Additional opaque data structure used in hardware raytracing that subdivides a triangle further into “micro-triangles.”
Its main purpose is to store fine grain information1 about the alpha values within a triangle. This enables better information and clearer choices about which shaders to invoke.
Most triangles only contain regions that are either fully opaque or fully transparent.2 With the usage of OMMs, ray queries can therefore immediately be resolved to a miss or a hit, reducing or removing the need to fall back to a AnyHit shader.
What made the usefulness of OMMs immediately obvious to me is this screenshot of PIX’s acceleration structure viewer. Greenery like leaves have a lot of complicated detail only resolved by transparency, and it’s usually either “part of leaf” or “nothing, just air.” Providing OMMs for these triangles will definitely make ray queries against them faster.
Footnotes
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Limited to 1 or 2 bits in DXR. See DirectX-Specs > Raytracing > OMM encoding. ↩