Multisample Anti-Aliasing - Multisampling has been enhanced to generalize coverage based transparency and make multisampling work more effectively with multi-pass rendering. 

* To achieve this, all multisample semantics are defined as if the pixel shader always runs once per sample (sample-frequency), computing a separate color per sample. 

* If a pixel shader doesn't use any per-sample attributes, then it will compute the same value for each covered sample in a pixel. 

- In that case, it is equivalent to the hardware executing the shader once per pixel (pixel-frequency), replicating the result to all covered samples. Naturally, running at pixel-frequency always produces the same results as running the same shader at sample-frequency, when the attributes are sampled at a pixel-frequency. 

- The PSInvocations pipeline statistic increments at sample-frequency unless the shader is running at pixel-frequency.

Rasterization Rules - The rules for rasterization have changed for lines, in addition, new functionality has been added.

MultisampleEnable only affects line rasterization (points and triangles are unaffected), and is used to choose a line drawing algorithm. This means that some multisample rasterization from Direct3D 10 are no longer supported.

New sample-frequency pixel shader execution with primitive rasterization.

Attach the SV_SampleIndex semantic to a pixel shader input to invoke a pixel shader at sample frequency, the input value is then used as a sample index when sampling the render target.

You can interpolate a pixel shader input in several ways. To interpolate at:

A pixel center, don't use any semantic.

A sample, use the SV_SampleIndex semantic.

A centroid location, use the _centroid modifier.