Subdivision surfaces in blacksmith3d4/17/2023 ![]() ![]() ![]() Unfortunately, fillets tend to create many more polygons. When approximating a smooth surface with polygons, it is common to apply a fillet operator onto a sharp edge in order to convert it into a rounded edge. This means that near that feature, no actual subdivision will be applied, and the resulting limit surface will be unchanged from the control mesh. In cases where a sharp feature needs to be preserved, an edge or vertex can be tagged as a crease or a corner. However, due to the nature of triangles, derivatives can be discontinuous from face to face, which may lead to texturing artifacts that require extra effort to solve. Loop subdivision surfaces is a triangle based scheme, which is optimized for control meshes that are entirely triangles: they tend to require less memory than Catmull-Clark scheme.They generally good for most situations and have no special concerns when it comes to texture filtering. It is a quadrilateral based subdivision scheme, and work best with control meshes that is comprised mostly of quads - any non-quad geometry is immediately converted to quadrilaterals on the very first subdivision step. Catmull-Clark subdivision surfaces are the industry standard.RenderMan's implementation of subdivision meshes include two subdivision schemes: In practice, this is usually not a significant problem. Subdivision meshes have some topological constraints in particular, faces cannot have holes, and they cannot represent non-manifold surfaces (surfaces where an edge may be incident to more than two faces - imagine two cubes sharing a single edge).In RenderMan, polygons cannot do this because they don't retain enough data. Subdivision meshes have the ability to perform watertight dicing in order to close potential holes or cracks caused by displacement. ![]() Note that the renderer tries very hard to mitigate cases where subdivision meshes are over-modeled by converting their underlying representation to what is essentially a polygonal representation up front in a "pre-tessellation" step, saving memory by discarding the subdivision data. In that case, the polygonal representation may suffice (i.e. If the asset is significantly over-modeled to begin with, it may be the case that the limit surface may not differ significantly from the control mesh and the extra memory needed to perform the subdivision was wasted. The renderer needs to retain extra memory on a subdivision surface in order to be able to perform subdivision. However, polygons are usually considerably cheaper if the renderer doesn't need to perform any subdivision.In such cases, it is obvious that the subdivision mesh is the better representation. The number of polygons that may be needed to represent a smooth surface may be significantly higher (and require more memory) than the equivalent subdivision mesh.If the asset is meant to be a smooth surface, then the tradeoffs become more complicated: simple geometry with hard silhouettes), then polygons are probably all that are needed. Comparison with PolygonsĪ common question to consider when modeling assets is: should a polygonal asset be rendered as is, or should be it converted to a subdivision surface? If the asset is not smooth in nature (i.e. ![]() Moreover, even if the micropolygons are large due to the micropolygon length setting, their vertices are computed directly from the limit surface for the highest accuracy. Instead, it adaptively tessellates into micropolygons whose size is by default computed in term of pixels on the screen. Unlike many other renderers, RenderMan's implementation of subdivision surfaces does not apply a fixed number of subdivision steps to a polygonal mesh. But where polygonal surfaces require large numbers of points to appear smooth, a subdivision mesh's limit surface is guaranteed to be smooth - meaning that polygonal artifacts or faceting are never present, no matter how the surface animates, or how closely it is viewed. Unlike NURBS, the control mesh is not confined to be rectangular, so in this respect, the control mesh is very similar to polygonal surface. Like most other types of geometry, a subdivision mesh is described by its control mesh. ![]()
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