Image is formed by projecting a 3D scene onto a 2D plane. This means understanding image is an underconstrainted inverse (ill-posed) problem.

Image Intensity

The image intensity \(I\) is described by the function \(f\) as follow:
$$
I = f(\text{Illumination}, \\
\quad \text{Surface Orientation}, \\
\quad \text{Surface Reflectance})
$$
This simple model shows that the photon emitted from the light source interact with objects (scattering, reflection, absorption) and reach the camera. In other words, to interpret image intensities, we need to understand radiometric concept (Radiance, Surface irradiance, Surface radiance, Image irradiance) and object reflectance properties (Lambertian, BRDF, BSSRDF).