"The two previous 3D games Hovertank and Catacombs 3D were done in an object space rendering where it drew limited polygons. They were one-dimensional in terms that they were just line segments that were restricted on axises. We had something that resembled a polygon rasterizer and a polygon clipper and those were both done in four to six weeks a piece. I had really quite a bit of difficulty with it. Going back in time twenty years, there weren't all of the references in existence, books and tutorials on this subject. I was having a hard time getting some of that stuff to be as robust and reliable as it needed to be. You could get a few freak out cases in Catacombs 3D and one of my real goals was to simplify it enough that it would be really rock and solid."
--John Carmack, "Wolfenstein 3D - Director's Commentary"
Rasterisation (or rasterization) is the task of taking an image described in a vector graphics format (shapes) and converting it into a raster image ( A series of pixels, dots or lines that when they come together on a display, they recreate the image). The rasterised image may then be displayed on a computer display, video display or printer, or stored in a bitmap file format. Rasterisation may refer to either the conversion of models into raster files, or the conversion of 2D rendering primitives such as polygons or line segments into a rasterized format. The latter is used to convert the polygonal vector world created for Catacomb 3D and present it as a pixel display that matched with the raster sprites used for game objects.
This approach was later replaced by raycasting in the engine's successor, first used in Wolfenstein 3D (1992). Compared with other rendering techniques such as ray tracing, rasterisation is extremely fast. However, rasterization is simply the process of computing the mapping from scene geometry to pixels and does not prescribe a particular way to compute the color of those pixels. Shading, including programmable shading, may be based on physical light transport, or artistic intent.