Add fog documentation

markdown/landing/index.md

@@ -48,6 +48,7 @@ Documentation and by extension headers/categories are a work in progress.
 - [Customizing Mystery Box Weapons](../mapping/mbox2-format.md)
 - [Advanced Model Exporting](../mapping/model-exporting.md)
 - [Model Alpha Transparency](../mapping/alpha-transparency.md)
+- [Fog](../mapping/fog.md)
 
 ### Map Entities
 - [Barricades](../mapping/barricades.md)

markdown/mapping/fog.md

@@ -0,0 +1,45 @@
+% Fog - NZ:P Mapping Documentation
+# Fog
+
+## Introduction
+
+![Christmas Special's white haze on PlayStation Portable (left) and FTE (right).](../res/images/fog_comparison.webp)
+
+_Nazi Zombies: Portable_ uses it's own 5-parameter, linear fog system. This was originally conceived this way as the PlayStation Portable's hardware fog is *linear* and not *exponential* as is traditional for Quake and OpenGL itself. The Nintendo 3DS also adheres to this standard thanks to [picaGL](https://github.com/masterfeizz/picaGL/tree/revamp)'s linear LUT fog. Other platforms manipulate these fog values to approximate them into standard `exp2` fog modes. Exponential fog is significantly less granular and as a result this approximation will never be spot-on. 
+
+## Parameters
+
+```
+usage:
+    fog (start) (end) (red) (green) (blue)
+```
+*Excerpt from `fog` command in the console.*
+
+When defining fog, you can provide a `start` and `end` distance, in units, of the fog "wall" that appears. The distance between `start` and `end` will impact intensity, and of course, the farther away `start` is from zero, the more visibility you have before the fog begins.
+
+You can change the color of the fog given individual `red`, `green`, and `blue` color values. These values range from `0`-`100`, with `100` being maximum intensity. Both pure black fog with `0 0 0` and pure white fog with `100 100 100` are supported.
+
+As an example, below is the fog output for *Nacht der Untoten*:
+
+```
+current values:
+    "start" is "240"
+    "end" is "1550"
+    "red" is "54"
+    "green" is "55"
+    "blue" is "59"
+```
+
+## Translation
+
+### Linear to Exponential
+
+Platforms that do not support linear fog will algorithmically approximate a decent exponential fog density to use instead. The formula is as follows:
+
+```
+fog_density_gl = ((fog_start / fog_end)/3.5f)
+```
+
+### Exponential to Linear
+
+FTE will still fallback to old `exp2` fog behaviors if provided them, however all other platforms will ignore fog if any of it's provided values are less than zero. This is intended behavior.