// Draws a line from "startVertex" var to the curent mouse position.
var mat : Material;
private var startVertex : Vector3;
private var mousePos : Vector3;
startVertex = Vector3(0,0,0);

function Update() {
    mousePos = Input.mousePosition;
    // Press space to update startVertex
    if(Input.GetKeyDown(KeyCode.Space)){
        startVertex = Vector3(mousePos.x/Screen.width, mousePos.y/Screen.height, 0);
    }
}

function OnPostRender() {
    if (!mat) {
        Debug.LogError("Please Assign a material on the inspector");
        return;
    }
    GL.PushMatrix();
    mat.SetPass(0);
    GL.LoadOrtho();
    GL.Begin(GL.LINES);
    GL.Color(Color.red);
    GL.Vertex(startVertex);
    GL.Vertex(Vector3(mousePos.x/Screen.width, mousePos.y/Screen.height, 0));
    GL.End();
    GL.PopMatrix();
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public Material mat;
    private Vector3 startVertex;
    private Vector3 mousePos;
    void Update() {
        mousePos = Input.mousePosition;
        if (Input.GetKeyDown(KeyCode.Space))
            startVertex = new Vector3(mousePos.x / Screen.width, mousePos.y / Screen.height, 0);
        
    }
    void OnPostRender() {
        if (!mat) {
            Debug.LogError("Please Assign a material on the inspector");
            return;
        }
        GL.PushMatrix();
        mat.SetPass(0);
        GL.LoadOrtho();
        GL.Begin(GL.LINES);
        GL.Color(Color.red);
        GL.Vertex(startVertex);
        GL.Vertex(new Vector3(mousePos.x / Screen.width, mousePos.y / Screen.height, 0));
        GL.End();
        GL.PopMatrix();
    }
    void Example() {
        startVertex = new Vector3(0, 0, 0);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public mat as Material

    private startVertex as Vector3

    private mousePos as Vector3

    def Update():
        mousePos = Input.mousePosition
        if Input.GetKeyDown(KeyCode.Space):
            startVertex = Vector3((mousePos.x / Screen.width), (mousePos.y / Screen.height), 0)

    def OnPostRender():
        if not mat:
            Debug.LogError('Please Assign a material on the inspector')
            return
        GL.PushMatrix()
        mat.SetPass(0)
        GL.LoadOrtho()
        GL.Begin(GL.LINES)
        GL.Color(Color.red)
        GL.Vertex(startVertex)
        GL.Vertex(Vector3((mousePos.x / Screen.width), (mousePos.y / Screen.height), 0))
        GL.End()
        GL.PopMatrix()

    def Example():
        startVertex = Vector3(0, 0, 0)