Update 2025-05-03-Microgeometry.md

Author: Assassin-plus

_posts/2025-05-03-Microgeometry.md

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 # Microgeometry
 
+As we discussed earlier in Section 9.1.3, surface irregularities much smaller than a pixel cannot feasibly be modeled explicitly, so the BRDF instead models their aggregate effect statistically. For now we keep to the domain of geometrical optics, which assumes that these irregularities either are smaller than a light wavelength (and so have no effect on the light's behavior) or are much larger. The effects of irregularities that are in the "wave optics domain" (around 1?100 wavelengths in size) will be discussed in Section 9.11.
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+Each visible surface point contains many microsurface normals that bounce the reflected light in different directions. Since the orientations of individual microfacets are somewhat random, it makes sense to model them as a statistical distribution. For most surfaces, the distribution of microgeometry surface normals is continuous, with a strong peak at the macroscopic surface normal. The "tightness" of this distribution is determined by the surface roughness. The rougher the surface, the more "spread out" the microgeometry normals will be.
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+The visible effect of increasing microscale roughness is greater blurring of reflected environmental detail. In the case of small, bright light sources, this blurring results in broader and dimmer specular highlights. Those from rougher surfaces are dimmer because the light energy is spread into a wider cone of directions.
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