High Performance Graphics (2014) Jonathan Ragan-Kelley and Ingo Wald (Editors) Coarse Pixel Shading

Figure 1: The CITADEL 1 scene, rendered at 2560×1440 with pixel-rate shading (PS) on the left and coarse pixel shading (CPS) on the right, using a coarse pixel size of 2×2. CPS almost halves the number of shader invocations, yet shows few perceivable differences on a high pixel density display, with a structural similarity index (SSIM) of 90.6%. In contrast, an image rendered at 1280×720 and upscaled exhibits blurring at silhouette edges, and lower overall quality, with an SSIM of 86.9%. We present a novel architecture for flexible control of shading rates in a GPU pipeline, and demonstrate sub-stantially reduced shading costs for various applications. We decouple shading and visibility by restricting and quantizing shading rates to a finite set of screen-aligned grids, leading to simpler and fewer changes to the GPU pipeline compared to alternative approaches. Our architecture introduces different mechanisms for programmable control of the shading rate, which enables efficient shading in several scenarios, e.g., rendering for high pixel den-sity displays, foveated rendering, and adaptive shading for motion and defocus blur. We also support shading at multiple rates in a single pass, which allows the user to compute different shading terms at rates better matching their frequency content