What is the significance of a semantics and vertex layouts in D3D11?

Question

What is the purpose of semantics?

if I had a vertex layout like this:

struct VS_Input
{
    float4 position : COLOR;
    float4 color : POSITION;
};

Would it actually matter that I reversed the semantics on the two members?

If I have to send Direct3D a struct per vertex, why couldn't it just copy my data as is?

If I provide direct3D with a vertex with a layout that doesn't match that of the shader, what will happen? for example, if I pass the following vertex into the above shader?

struct MyVertex
{
    Vec4 pos;
    Vec2 tex;
    Vec4 col;
};

In the D3D documentation, it said that a warning will be produced, and that my data will be "reinterpreted"

Does that mean "reinterpreted" as in reinterpret_cast<>? like, my shader will try to use the texture coordinates and half of the color as the color in the shader? or will it search my vertex layout for the element that matches each semantic and shuffle the input into the right places to make the shader work?

And if the above is not true, then why does D3D require an explicit vertex layout?

Answer 1

Semantics are used to bind your vertex buffers to your shader inputs. In D3D11 you have buffers which are just chunks of memory to store data in, shaders which have an input signature describing the inputs they expect and input layouts which represent the binding between buffers and shaders and describe how the data in your buffers is to be interpreted. The role of the semantic is just to match elements in the buffer layout description with the corresponding shader inputs, the names are not really important as long as they match up.

It's up to you to correctly specify the layout of your vertex data when you create an input layout object. If your input layout doesn't match the actual layout in memory of your data then it will be effectively like using reinterpret_cast and you'll render garbage. Providing your semantics match up correctly between your input elements and your shader input however they will be correctly bound and things like the order of elements don't matter. It's the semantics that describe how data elements from the vertex buffer are to be passed to the inputs of a shader.