mode	Primitives to draw: can be TRIANGLES, TRIANGLE_STRIP, QUADS or LINES.

Begin drawing 3D primitives.

In OpenGL this matches glBegin; on other graphics APIs the same functionality is emulated. Between GL.Begin and GL.End it is valid to call GL.Vertex, GL.Color, GL.TexCoord and other immediate mode drawing functions.

You should be careful about culling when drawing primitives yourself. The culling rules may be different depending on which graphics API the game is running. In most cases the safest way is to use Cull Off command in the shader.

See Also: GL.End.

	// Draws a Triangle, a Quad and a line
	// with different colors
	var mat : Material;
	function OnPostRender() {
		if (!mat) {
			Debug.LogError("Please Assign a material on the inspector");
			return;
		}
		GL.PushMatrix();
		mat.SetPass(0);
		GL.LoadOrtho();
		GL.Begin(GL.TRIANGLES); // Triangle
		GL.Color(Color(1,1,1,1));
		GL.Vertex3(0.50,0.25,0);
		GL.Vertex3(0.25,0.25,0);
		GL.Vertex3(0.375,0.5,0);
		GL.End();
		GL.Begin(GL.QUADS); // Quad
		GL.Color(Color(0.5,0.5,0.5,1));
		GL.Vertex3(0.5,0.5,0);
		GL.Vertex3(0.5,0.75,0);
		GL.Vertex3(0.75,0.75,0);
		GL.Vertex3(0.75,0.5,0);
		GL.End();
		GL.Begin(GL.LINES); // Line
		GL.Color(Color(0,0,0,1));
		GL.Vertex3(0,0,0);
		GL.Vertex3(0.75,0.75,0);
		GL.End();
		GL.PopMatrix();
	}

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.LoadOrtho();
        GL.Begin(GL.TRIANGLES);
        GL.Color(new Color(1, 1, 1, 1));
        GL.Vertex3(0.5F, 0.25F, 0);
        GL.Vertex3(0.25F, 0.25F, 0);
        GL.Vertex3(0.375F, 0.5F, 0);
        GL.End();
        GL.Begin(GL.QUADS);
        GL.Color(new Color(0.5F, 0.5F, 0.5F, 1));
        GL.Vertex3(0.5F, 0.5F, 0);
        GL.Vertex3(0.5F, 0.75F, 0);
        GL.Vertex3(0.75F, 0.75F, 0);
        GL.Vertex3(0.75F, 0.5F, 0);
        GL.End();
        GL.Begin(GL.LINES);
        GL.Color(new Color(0, 0, 0, 1));
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(0.75F, 0.75F, 0);
        GL.End();
        GL.PopMatrix();
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public mat as Material

	def OnPostRender() as void:
		if not mat:
			Debug.LogError('Please Assign a material on the inspector')
			return
		GL.PushMatrix()
		mat.SetPass(0)
		GL.LoadOrtho()
		GL.Begin(GL.TRIANGLES)
		GL.Color(Color(1, 1, 1, 1))
		GL.Vertex3(0.5F, 0.25F, 0)
		GL.Vertex3(0.25F, 0.25F, 0)
		GL.Vertex3(0.375F, 0.5F, 0)
		GL.End()
		GL.Begin(GL.QUADS)
		GL.Color(Color(0.5F, 0.5F, 0.5F, 1))
		GL.Vertex3(0.5F, 0.5F, 0)
		GL.Vertex3(0.5F, 0.75F, 0)
		GL.Vertex3(0.75F, 0.75F, 0)
		GL.Vertex3(0.75F, 0.5F, 0)
		GL.End()
		GL.Begin(GL.LINES)
		GL.Color(Color(0, 0, 0, 1))
		GL.Vertex3(0, 0, 0)
		GL.Vertex3(0.75F, 0.75F, 0)
		GL.End()
		GL.PopMatrix()