![]() This means that if two rays of light hit a viewer in the right configuration, is as if no light at all is received. When the opposite happens, they can literally destroy each other. When two waves are in phase, it means that their peaks and troughs are perfectly aligned: in this case, the resulting wave is amplified. The animation below shows how two simple sinusoidal waves can either amplify or cancel each other out depending on their phase. If two rays of light reach a viewer, the final colour depends on how their waves interact with each other. The more rays a surface emits, the brighter it is. If two rays of light reach a viewer, their intensity is simply added. A result of this massive simplification is that light is subjected to an additive composition. Certain phenomena can only be understood if we accept the fact that light, under certain conditions, behaves like a wave. However, it prevents us from replicating behaviours that many materials exhibit iridescence included. Modelling rays of light as made out of particles is very convenient. You can read more about these effects (and how they are simulated) in Basic Theory of Physically Based Rendering by Morsmoset. This behaviour is often called subsurface scattering and is often computationally very intensive to simulate. This means that a percentage of all the incoming light can be re-emitted by the material surface at any arbitrary point and angle. Light can penetrate the surface of an object, bouncing inside it and finally escaping with a different angle (image above). The diffuse component of a surface also comes from a secondary source: refraction. There is indeed something else which is responsible for this effect. Even if we could achieve a perfectly smooth surface, white marble will still exhibit a white diffuse component. White marble is a good counter example: no level of polish will make it look black. If a surface only exhibits specular reflection, it means that it would look black when fully polished. In this section we have said that diffuse reflection can be fully explained by assuming specular reflection occurring on a surface with misaligned micro-facets. ![]() You can read more about this on the Unity page that explains the Smoothness property available in its Standard Shader. The disalignment of the micro-facets is often modelled by physically based shaders with properties such as “ Smoothness” or “ Roughness“. The presence of those micro-facets scatters rays in all directions, de-facto diffusing the incoming light. One can model a rough surface as made of tiny mirror, each one fully characterised by a specular reflection. ![]() A previous tutorial, Physically Based Rendering and Lighting Models, explains the Lambertian and Blinn-Phong reflectance models used for the diffuse and specular reflections, respectively.ĭespite looking different, diffuse reflection can be explained with specular reflection alone. Most modern engines (like Unity and Unreal) used to model those two behaviours with different sets of equations. This gives objects a uniform, diffuse coloration. When a ray of light hits a diffusive surface, it is scattered more or less uniformly in all directions. In reality, most objects exhibit another type of reflection, called diffuse. Objects rendered with such a technique looks like mirrors. Moreover, if the light comes from the direction L, it can only be seen if the viewer is looking at it from the direction R. This type of reflection is also called specular, which means “mirror-like”. Those surfaces act like ideal mirrors, perfectly reflecting light. Generally speaking, when a ray of light hits a surface, it bounces off with the same incidence angle. Most shading models treat light as made of homogeneous particles, all behaving like ideal billiard balls. The scientific literature often refers to a “ ray of light“, which is a way to indicate the path that photons traverse when travelling through space and interacting with objects. Introduction Reflection: Lights and Mirrors:
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