A method preserves shapes in a solid model when distributing material during topological optimization. A 3D geometric model of a part having a boundary shape is received. The geometric model is pre-processed to produce a variable-void distance field and to produce a frozen distance field representing the boundary shape. The geometric model is apportioned into a plurality of voxels, and a density value is adjusted for each voxel according to an optimization process. An iso-surface mesh is extracted from the voxel data, and an iso-surface distance field is generated from the extracted iso-surface mesh. A distance field intersection is derived between the iso-surface distance field and the variable-void distance field. A distance field union is performed between the distance field intersection and the frozen distance field, and a result iso-surface mesh is produced from the distance field union.