The principle objective of enhancement techniques is to process a given image so that the result is more suitable than the original image for a specific application. The work " specific " is important because enhancement techniques are to a great extent problem oriented. The methods used for enhancing x-ray images may not be suitable for enhancing pictures transmitted by a space probe.
Spatial domain method: refers to the image plane itself and methods in this category are based on direct manipulation of the pixels an image.
Frequency domain techniques
Let
be an image formed by the convolution of an image 
and a position invariant operator 
i.e.
From convolution theorem, we have
where G, H, F are F T of g, h and f respectively.
The transform 
is referred to as the transfer function of the process.
Note: We are interested in the frequency domain relation because discrete convolution is often more efficiently carried out in the frequency-domain via an FFT algorithm. In discrete convolution wraparound error (aliasing) can be taken care of by assuming the functions to be periodic with periods chosen in a specified way. In an image, wraparound error gives rise to distortion around the edges. In practice this error is tolerable, even when images are not extended as required.
In a typical image enhancement problem is given and the goal after computation of 
is to select 
so that the desired image given by,
exhibits some high lighted features of .For eg. edges in can be accentuated by using a function 
which emphasizes the high frequency components of 
.
