Anti Aliasing in DirectX 11

Document Changelog: Created by Daniel Wright.

Overview

UE3 supports full scene anti aliasing in DirectX 11 through MSAA. MSAA is a hardware feature that only shades pixels once, but evaluates the depth test multiple times per pixel.

Enabling MSAA

MSAA is not enabled by default, but can be enabled by setting MaxMultiSamples in the engine ini to a value of 4. Alternatively you can enable MSAA without restarting the editor or game with the command 'scale set MaxMultiSamples 4'. The DirectX 11 renderer only supports MaxMultiSamples values of 1 (no AA) and 4 (4x MSAA).

MSAA with Deferred Shading

The DirectX11 renderer does many operations in a deferred pass, which do not work nicely with MSAA untouched. Each deferred pass (lighting, shadowing, fog, SSS, reflections, etc) has been updated to super sample the shading along geometry edges, which reduces aliasing.

Material Properties

Materials have a property called bEnableMaskedAntialiasing which allows the edges of a masked material created by the Opacity Mask to be antialiased when MSAA is enabled. This is useful for hair and foliage, but comes at a rendering cost.

This image shows magnified character hair without bEnableMaskedAntialiasing on the left, and with bEnableMaskedAntialiasing enabled on the right (4xMSAA for both).

The material blend mode BLEND_DitheredTranslucent implements a different material feature when MSAA is enabled. It causes the Opacity Mask to be evaluated with a dithered pattern that makes it look like transparency, even though the material is actually masked.