F Digital Images

A simple image formation model

· In a mathematical view, a monochromatic image is a two-dimensional function, f(x, y), where xand yare spatial (plane) coordinates, and the amplitude of f at any pair of coordinates (x, y) is called the intensityor gray level of the image at that point.
· The values of a monochromatic image (i.e. intensities) are said to span the grayscale.
· When x,y, and the amplitude value of f are all finite, discrete quantities, the image is called a digital image.
The function f(x, y) must be nonzero and finite; that is,

0 < f(x, y) < ∞

©Asst. Lec. Wasseem Nahy Ibrahem Page 4
Image Processing Lecture 2

The function f(x, y) is the product of two components: 1) the amount of

source illumination incident on the scene i(x, y) and 2) the amount of illumination reflected by the objects in the scene r(x, y):
f(x, y)=i(x, y)r(x, y) where 0<i(x, y)<∞ and 0<r(x, y)<1.
Note that the equation 0<r(x, y)<1 indicates that reflectance is bounded by 0 (total absorption) and 1 (total reflectance).
The nature of i(x, y) is determined by the illumination source, and r(x, y) is determined by the characteristics of the imaged objects.
As mentioned earlier, we call the intensity of a monochrome image at any coordinates (x_a, y_b) the gray level ( l ) of the image at that point. That is,
l = f(x_a,y_b)

_From the above equations, it is evident that l lies in the range L_min ≤ l ≤ L_max
Where L_min is positive, and L_max is finite.

The interval [L_min , L_max] is called the grayscale. Common practice is to shift this interval numerically to the interval [0, L-1], where l = 0 is considered black and l = L-1 is considered white on the gray scale. All intermediate values are shades of gray varying from black to white.