Editing Texture mapping (section)

===Restricted camera rotation===
[[File:Freedoom 2018.png|300px|thumb|right|[[Doom engine]] did not permit ramped floors or slanted walls. This requires perspective correction only once per each horizontal or vertical span, rather than per-pixel.]]

The ''[[Doom engine]]'' restricted the world to vertical walls and horizontal floors/ceilings, with a camera that could only rotate about the vertical axis. This meant the walls would be a constant depth coordinate along a vertical line and the floors/ceilings would have a constant depth along a horizontal line. After performing one perspective correction calculation for the depth, the rest of the line could use fast affine mapping. Some later renderers of this era simulated a small amount of camera [[Pitch (orientation)|pitch]] with [[Shear (transformation)|shearing]] which allowed the appearance of greater freedom whilst using the same rendering technique.

Some engines were able to render texture mapped [[heightmaps]] (e.g. [[Nova Logic]]'s [[Voxel Space]], and the engine for [[Outcast (video game)|Outcast]]) via [[Bresenham's line algorithm|Bresenham]]-like incremental algorithms, producing the appearance of a texture mapped landscape without the use of traditional geometric primitives.<ref>"[https://web.archive.org/web/20131113094653/http://www.codermind.com/articles/Voxel-terrain-engine-building-the-terrain.html Voxel terrain engine]", introduction. In a coder's mind, 2005 (archived 2013).</ref>