Contrast stretching Equation problem

1. Why is contrast stretching unreliable?
2. What happens in contrast stretching?
3. How contrast stretching and thresholding can be achieved?

Why is contrast stretching unreliable?

Contrast Stretching and Histogram Equalisation have the same goal: making the images to use entire range of values available to them. This is why Contrast Stretching is un-reliable, if there exist only two pixels have 0 and 255 intensity, it is totally useless.

What happens in contrast stretching?

Contrast stretching (often called normalization) is a simple image enhancement technique that attempts to improve the contrast in an image by 'stretching' the range of intensity values it contains to span a desired range of values, the full range of pixel values that the image type concerned allows.

How contrast stretching and thresholding can be achieved?

After n repeated applications of the contrast stretching transformation function, on an input image we get an image having only two grey levels :'0' and '255'. This is a Binary Image. Where as when Thresholding is applied once to an input image it gives Binary output Image.