Subpixel Reconstruction Anti-Aliasing (SRAA)

Subpixel Reconstruction Anti-Aliasing (SRAA) is an additional anti-aliasing technique that restores small image details. It also provides a net speedup and minimal overhead compared to supersampling. It is recommended to combine this technique with TAA for better anti-aliasing quality.

SRAA is used in deferred rendering to achieve high-quality anti-aliasing by sampling geometry at higher resolution than shading, since shading usually changes more slowly than geometry.

There are two stages in SRAA:

1. Depth prepass multisampling generates depth position information at subpixel resolution. Subpixels are subdivision cells of a pixel. Depth prepass samples the geometry in the screen-space and stores samples’ position information in a G-buffer.
2. Reconstruction pass uses the custom filter after the deferred shading to refine the shading results and output a screen-resolution, antialiased frame. In the picture below you can see how the SRAA reconstructs one subpixel. At each geometric sample, all shaded neighbors in a fixed radius are considered and their values are interpolated using our custom filter with weights to restore subpixel details. A neighboring sample with significantly different depth is considered to be across a geometric edge (blue line), and receives a low weight. The filter weights estimate distance between source and target samples by comparing their depth values.

UNIGINE's implementation of SRAA uses a depth threshold method to identify areas of the frame with edges. This way we eliminate the blur problem that comes from applying a smoothing filter to areas with no depth variation. Despite the fact that this approach uses only depth it keeps acceptable quality and represents a good trade-off for the sake of performance.

Pros and Cons of SRAA:
     + Subpixel details and geometry reconstruction.
     - Costs performance.

Subpixel Reconstruction Anti-Aliasing (SRAA) settings are available for adjustment via the Render -> Antialiasing section of the Settings window, along with Fast approXimate Anti-Aliasing (FXAA), Temporal Anti-Aliasing (TAA) and Supersampling settings. The following settings are available when the Custom preset for Anti-Aliasing is selected.

SRAA Settings#

SRAA	Toggles SRAA on and off. Notice Can be controlled by render_sraa console command.
Samples	The number of depth geometry samples per pixels. Greater values enhance the anti-aliasing effect but can cause drops in performance. Notice Can be controlled by render_sraa_samples console command.
Depth Threshold	Depth threshold for edges detection that specifies the area for the SRAA processing. Turn on the debug mode and adjust this parameter in such a way that covers the required edges but at the same time leaves out the unnecessary geometry in the scene. Notice Can be controlled by render_sraa_depth_threshold console command.
Show Debug	Shows the geometry edges smoothed by the SRAA. Notice Can be controlled by render_sraa_debug console command.
Use TAA	Specifies if the SRAA should use the shading sample from the previously rendered frame to achieve correct anti-aliasing. It is recommended to use this option by default. Notice Uses camera jittering, so it works only when the TAA is enabled. Can be controlled by render_sraa_temporal console command.

Known Limitations#