Deferred Rendering - Practical Rendering and Computation with Direct3D 11 - page 532

makes them suitable not just for forward rendering, but also for deferred rendering, or
any non-traditional rendering pipelines. They can also be quite a bit cheaper than MSAA,
since MSAA can greatly increase the bandwidth usage and memory footprint (especially
when deferred rendering is used). This also helps screen-space techniques scale better with
higher resolutions. One added benefit from these techniques is that they can be applied af-
ter HDR tone mapping. When an MSAA resolve happens before tone mapping, the quality
often suffers in high-contrast areas, because tone mapping uses a non-linear operator. This
manifests as triangle edges that appear to have no anti-aliasing applied to them (Persson,
Post-tonemapping resolve for high quality HDR anti-aliasing in D3D10, 2008). Moving
the resolve to after the tone mapping step can greatly reduce the artifacts that occur in high
contrast areas.
The primary disadvantage of these techniques is that they typically don't have any
sort of subpixel information available to them, which makes it difficult or impossible to
reconstruct signals for small or thin geometry. It also means that geometry edges can't
snap to subpixel positions, as they can with MSAA, which makes it difficult to alleviate
temporal aliasing artifacts.
Edge Blur
One of the oldest and simplest implementations of screen-space AA involves detect-
ing "edges" in the scene and applying a blur to the pixels (Policarpo & Fonseca, 2005).
Typically, an edge-detection operator such as a Sobel filter is used to detect edges in screen
space, with the detection applied to scene depth and normal vectors. Using normal vec-
tors and depth, rather than color or luminosity, restricts edge detection to triangle edges,
preventing triangle interiors from inadvertently being blurred. Only depth is required to
detect edges at mesh silhouettes, while use of normal vectors is required to detect edges
where triangles intersect. However, even with normal vectors, this approach will still fail
for coplanar triangles that have different material properties.
The obvious downside to this technique is that quality is generally rather poor.
Applying a box filter or Gaussian filter to pixel colors can reduce aliasing artifacts some-
what, but it can also destroy detail in the process.
Morphological Anti-Aliasing
Morphological anti-aliasing (Reshetov, 2009), commonly abbreviated as MLAA, is an-
other screen-space technique that uses sophisticated pattern recognition techniques to iden-
tify triangle edges. Rather than using a simple edge detection operator like Sobel that oper-
ates on a single pixel at a time, the algorithm works by splitting the screen into tiles and
detecting all edge patterns within a tile. These patterns are then used to estimate the actual
triangle coverage for a pixel, which is then used to blend the colors on both sides of the
edges. Since the algorithm reconstructs the actual triangle edges and doesn't just rely on a