// Draw a yellow line from the botton left to the
// top right of the screen
var mat : Material;

function OnPostRender() {
    if (!mat) {
        Debug.LogError("Please Assign a material on the inspector");
        return;
    }
    GL.PushMatrix(); // Save the current state
    mat.SetPass(0);
    GL.LoadPixelMatrix();
    GL.Color(Color.yellow);
    GL.Begin(GL.LINES);
    GL.Vertex3(0,0,0);
    GL.Vertex3(Screen.width, Screen.height,0);
    GL.End();
    GL.PopMatrix(); // Pop changes.
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Material mat;
    void OnPostRender() {
        if (!mat) {
            Debug.LogError("Please Assign a material on the inspector");
            return;
        }
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadPixelMatrix();
        GL.Color(Color.yellow);
        GL.Begin(GL.LINES);
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(Screen.width, Screen.height, 0);
        GL.End();
        GL.PopMatrix();
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public mat as Material

    def OnPostRender():
        if not mat:
            Debug.LogError('Please Assign a material on the inspector')
            return
        GL.PushMatrix()
        mat.SetPass(0)
        GL.LoadPixelMatrix()
        GL.Color(Color.yellow)
        GL.Begin(GL.LINES)
        GL.Vertex3(0, 0, 0)
        GL.Vertex3(Screen.width, Screen.height, 0)
        GL.End()
        GL.PopMatrix()