Progressive refinement

Progressive refinement is a ray tracing algorithm that quickly reveals coarse structure of an image, and gradually reveals additional detail over time.

The first pixel is rendered as a single rectangle occupying the entire work area. The second through fourth each occupy a quarter of the work area. Sufficient progression will refine the image until the rendered rectangles correspond to a target resolution (for example, a screen resolution).

Rectangles are laid out with an overlap pattern so as to avoid unnecessary rendering.