Hi, 2008/2/4, Hamish <[EMAIL PROTECTED]>: > > Benjamin Ducke: > > > If we have 3D input points for the TIN construction, then > > > the resulting triangles should be 3D face primitives not > > > areas (of which I assumed they were 2D primitives in the > > > GRASS vector model), shouldn't they? > > Martin Landa wrote: > > AFAIU, face is 3D boundary, kernel is 3D centroid for volume (3D > > corpus, the topological composition of faces and kernel). So > > resulting triangles should be zero-height volumes (volumes > > constracted from one face and kernel). ?? > > > > > We would then also be talking about kernels, not centroids, > > > which should be placed in the 3D geometric center of each > > > traingular face? > > > > yes. Hamish: > For a long time it has been unclear to me, do kernels live in the > middle of each face like an area centroid with 3D coordinates, or is it > for true 3D topology like a single point somewhere within a closed > volume defined by many faces.
AFAIU, kernel as 3D centroid is a single point within volume (defined by many faces). > ie would a cube get a single kernel in the center of it* or six > kernels, one on each face? I guess one single kernel in center. > [*] if kernel-as-center-of-volume that could hold attribute info like > building address or geological mineral type. For kernel-on-face I can't > think of what kind of attribute you would want to attach to a wall > besides something obvious like "north facing". > > Thus I'm leaning to the kernel-as-center-of-volume meaning. I agree with this point. > In that case a TIN describing a DEM might just get a single kernel, in > the middle of XY space and placed below the surface to describe the > interior side. (?) > > But it would probably only be meaningful for a closed TIN "sphere" of > the entire geoid surface. Or just to ignore kernels, for 3d output support only "Output tessellation as a 3D graph (faces), not volumes" ? Martin -- Martin Landa <[EMAIL PROTECTED]> * http://gama.fsv.cvut.cz/~landa * _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
