Rendering Depth pass/ Mist pass

1. Set layers you want to render.
2. Turn off all lights.
3. Turn off AO.
4. From Render Shading, turn off Tex, Shad, SSS, EnvMap and Ray Tracing (will speed up render).
5. Select camera and go to settings.
6. Under Display, turn on Mist.
7. Go to World tab.
8. Check Mist on and setup as you need.
9. Drop intensity to 0.
10. Change World horizon color to pure white (or what ever you want).