## Splinters

Making a surface mesh breakable clips it with the Tet Mesh. The fracture geometry is very angular and straight. This fine for crystalline materials but not for things made of other materials like wood or bricks. With DMM "splinters" are used to describe detailed fracture geometry. Splinter "cages" are optionally attached to DMM objects to assign splinters to the object after a surface mesh is attached, and this affects how the attached surface mesh is prepared for fracture.

A library of splinter cages is provided as a set of .obj files. Use the "Import..." menu item (under the "File" menu of Maya) to load a splinter cage from an .obj file or simply drag the .obj files into Maya (provided that the .obj export plugin is loaded). Once loaded a splinter cage is positioned and oriented and can be expanded or contracted in any dimension to encase the entire surface mesh for the DMM Object. The splinter cage can be duplicated if it needs to be larger without making it less dense. After placing the resulting copy beside the original too small splinter cage, it must be combined with the original because a DMM Object can have only one single polymesh as a splinter cage.

To associate a splinter mesh with the DMM Object after it is positioned properly, select both the DMM Object and the splinter cage then select "DMM Asset >> Add Splinters to DMM Object" menu item. Alternatively, click on the appropriate shelf icon.
Splinter cages can also be made from a 2D texture map (currently only .bmp format supported). The splinter cage is an extrusion of the pattern found in the texture map. A single layer splinter cage made from a 2D texture map is good for brick walls and glass. For materials that tend to have more depth, like wood, several 2D texture maps can be used to produce multiple layers. The layers are combined into a single mesh, then the result can be attached to a DMM Object as a splinter cage.
Choose "DMM Asset >> Create Splinters from Image..." menu item and pick an appropriate bitmap. A Splinter Cage will be added as a polymesh to the scene. The appropriate icon of the DMM shelf can also be used.

Splinters are seen in the output simulated surface mesh when fracture occurs and only in the area of fracture. Setting the DMM Object node "Start Splintered" attribute as true will show splinters even before fracture occurs.

It's important to realize that, once there are fractures, each involved splinter cell is controlled by one tetrahedron (which is the one with its centroid the closest to the splinter cell centroid). If the splinter cells are much bigger than the tetrahedra, then only a few tetrahedra will end up controlling the splinters cells. This can lead to a visual result that differs quite a lot from the simulation.
In the example below, the splinters are drawn in blue and the tetrahedra are drawn with a semi-transparent red shader, and for better visual differenciation the edges are green. The two results are quite different because the splinter cells are much bigger than the tetrahedra. It is recommended to keep the volume of each splinter cell equal to or smaller than the volume of a few combined tetrahedra. A good rule of thumb is to have about three splinter cells per tetrahedron.

Requirements for Splinter Cages Each cell needs to be watertight Components can overlap and can have gaps between them Components can share faces Each face has to be convex and planar The convex hull of each splinter cell is used for the clipping (except if the cell is an extrusion of a 2D shape such as when getting the Splinters from a bitmap, then the non-convex shape is used for clipping) Can be an instance Notes: When using Splinters, the Input Surface Mesh needs to have it's faces convex and planar. The parts of Surface Mesh sticking outside the Tet Cage won't be clipped off, but the further outside the piece of the surface mesh is, the more it will deform in unexpected ways as the Tet Mesh deforms. A library of 2D and 3D splinters is installed with the plugin in My Documents\DMM Libraries\splinter Although splinter cells may overlap, in regions of overlap extra overlapping faces will be created in the perpared surface mesh. So it is best to keep any overlaps to a minimum.

Next: Maya Forces