• 03/25/2021
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DirectX12 Sampler Feedback

Sampler Feedback is a hardware-accelerated feature introduced in DirectX12.2 and supported by Xᵉ-LP and later. Conceptually, it is the reverse of texture sampling: the shader Sample() intrinsic reads a number of texels from a texture and returns an average value. The new WriteSamplerFeedback() shader intrinsic writes to a binary resource "marking" the texels that would have been read. This enables two important usages:
  1. Sampler Feedback Streaming
    : when objects are drawn, we can simultaneously collect data about what texture data was required to draw the scene. By dynamically streaming only the visible portions of resources just-in-time, we can draw a scene that accesses more textures than could simultaneously fit in physical graphics memory, if they all had to be entirely physically resident. Sampler Feedback includes a "min mip map" feature to facilitate this usage: a single-layer texture where each texel represents a region of the streaming texture. Each texel is an integer (byte) representing the minimum mip that was sampled in that region. If the sampler feedback region size is set to match the tile size of a reserved resource, the result is a map that informs what tiles to stream. Min-mip-maps are very small, e.g. for a 16kx16k BC7 reserved resource, the corresponding sampler feedback min-mip-map with region size 256x256 is only 64x64 bytes or 4KB in size: much smaller than the 350MB texture it represents.
  2. Texture Space Rendering
    : Texture-Space Rendering was pioneered by Intel® in a Siggraph 2014 research paper [Clarberg et al. 2014]. At that time, we observed that the cost of pixel shading is tightly coupled to both the geometric complexity and the screen resolution which has only increased over time as workloads enable richer visual environment using tessellation, complex lighting calculations using pixel shading and higher resolution displays. Instead of shading directly through traditional pixel shaders, the application uses Sampler Feedback to mark a mask which corresponds to the texels it requires. This mask can then be referenced in a subsequent compute shader pass and the required texels can be shaded at a resolution and frequency determined by the developer in texture space as opposed to a fixed screen space resolution and frame frequency. Once shading to texels is completed, in a separate lighter weight pass, pixels in screen-space are mapped into texture space, and the corresponding texels are sampled and filtered using standard texture lookup operations.
    This feature allows pixel shading to be largely independent of the geometric complexity or screen resolution and adds new texture LOD methods to control level of shading detail. These abilities are introduced to temporally decouple the shading pipeline from the traditional sample, pixel or coarse shading rates. AMFS can provide sizable gains over traditional pixel shader approaches as shader complexity, geometry or MSAA samples increases.

Product and Performance Information

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Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.