Instance vs Loops in HLSL Model 5 Geometry Shaders

Question

I'm looking at getting a program written for DirectX11 to play nice on DirectX10. To do that, I need to compile the shaders for model 4, not 5. Right now the only problem with that is that the geometry shaders use instancing which is unsupported by 4. The general model is

[instance(NUM_INSTANCES)]

void Gs(..., in uint instanceId : SV_GSInstanceID) { }

I can't seem to find many documents on why this exists, because my thought is: can't I just replace this with a loop from instanceId=0 to instanceId=NUM_INSTANCES-1?

The answer seems to be no, as it doesn't seems to output correctly, but besides my exact problem - can you help me understand why the concept of instancing exists. Is there some implication on the entire pipeline that instancing has beyond simply calling the main function twice with a different index?

Answer 1

With regards to why my replacement did not work: Geometry shaders are annotated with [maxvertexcount(N)]. I had incorrectly assumed this was the vertex input count, and ignored it. In fact, input is determined by the type of primitive coming in, and so this was about the output. Before, if N was my output over I instances, each instance output N vertices. But now that I want to use a loop, a single instance outputs N*I vertices. As such, the answer was to do as I suggested, and also use [maxvertexcount(N*NUM_INSTANCES)].

To more broadly answer my question on why instances may be useful in a world that already has loops, I can only guess

• Loops are not truly supported in shaders, it turns out - graphics card cores do not have a concept of control flow. When loops are written in shaders, the loop is unrolled (see [unroll]). This has limitations, makes compilation slower, and makes the shader blob bigger.
• Instances can be parallelized - one GPU core can run one instance of a shader while another runs the next instance of the same shader with the same input.