Online BRDF Visualization
A bidirectional reflectance distribution function (BRDF) is a function which defines how incoming light $\omega_i$ is reflected to every outgoing direction $\omega_o$ on the hemisphere $f_r(\omega_i, \omega_o)$. A BRDF can be measured by capturing a surface from various known view and light positions. The information can then be stored in a texture where the x and y coordinates represent the corresponding view and light direction. This is mostly only done for materials that do not vary over the surface due to the information density. Therefore, often analytical BRDF models are used. Parameters are then used to build a function that can be queried for all incoming and outgoing directions. One popular model is the Cook-Torrance model, which parametrizes the BRDF with a diffuse and specular color plus a roughness value. The model is split into a diffuse and specular lobe. Below is an interactive visualizer, where you can change the diffuse, specular, and roughness parameters.
Interpolating between random BRDFs is also quite soothing:
If you want to use this interactive visualizer for teaching, feel free to use it: link.