How do I draw in the inspector?

I want to highlight a part of an image in the inspector. So far I'm having a lot of trouble drawing anything properly at all.

I have the following code:

Code (csharp):

1.  
2. import UnityEngine
3. import UnityEditor
4.  
5. [CustomEditor(Thing)]
6. class ThingEditor (Editor):
7.     preview as RenderTexture
8.  
9.     def OnInspectorGUI():
10.         unless preview:
11.             preview = RenderTexture(256.0f, 256.0f, 24, RenderTextureFormat.ARGB32)
12.             preview.Create()
13.  
14.         if Event.current.type == EventType.MouseDown and Event.current.button == 0:
15.             Graphics.SetRenderTarget(preview)
16.  
17.             GL.PushMatrix()
18.             GL.LoadOrtho()
19.             GL.Clear(true, true, Color(0,0,0,0))
20.             GL.Color(Color.red)
21.             GL.Begin(GL.TRIANGLES)
22.             GL.Vertex3(0.25F, 0.3F, 0)
23.             GL.Vertex3(0.25F, 0.1351F, 0)
24.             GL.Vertex3(0.5F, 0.3F, 0)
25.             GL.End()
26.             GL.PopMatrix()
27.  
28.             newPreview = true
29.  
30.         GUI.DrawTexture(GUILayoutUtility.GetRect(64.0f, 64.0f), preview)
31.  
32.         Repaint() if newPreview

The results are as follows, after clicking 0, 1 and 2 times on the inspector window:



Why does it flip the second time I click in the inspector?
I have tried a lot of things to make in flip on the first click, with no success.

http://unity3d.qatohost.com/questions/246853/how-to-draw-in-the-inspector.html

 

The behaviour seems to become undefined if the renderTarget is not reset

Code (CSharp):

1.  
2. var oldTarget = RenderTexture.active; // always null?
3. Graphics.SetRenderTarget (preview);
4.  
5. // ...
6.  
7. Graphics.SetRenderTarget (oldTarget);

 

The finest of necroposts!

 

The answer is targeted at people (like me) who search for this problem and find this thread, which has a solution now.

 

Well, it's also a bad answer. Maybe it'd have been good in 2012, but these days we have the Handles class to draw in the inspector.

 

suggesting a less capable higher-level api doesn't answer the problem.