Combining fast grid traversal with local SDF raymarching accelerates scenes where objects occupy sparse grid cells

For scenes with objects arranged in a regular grid (e.g. voxels, rounded boxes), pure SDF raymarching is slow because the marcher must traverse all empty space one SDF-step at a time. A hybrid approach uses fast grid ray-casting (a DDA-style traversal, done with a few additions and integer mask operations) to leap over empty cells, then switches to SDF raymarching only inside a non-empty cell to find the precise surface intersection. If no intersection is found within the cell, control returns to the grid traverser. This dramatically reduces total march iterations for sparse scenes. IQ demonstrated it in Rounded Voxels (2013), combining integer-mask grid traversal with SDF raymarching for the rounded-box detail inside each voxel.

Examples

Pseudocode: while(cell is empty) { advance ray to next grid-cell boundary (DDA step); } then march the SDF inside the non-empty cell until hit or exit.

Assessment

Explain why plain SDF raymarching is slow for sparse voxel scenes, and describe the two-phase hybrid approach that solves it.