GL.Begin

Parameters

Description

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.

Additional resources: GL.End.

using UnityEngine;

public class Example : MonoBehaviour
{
    // Draws a Triangle, a Quad and a line
    // with different colors

    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); // Triangle
        GL.Color(new Color(1, 1, 1, 1));
        GL.Vertex3(0.50f, 0.25f, 0);
        GL.Vertex3(0.25f, 0.25f, 0);
        GL.Vertex3(0.375f, 0.5f, 0);
        GL.End();

        GL.Begin(GL.QUADS); // Quad
        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); // Line
        GL.Color(new Color(0, 0, 0, 1));
        GL.Vertex3(0, 0, 0);
        GL.Vertex3(0.75f, 0.75f, 0);
        GL.End();
        GL.PopMatrix();
    }
}