Resize filters.

[Neil]

I use a thumbnail creator to post my pics to my personal web page. I re-size the pictures and was wondering what the various (11) resize filters did.

The filter that supposedly gives the best quality but is slowest is Lanczos3. What does all this mean?

 

[DougPaul]

When you downsample an image, you cannot just take a subset of the the pixels. (Well, you can but you will get inferior results...) For example, let us downsample an image by a factor of 16 by selecting every 16th pixel. If there is a picket fence in the picture (or a similar repeating structure), the selected pixels might miss the fence boards and show only background. Thus the fence might disappear. More likely, some of the pixels might hit the fence and some might miss. If the spacing of the pixels and the spacing of the fence boards have certain relations (eg the fence repeats every 15 pixels), you will get a moire pattern.

Basically, you must have a certain pixel density to show a certain amount of detail and if you do not have that density the picture will be degraded. (This process is called aliasing in signal processing.) To prevent aliasing, you must smooth the picture (remove the fine detail) before downsampling. And if you downsample by on non-integral factor, you must also interpolate to estimate values between the pixels. There are a number of algorithms for smoothing and interpolating--which ones you choose will affect the quality of the downsampled image.

A reference comparing a number of methods: http://www.xs4all.nl/~bvdwolf/main/foto/down_sample/example1.htm.

Doug