When SV_Position is declared for input to a Pixel Shader, it can have one of two interpolation modes specified: linearNoPerspective or linearNoPerspectiveCentroid, where the latter causes centroid-snapped xyzw values to be provided when multisample antialiasing. 

When used in a pixel shader, SV_Position describes the pixel location. Available for read/write in the vertex shader and geometry shader. The pixel shader can use it as read only to get the pixel center with a 0.5 offset.

透视校正插值（Perspective-Correct Interpolation）