Layering

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The key concept for combining hardware with software rendering is layering. It refers to building a final image from multiple sub-render passes. The concept is already common in pure hardware rendering: a surface might be too complex to be rendered in one pass, so a first render pass may lay down the base color, another adds glossy, glow, or fur effects, and a final pass puts highlights on top. Each layer accumulates color in the frame buffer. Newer hardware also permits combining successive layers in ways other than accumulation of the frame buffer by providing a feedback path from the previous layer result to the current layer calculation.

In this document, the term Layering is extended to also cover pre-rendering of shadow maps and and other maps as listed above. Although the result is stored as textures and not accumulated in the image frame buffer, the feedback paths in recent hardware designs are beginning to blur this distinction.

Layering does not necessarily involve rendering the entire scene. It is more common to group sections of the scene by object or by material. Many objects contain multiple materials, and it is fairly expensive to switch materials in the graphics hardware because that may involve reloading of textures, lights, shaders, transformation matrices, and other context information. Reloading is far slower than rendering triangles. For this reason, object sections are normally sorted by material, in addition to normal depth sorting to avoid sending objects that are hidden behind others at all.

Effectively, hardware rendering is a fairly long sequence of rendering separate object portions, many of them multiple times, each time resulting in some form of pixel rectangle. The rendering operations form a dependency graph, with pixel rectangles flowing along the edges of the graph:

Rendering begins at the bottom of the graph because all inputs to a node must be available before beginning to calculate the node. Object 1 consists of two layers, one computing illumination (and hence needing to know which points are in shadow) and the other adding some extra information such as glows. Object 2 is only a single pass illuminated only by one light, but using a chrome reflection model that requires an environment map. Both objects are combined to calculate the final output image. In practice, such graphs are far larger than this trivial example.

The design goal of hardware support in mental ray 3.3 is that some of the graph nodes are computed in software, and others in hardware. For example, to add global illumination to the above graph, a software node would compute the global illumination map (using final gathering or photon mapping), then another software node would create a light map to bake the indirect light into a texture, which is then used by a hardware node to add to the direct light contribution computed by hardware shading:

The shaded nodes are software nodes; the unshaded nodes are hardware nodes. Note that it is often possible, as in the case of global illumination maps, to compute the map once and re-use it for multiple frames.