Smallpt Rewritten with massive speed up and easier to understand.
Tent Filter was expected to give a smoother image, but apparently it don't.
onb used in so many projects.
1.https://backend.orbit.dtu.dk/ws/portalfiles/portal/126824972/onb_frisvad_jgt2012_v2.pdf
onb Revisited
2.https://graphics.pixar.com/library/OrthonormalB/paper.pdf
3.Realistic Ray Tracing Peter Shirley.
4.Smallpt by Kevin Beason.