what is the need of function wavedec2 ???

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Answer 1

[John D'Errico]

There is no need for it at all, if you have no use for it. Personally, I have no need for it, nor would I ever plan to have one, because I have no plans to use wavelet transformations.

http://www.mathworks.com/help/wavelet/ref/wavedec2.html

However, if you wish to decompose a matrix into a wavelet decomposition, then it would seem to be an invaluable tool.

So why not read the documentation for the tool above? If that is insufficient for you, then you need to do some reading about what is a wavelet transformation.