Gray scale function
Grayscale images are defined as follows. The level of gray of each pixel is given by a number between 0 and 255. Thus, there are the total of 256 = 28 levels of gray. These numbers together form the gray scale function of the image. An example is on the right.
Another example with larger (162x205) image is below.
The image on the left is accompanied by its gray scale function. Lighter means higher values and darker means lower values.
Gray scale images are analyzed by thresholding the gray scale function.
For more see Images as functions of two variables.