Editing Doom engine (section)

===Drawing the walls===
All of the walls in ''Doom'' are drawn vertically; it is because of this that it is not possible to properly look up and down. It is possible to perform a form of look up/down via [https://zdoom.org/wiki/Y-shearing "y-shearing"], and many modern ''Doom'' source ports do this, as well as later games that use the engine, such as ''[[Heretic (video game)|Heretic]]''. Essentially this works by moving the horizon line up and down within the screen, in effect providing a "window" on a taller viewable area. By moving the window up and down, it is possible to give the illusion of looking up and down. However, this will distort the view the further up and down the player looks.

The ''Doom'' engine renders the walls as it traverses the BSP tree, drawing subsectors by order of distance from the camera so that the closest segs are drawn first. As the segs are drawn, they are stored in a linked list. This is used to clip other segs rendered later on, reducing overdraw. This is also used later to clip the edges of sprites.

Once the engine reaches a solid (1-sided) wall at a particular x coordinate, no more lines need to be drawn at that area. For clipping the engine stores a "map" of areas of the screen where solid walls have been reached. This allows far away parts of the level which are invisible to the player to be clipped completely.

The ''Doom'' graphic format stores the wall textures as [[row- and column-major order|sets of vertical columns]]; this is useful to the renderer, which essentially renders the walls by drawing many vertical columns of textures.