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## Choosing a MIR algorithm

VisIt currently provides three MIR algorithms: Tetrahedral, Zoo-based, and Isovolume. Each MIR algorithm reconstructs the interfaces between materials using a different method and one method may work better or worse than another based on the complexities of the input data. You can select your preferred MIR algorithm by choosing from the Algorithm combo box in the Material Reconstruction Options Window. Note that if you have plots that have already been generated, the new material options will not take effect for those plots unless you clear the plots and redraw them.

The Tetrahedral algorithm breaks up each mixed cell into tetrahedra and computes the interfaces through the original cell by recursively subdividing the tetrahedra until the approximate volume fractions, which determine the amount of material in a cell, are reached. The Tetrahedral MIR algorithm results in a high cell count so it is not often used.

Figure 16

The Zoo-based MIR algorithm breaks up mixed cells into elements based on supported finite elements (tetrahedra, prisms, pyramids, wedges, cubes). The resulting reconstruction results in far fewer cells than other methods while also producing superior material boundaries. The Zoo-based algorithm is the default because of the quality of the material boundaries and because the zoo-based cell representation saves memory and ultimately leads to faster pipeline execution due to the smaller cell count.

Figure 17

The Isovolume algorithm computes an isovolume containing portions of cells that contain a user-specified fraction of materials. The Isovolume approach to MIR does not generally produce very good looking results since there are gaps where several materials join. However, the Isovolume algorithm does do a better job than the other two algorithms when it comes to finding cells that contain very small fractions of a certain material when the cells are heavily mixed. If you use the Isovolume MIR algorithm, you can specify the amount of material required to be present before VisIt creates a material interface for a material. The amount of material is specified as a volume fraction in the range [0,1]. Specifying smaller values in the Volume Fraction for Isovolume text field will find materials that may be omitted by other MIR algorithms.