Turns a three-dimensional volume into a flat picture. This is the last stage of every program in the two 3D chapters.
A ray is fired into the volume for each pixel on the screen and walked forward until it hits something. There are two modes, and they are genuinely different methods with different strengths.
Isosurface mode steps along the ray at regular intervals, watching for the point where the volume crosses from empty to solid. Having found the step where it happened, it then narrows down on the exact crossing by repeatedly halving the interval, which pins the surface to well under the size of one cell. That is what gets a smooth surface out of a coarse volume.
Voxel mode instead walks cell by cell in the exact order a straight line crosses a grid, and shades whichever face it came in through. Blocky, and it never misses a thin wall.