CS 450: COMPUTER GRAPHICS TRANSPARENT SURFACES SPRING 2015 DR. MICHAEL J. REALE

INTRODUCTION A couple of definitions to get us started: Opaque = cannot see through Transparent = can see through  produces reflected and transmitted light Translucent = transparent, but transmitted light is diffused in all directions Example: frosted glass For transparent surfaces, we’ve just done alpha blending, but a true transparent surface is more complicated…

LIGHT REFRACTION When light hits a transparent surface: Some light is reflected Some light is transmitted (i.e., goes through surface) Angle of incidence = angle from normal that light hits surface Angle between L and N  θ i Angle of refraction = angle from negative normal that light travels through surface Angle between T and –N  θ r HOWEVER, light travels at different speeds through different materials  angle of incidence IS PROBABLY NOT THE SAME as the angle of refraction

LIGHT REFRACTION: SNELL’S LAW The angle of refraction depends on: Angle of incidence (angle of incoming direction of light relative to the normal) Indices of refraction for the incident and refracting materials  η i and η r, respectively Other factors (temperature of materials, wavelength of light, etc.) Index of refraction = (speed of light in vacuum) / (speed of light in material) The relationship between the indices of refractions and the angles of incidence and refraction is given by Snell’s Law:

LIGHT REFRACTION: MORE COMPLEXITIES Refraction can depend on the wavelength of the light (so some frequencies may travel slower or faster, resulting in a different angle) Example: rainbow coming out of a prism! Refraction also may be different depending on the direction  anisotropic materials (e.g., crystalline quartz, calcite) may have double refraction = two refracted light rays are generated

INDICES OF REFRACTION Common indices of refraction: Vacuum = 1.0 Air = about 1.0 (good enough) Heavy crown glass = 1.61 Water = 1.33 Ice = 1.31

LIGHT REFRACTION: BORDERS Refraction occurs at a transition from one material to another So, for, say, a pane of glass, the light ray will be refracted twice: Once entering the glass Once leaving the glass Turns out that the final outgoing light path is parallel to the original but shifted SO…can approximate by shifting light path instead of doing trigonometric functions

GETTING THE DIRECTION OF REFRACTION Using Snell’s law and the N and L vectors:

TRANSPARENCY IN PRACTICE A lot of times refraction is ignored  just use alpha blending Effectively assumes object is very thin  no shift caused by refraction Raytracing can model refraction Shoot ray through material and change direction because of refraction

TRANSLUCENT MATERIALS? Translucent materials a little more complicated, because the light is being scattered as it goes through the object Can be simulated by: Distribute all intensity contributions from background objects over finite area Raytracing Expensive to simulate frosted-glass-room-dividers/amazing-home-interior-decoration-using- swing-door-frosted-glass-room-dividers-along-with-single-glass- interior-door-and-cherry-wood-home-flooring/