There are three types of sampling methods in Image Processing:

Ideal Sampling

In ideal sampling (Instantaneous sampling) pulses from the analog signal are sampled. This is an idea sampling method and cannot be easily implemented.

Natural Sampling

Natural sampling is a practical method of sampling in which pulse have finite width equal to T. The result is a sequence of samples that retain the shape of the analog signal.

Flat Top Sampling

In comparison to natural sampling, flat top sampling can be easily obtained. In this sampling technique, the top of the samples remains constant by using a circuit. This sampling method is commonly used.