I happened to stumble across this interesting program.  It claims to produce excellent image enlarging capabilities using it's patented S-Spline XL resampling algorithm - so I decided to compare it against traditional resampling methods.

Just For your information, most applications, such as Internet Explorer, MS Office, PSP etc use a Bilinear filter when resizing images.  A Bilinear filter is pretty much a simple blur.
Typically, Lanczos is considered to be the best resampling algorithm, however, it's rather slow compared to other methods.


Here's my test results - enlarging an image by a factor of 3.

Sample Image:


No resampling (pixel resize / nearest neighbour):


Bilinear resampling:


Bicubic resampling:


Lanczos resampling:


S-Spline XL resampling:



Wow, S-Spline XL looks really good actually :P  Only if you could use it more...

If you tend to upscale pictures, PhotoZoom ain't free though.  You can get a trial version here - a serial number ain't hard to find though :P
PhotoZoom comes with a Photoshop plugin as well.


Oh, and some random downscaling tests:

No resampling:


Bilinear resampling:


Lanczos resampling:


S-Spline XL resampling: