Scanline rendering

The scanline algorithm works down the image, one row at a time. An active edge table tracks which polygon edges intersect the current row, and for each row it resolves which surface is frontmost along the line. The technique solved hidden surfaces before the z-buffer became cheap enough, and was formulated by Wylie, Watkins and others in the late 1960s.

The name comes from the cathode-ray tube's sweep: the screen was drawn row by row, so it was natural to compute the image the same way. Unlike ray tracing, which casts a ray per pixel, and modern per-triangle rasterization, scanline works across a whole row at once.