Refraction

Refraction is the bending of light as it passes from one medium into another. It is why a spoon in a glass of water looks broken. In rendering, how strongly a transparent material bends light is set by its index of refraction (IOR). Air is very close to 1, water is about 1.33 and ordinary glass is usually taken as around 1.5.

Refraction in architectural scenes

Window glass, glass balustrades, shower screens, aquariums, pool water, crystal chandeliers and carafes all need an engine that handles refraction properly to look convincing. Thick glass and water slightly shift and bend whatever sits behind them, and that distortion is part of what tells the eye a scene is real.

Practical points

• Glass needs real thickness in the model; a single-sided plane will not refract correctly.
• For thin panes such as window glass, many engines offer a “thin glass” option that skips refraction, which cuts render time and is usually good enough.
• For tinted glass, absorption or volume colour is generally used instead of surface colour, so thicker parts look darker.
• Refractive surfaces add noise and may need more samples or a denoiser.

Refraction is closely tied to caustics: light bent by glass and water forms bright patterns on nearby surfaces.