Scale

Compile it for Linux:

Checkout as explained here.
Then:

cd fourier
make scale


How to get a scale binary for windows:

Download  a compiled binary here: scale.zip

Example how to use scale:

This picture is 30x30 size. Now scale it up using scale.



Scaled up using this command:

scale -i  scaled30x30.tif -x 100 -y 100

Result (always written to out.tif):



Scaled down again to 30x30:



The point is that scaling up in the frequency domain means extending using zeros in the high frequency parts. If you scale such image down later you just remove the parts that you added before. The picture is exactly the same as before scaling up and down. The process of  scaling up and down is completely reversible. Other processes of image scaling always loose information if you scale up not to multiples of the original size.
However this implementation is not smart enough to keep the relative differences of the colors while doing this.  This could easily be added for anybody who cares.

The Sourcecode for the program is here .

It can be accessed via cvs as it is explained here.

The project sourceforge page is here.