====== World Layout ======

===== Coordinate System =====

The engine uses a right-handed coordinate system, with $X$ to the right, $Y$ down, and $Z$ forward.

The mapping between OpenGL and the engine's coordinate system is $(x, y, z)_\mathrm{gl} = (x, -y, -z)$.

==== Units ====

The most important "metering" in the engine is the //sector size//, which defines a length of $1024$ units,
and approximately equals two meters in the real world. 

Another significant, but less important, metering is the //click//, which is a quarter sector size ($256$ units).
It is used to define floor and ceiling slants, and is also a "rule of thumb" 

==== Sectors ====

The whole engine world is divided into //sectors//.  A sector (which defines a $1024 \times 1024$ rectangular area)
defines
  * ceiling and floor layout,
  * behaviour, and
  * the adjoining room for that sector.

===== Angles =====

{{:engine:angle.png?nolink |}}

"AU" is short for "Angular Unit". $1$ AU equals the $2^{16}$th of a full circle and ranges from $-2^{15}$ ($-32768$) to
$2^{15}-1$ ($32767$), representing the interval $[-180^°; 180^°[$.

The conversion functions are:
\\
$\varphi_{\mathrm{rad}} = \varphi_{\mathrm{deg}} \cdot \pi / 180 = \varphi_{\mathrm{AU}} \cdot \pi / 2^{15}$
\\
$\varphi_{\mathrm{deg}} = \varphi_{\mathrm{rad}} \cdot 180 / \pi = \varphi_{\mathrm{AU}} \cdot 180 / 2^{15}$
\\
$\varphi_{\mathrm{AU}} = \varphi_{\mathrm{deg}} \cdot 2^{15} / 180 = \varphi_{\mathrm{rad}} \cdot 2^{15} / \pi$

==== YPR Rotation ====
The engine uses Euler angles to express rotations, and applies them in ZXY order.


===== Rooms & Portals =====

The world is divided into so-called //rooms//, which are basically subdivisions of the whole level.
A room may be physically incomplete, i.e. a large physical room may be artificially split into two engine rooms.
Each room is connected to its direct neighbors through //portals//.  The engine uses these visibility portals
to determine which room should be rendered and which not.

See https://en.wikipedia.org/wiki/Portal_rendering for a more in-depth explanation.