Question

Explain about Photometric image formation.

Answer 1

The brightness of a pixel is proportional to the amount of light directed by the surface patch which projects on to the pixel. Light reflected from an object is characterized as being either diffusely reflected or specularly reflected. The intensity of diffusely reflected light in all directions is proportional to the cosine of the angle of the incident light and the reflectance or albedo of the surface (Lambert's law). On the other hand, specularly reflected light is reflected only along a direction where it is equal to the angle of incident light. In real life surfaces are a mixture of Lambertian (i.e. diffusely reflecting or satisfying Lambert's law) and specular (from the Latin speculum meaning mirror). An example, sort of canonical in the image processing literature is shown in Figure 1. Each pixel has an intensity .


Figure 1: Showing a digitized image from a camera
A basic operation that one can perform with images is thresholding. This operation consists in determining the maximum and minimum of the intensity values in the image denoted and replacing the pixel intensity by



where



is a parameter used for scaling the threshold



These demos are visualized in
http://robotics.eecs.berkeley.edu/sastry/ee20/index.html for different values of the threshold parameter