So these days i'm writing a D3D12/Vulkan abstraction for a project and i've hit a wall tackling resource binding. In an older renderer i wrote, i put all of my per-object uniforms into one big Uniform Buffer/Constant Buffer, copied all the data in one go and bound ranges of it using glBindBufferRange (GL) and XSSetConstantBuffers1 (D3D11) for each object in the scene. It seemed to be a more efficient approach than copying between draws.

The same thing can be done in Vulkan by creating a dynamic uniform buffer and providing Dynamic Offsets to vkCmdBindDescriptorSets.

But when it comes to Direct3D 12, i haven't seen an equivalent approach yet. The only thing that i came up with so far is to create multiple ConstantBufferViews for a Constant Buffer into a Descriptor Table and bind it by adding the appropriate offset to the Descriptor Table's address.

// Get the descriptor increment size
const UINT cbvSrvDescriptorSize = pDevice->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV);

// Handle for the first object's constant buffer view
CD3DX12_GPU_DESCRIPTOR_HANDLE cbvSrvGpuHandle1(pCbvSrvHeap->GetGPUDescriptorHandleForHeapStart(), 0);
pCommandList->SetGraphicsRootDescriptorTable(0, cbvSrvGpuHandle1);

// Draw first object...

// Add offset to get the handle for the second object's constant buffer view
CD3DX12_GPU_DESCRIPTOR_HANDLE cbvSrvGpuHandle2(pCbvSrvHeap->GetGPUDescriptorHandleForHeapStart(), cbvSrvDescriptorSize);
pCommandList->SetGraphicsRootDescriptorTable(0, cbvSrvGpuHandle2);

// Draw second object...

While this approach works fine, it doesn't necessarily match Vulkan's dynamic offset approach, so i can't make a clean abstraction over the two.

So how would you guys go about handling this? And i'd love to know your approaches to abstracting the Vulkan/D3D12 binding model in general.

34 minutes ago, Hodgman said:

When placing dynamic constants in that memory, I can then fetch a pre-created CBV that corresponds to the particular allocation. I place these CBV's in a non-shader-visible descriptor heap, and at binding time I use CopyDescriptors to copy the appropriate CBV's into a dynamic descriptor table, and then bind that single descriptor table to a root argument.

So you're copying per-object constants between draw calls instead of copying everything at once into a single large buffer? And wouldn't copying descriptors in every frame have a lot of overhead?

How do you handle dynamic constants in your Vulkan path?

7 minutes ago, MJP said:

Two other approaches that you can take:

1. Create the CBV descriptor on-the-fly with the desired offset. Creating descriptors is fast, since there's no allocation or heavy resource creation
2. Use a "root" CBV, which let's you pass an arbitrary GPU virtual address. 

I'd recommend #2, since it's easy and cheap. Nvidia also prefers it for various reasons.

I didn't know creating CBV descriptors was cheap. I should give that a try. I doubt i could use CBV's in the Root Signature for everything though, since they take up a lot of space. Now i have to figure out a way to neatly wrap this with Vulkan. Thank you!