  A method is presented to include irregular domain boundaries in a geometric
multigrid solver. Dirichlet boundary conditions can be imposed on an irregular
boundary defined by a level set function. Our implementation employs
quadtree/octree grids with adaptive refinement, a cell-centered discretization
and pointwise smoothing. Boundary locations are determined at a subgrid
resolution by performing line searches. For grid blocks near the interface,
custom operator stencils are stored that take the interface into account. For
grid block away from boundaries, a standard second-order accurate
discretization is used. The convergence properties, robustness and
computational cost of the method are illustrated with several test cases.
