What is the meaning of 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 really be important for me to change the semantics to two members?

If I need to send a Direct3D structure to the top, why can't it just copy my data like?

If I provided direct3D a vertex with a layout that does not match the layout of the shader, what will happen? for example, if I pass the next vertex to the specified shader?

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

The D3D documentation says that a warning will be prepared and that my data will be "reinterpreted"

Does this mean that "reinterpreted", as in reinterpret_cast <>? for example, will my shader try to use texture coordinates and half color as the color in the shader? or will he look for my vertex layout for an element that matches each semantics and moves the input to the right places to make the shader work?

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

Answer 1

Semantics are used to bind your vertex buffers to shader inputs. In D3D11, you have buffers that are just chunks of memory for storing data in shaders that have an input signature that describes the inputs they expect and input layouts that represent the binding between buffers and shaders and that describe how to interpret the data in your buffers, the role of semantics is to match the elements in the description of the buffer layout with the corresponding shader inputs, the names are not very important if they match.

You need to correctly specify the layout of your vertex data when creating the input layout object. If your input layout does not match the actual memory location of your data, then reinterpret_cast will be effective and you will display garbage. Ensuring the correct semantics match between your input elements and your shader input, however, they will be correctly connected, and such things as the order of the elements do not matter. This is semantics describing how data elements from the vertex buffer should be passed to the input of the shader.