Scripting Reference
MonoBehaviour.OnPreRender

function OnPreRender () : void

Description

OnPreRender is called before a camera starts rendering the scene.

This function is called only if the script is attached to the camera and is enabled.

Note that if you change camera's viewing parameters (e.g. fieldOfView) here, they will only take effect the next frame. Do that in OnPreCull instead. OnPreRender can be a co-routine, simply use the yield statement in the function.

See Also: OnPostRender.

JavaScript 
// This script lets you enable/disable fog per camera.
// by enabling or disabling the script in the title of the inspector
// you can turn fog on or off per camera.

private var revertFogState = false;

function OnPreRender () {
    revertFogState = RenderSettings.fog;
    RenderSettings.fog = enabled;
}

function OnPostRender () {
    RenderSettings.fog = revertFogState;
}
using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    private bool revertFogState = false;
    void OnPreRender() {
        revertFogState = RenderSettings.fog;
        RenderSettings.fog = enabled;
    }
    void OnPostRender() {
        RenderSettings.fog = revertFogState;
    }
}
import UnityEngine
import System.Collections

class example(MonoBehaviour):

    private revertFogState as bool = false

    def OnPreRender():
        revertFogState = RenderSettings.fog
        RenderSettings.fog = enabled

    def OnPostRender():
        RenderSettings.fog = revertFogState