So, different portions of the screen or different draw passes can have different subsampling rates.įollowing is a table describing which MSAA level is supported with which coarse pixel size some are not supported on any platform, while others are conditionally enabled based on a cap The coarse pixel size can be varied after the render target is allocated. That is to say, a group of pixels can be shaded as a single unit and the result is then broadcast to all samples in the group.Ī coarse shading API allows apps to specify the number of pixels that belong to a shaded group. This is where shading can be performed at a frequency coarser than a pixel. The new model extends supersampling-with-MSAA into the opposite, “coarse pixel”, direction, by adding a new concept of coarse shading. In cases such as such as objects behind HUD elements, transparencies, blurs (depth-of-field, motion, etc.), and optical distortions due to VR optics- a lower shading rate may be desired compared to the rest of the image, but it’s not possible because the shading quality and cost are fixed over the entire image. These two choices don’t provide very fine control. Supersampling causes the pixel shader to be invoked once per sample, at a higher quality but also higher performance cost compared to per-pixel execution.Īpplications can control shading rate by choosing between per-pixel-based execution or MSAA-with-supersampling. The MSAA sample count must be known up front when the target is allocated, and can’t be changed thereafter. The MSAA sample count, which can be 1x, 2x, 4x, 8x, or 16x- governs the number of samples allocated per render target pixel. Without variable rate shading, the only means of controlling shading rate is MSAA with sample-based execution, or supersampling.įor context on what MSAA is- MSAA is a mechanism to reduce geometric aliasing and improve the rendering quality of an image as compared to non-MSAA. Today’s model: Multi-Sample Anti-Aliasing and Supersampling Visually, there are cases where shading rate can be reduced with little or no reduction in perceptible output quality, leading to “free” performance. Variable rate shading, or coarse pixel shading, is a mechanism to enable allocation of rendering performance/power at varying rates across the rendered image. ![]() Motivationĭue to performance constraints, graphics renderers cannot always afford to deliver the same quality level on every part of their output image. This document describes Direct3D 12 exposure of graphics hardware functionality. DirectX-Specs Variable Rate Shading About this Document
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