The mesh is created in three steps based on the detailed criteria: initial quads (estimated to roughly meet the criteria), refinement (subdivision to meet the criteria), and adjustment for trim boundaries.
Surfaces are meshed in a two step process. First a regular quad mesh is created and then that mesh is refined by splitting some quads into 4 smaller quads. The Maximum aspect ratio, Maximum edge length, and Minimum initial grid quads settings control the generation of the initial mesh. The Density (Rhino 4 only), Maximum angle, Maximum edge length, Minimum edge length, and Maximum distance, edge to surface settings determine which initial quads get split up into smaller quadrangles.
This is a new setting in Rhino 4. Uses a formula to control how close the polygon edges are to the original surface. Values between 0 and 1. Larger values result in a mesh with a higher polygon count. A very useful setting to use.
Smaller values result in slower meshing, more accurate meshes, and higher polygon count. The Maximum angle is the maximum allowable change between the surface normal at any point and the mesh vertex.
Geek Speak: The maximum angle between surface normals at neighboring vertices is less than or equal to Maximum angle. Two vertices are neighbors if they are at the ends of a facet edge.
What it means to you: When you are meshing surfaces that have flat and wiggly regions and you want fewer triangles in flat regions and more triangles in wiggly regions, then use max angle and set maximum edge length=0.
Surfaces are initially tessellated with a regular quadrangle mesh and then that mesh is refined. The initial quad mesh is constructed so that on average, the maximum aspect ratio of the quads is less than or equal to Maximum aspect ratio.
Smaller values result in slower meshing and higher polygon count with more equilateral polygons. This is approximately the maximum aspect ratio of the quads in the initial mesh grid. Setting Maximum aspect ratio to zero turns off the option. Zero means no limit. The default value is 6 and the suggested range is from 1 to 100. Scale independent.
The value is always in the current unit system.
Bigger values result in faster meshing, less accurate meshes and lower polygon count. Setting Minimum edge length to zero turns off the option. Limits the size of the polygon edges. Default is 0.01 and the usable range depends on the size of the model. Scale-dependent.
The maximum length of an edge will be less than or equal to Maximum edge length.
Smaller values result in slower meshing and higher polygon count with more equally sized polygons. Setting Maximum edge length to zero turns off the option, which is the default. When the Refine is checked, polygons are refined until all polygon edges are shorter than this value. This is also approximately the maximum edge length of the quads in the initial mesh grid. This option is scale dependent and can be used for making sure the polygons are approximately the same size.
The maximum distance from the center of an edge to the surface will be less than or equal to Maximum distance, edge to surface.
Smaller values result in slower meshing, more accurate meshes, and higher polygon count. Setting Maximum distance, edge to surface to zero turns off the option. The default value is zero and the usable range depends on the size of the model. When the Refine is checked, polygons are refined until the distance from a polygon edge midpoint to the NURBS surface is smaller than this value. This is also approximately the maximum distance from polygon edge midpoints to the NURBS surface in the initial mesh grid. This option is scale dependent and can be used as a general polygon mesh tolerance setting.
Minimum number of quads in the initial mesh.
Bigger values result in slower meshing, more accurate meshes and higher polygon count with more evenly distributed polygons. Setting Minimum initial grid quads to zero turns off the option, which is the default. Forces simple geometry to be meshed with more polygons. Zero means no minimum. The default is zero. Scale independent. This is the number of quads per surface in the initial mesh grid. In practice, Rhino will use at least this many polygons for each surface. This option is scale independent and can be used for making sure that surfaces with very subtle details are meshed with high enough polygon count.
If refine is not checked, the initial quadrangle mesh is returned.
The mesh is refined until the angle between surface normals along a polygon edge is smaller than this value. The default is 20 degrees and the suggested range is from 5 to 90 degrees. Setting Max angle to zero turns off the option. Scale independent.
No refinement results in faster meshing, less accurate meshes, and lower polygon count. Clearing this check box also means untrimmed individual surfaces and surface areas away from trim edges and joined edges are meshed with evenly sized quadrangles. When this Refine is checked, after initial meshing, Rhino uses a recursive process to refine the mesh until it meets the criteria defined by Maximum Angle, Minimum edge length and Maximum edge length and Maximum distance, edge to Surface options.
If checked then meshes for each surface in a polysurface do not necessary meet to form a watertight mesh. If not checked, then water tight meshes are created.
Causes dramatically faster meshing, lower polygon count and cracks between joined surfaces in the rendered image. By default, Jagged seams is not selected. This options means that all surfaces are meshed independently and the meshes of joined surface edges are not stitched together.
If simple planes is checked, then the above settings, except Jagged seams, are ignored for planar surfaces and the planar surface is meshed with as few polygons as possible.
Causes slower meshing and minimum polygon count on planar surfaces. Meshing can be slower especially for complex trimmed surfaces. By default, Simple planes is not selected. This option means all planar surfaces are meshed by meshing the surface edges and then filling the area bounded by the edges with triangles.
Welds coincident mesh vertices that came from seams between tangent surfaces of a polysurface.
In general, it is the target application and not the file format that matters. In general, Weld should not be checked; especially if you are exporting to an application that cares about texture coordinates or accurate vertex normals. If you are exporting to an application that has trouble with coincident vertices then check Weld. For simple formats like STL (which has no vertex normals and no texture coordinates), welding does nothing.
When polysurfaces are meshed, the packed texture coordinates are created. A packed texture is a partition of the unit square into disjoint sub-rectangles so that one bitmap can be used to apply independent textures to each face of the polysurface.
The Pack Textures option will pack the mesh textures of every mesh and render mesh in the selection set. If more than one object is selected, the packing "spreads" the texture over all the objects.
Previews the mesh that results from the current options.
The Polygon Mesh Options or STL Mesh Export Options dialog box appropriate for the action appears. These simple controls are an alternate way to control the way the mesh is made.