Scanline rendering is an algorithm for visible surface determination, in
3D computer graphics, that works on a row-by-row basis rather than a
polygon-by-polygon or
pixel-by-pixel basis. All of the polygons to be rendered are first sorted by the top y coordinate at which they first appear, then each row or
scanline of the image is computed using the intersection of a scanline with the polygons on the front of the sorted list, while the sorted list is updated to discard no-longer-visible polygons as the active scan line is advanced down the picture.