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Contrast

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Consider an object in a gray scale image. Its contrast measures how different it is from its background [1]. Simply put it's the difference between the max gray level and min gray level of the object. More precise definition is below.

Contrast of a dark object 
  = highest gray level adjacent to it - lowest gray level within it 
Contrast of a light object 
  = highest gray level within it - lowest gray level adjacent to it

For example, the intensity of object #12 is 192 and that of the rest of #11 is 128. Therefore, the contrast of object #12 = 192 - 128 = 64.

The following is a more compact formula:

Contrast = | border's intensity - object's intensity |

The object.

Suppose the curve on the left is the gray level function of the image. Then the tip marked with a horizontal line corresponds to an object. Finally, the height marked in red is the contrast of the object.

The images below show how the number of objects declines as the contrast threshold increases: 0, 30, 50, 70, 125.

Objects start to become visible to the human eye at contrast 5-10.

If we take the average intensity within the object instead of max/min we have the average contrast. The advantage of average contrast is that it is more 'robust'. Adding a single white pixel to a light object may dramatically increase its contrast, but not its average contrast.