroup = 1 object). Your hitgroup index, which is given by a system value in the shader, will be your texture index.

score 1 · Answer 2 · answered Jan 20 '21 at 10:23

Dynamic indexing of HLSL 5.1 might be the solution to this issue.

https://learn.microsoft.com/en-us/windows/win32/direct3d12/dynamic-indexing-using-hlsl-5-1

With dynamic indexing, we can create one heap containing all materials and use an index per object that will be used in the shader to take the correct material at run time
Therefore, we do not need multiple heaps of the same type, since it's not possible anyway. Only 1 heap per heap type is allowed at the same time

DXR Descriptor Heap management for raytracing

2 Answers2