Inputting Position Into the Pixel Shader

Question

In most of the programs I have seen that make use of vertex position data in the Pixel Shader, there is a tendency to process it as a float4 vector. This restriction does not appear to be present fin the other shaders. In the program that I am currently writing, for instance, float2's are inputted into the VS and float3's into the GS with no problem. But when I try to input this data into the PS, it rejects all forms except for float4. Are other vector types not allowed into the PS? If so, why?

Answer 1

In a pixel shader, the SV_Position is a system-generated value which must be a float4. When you use the SV_Position semantic in a vertex shader, it's basically just an alias for the old POSITION semantic and comes from the Input Assembler in whatever format the Input Layout specifies. The binding between a vertex and geometry shader has to agree, but can be whatever value.

In other words, it has a special meaning for a pixel shader because it's the pixel position as computed by the rasterizer stage.