Implementation

Be sure to read the Implementation page for DirectX Tool Kit for DirectX 11 as those topics also apply to DirectX Tool Kit for DirectX 12:

Naming conventions
The pImpl idiom
Calling-conventions
Default constructors/assignment operators
DirectXMath Parameter Conventions

SAL annotation

The DirectX Toolkit library makes extensive use of SAL2 annotations (_In_, _Outptr_opt_, etc.) which greatly improves the accuracy of the Visual C++ static code analysis (also known as PREFAST). The standard Windows headers #define them all to empty strings if not building with /analyze, so they have no effect on code-generation.

The main thing to note about Direct3D 12 is that the types D3D12_GPU_DESCRIPTOR_HANDLE and D3D12_CPU_DESCRIPTOR_HANDLE are actually implemented as structs, so SAL pointer-style annotation doesn't apply to them (_In_, _In_opt_, etc.).

Root Signatures

Rather than trying to share a single root signature across all shaders in the DirectX Tool Kit, the IEffect::Apply will call both SetGraphicsRootSignature and SetPipelineState as well as other required parameters. The overhead of resetting the root signature is minimal for the greatly improved usability and maintainability of the library.

Another design decision here is to use distinct SetGraphicsRootDescriptorTable entries for each texture and sampler descriptor. While the Direct3D 12 root signature allows you to create ranges for the tables, it means that all such descriptors must be continuous in the heap. Rather than impose this requirement on the application-level API, we use multiple tables.

All shaders have HLSL-described root signatures. While this isn't strictly for PCs since the driver already has to compile the IL into vendor-specific microcode so it can accommodate changes to the root signature, this is a significant performance hit on Xbox One XDK apps.

Samplers

When creating root signatures you have the choice of using static samplers built into the signature, or you must provide a sampler descriptor and set the sampler heap at render time. For SpriteBatch we support both a static-sampler mode and a sampler heap descriptor mode for ease of use, and the fact that this feature only added a couple of shader combinations. For Effects generally, we require sampler heap descriptors which must be set in addition to the texture heap descriptor to render.