Step 1. Hold and move the disk (red color) to the direction of the surface normal.
Step 2. Press space bar in keyboard to show the ground truth of the normal direction(in green color).
Step 3. Hold and move the disk (red color) to thd same direction of green disk.
Step 4. Press space bar to continue the practice.
How to enable WebGL:
FireFox: FireFox Enable WebGL
Safari: Safari Enable WebGL