At least this solved the problem as to why Lanzcos was three lobed while the rest of 4 lobes. Welsh was a later addition.
I have made Welsh, and Cosine default to 3 lobes, to join Lanzcos, while all the other filters remain 4 lobed by default. That includes Raw Sinc or Fast Sinc as 4 lobes by default though they probably should also should be 3 lobe.
Actually I don't think that last matters a lot. If you want a Raw Sinc, you should know what you you doing anyway.
Change in filter tables was uploaded to BOTH IMv6 and IMv7 SVN.
It makes the filter tables a little more complicated (using Lanczos to select SincFast with 3 lobe support, instead of a direct SincFast reference) but it is not a big change, and an extra comment hopefully makes it clear when the support value gets used.
I also added the following to the general Windowing Filter blurb...
All the functions (except 'Bartlett') form two basic catagories of windowing functions. However the catagories of windowing functions do not seem to be as important ast overall 'frequency response' you get from using a trigonometric defined curve.
A tapering 'bell' shaped curve, such as 'Hamming', 'Hann', 'Kaiser', 'Blackman', 'Bohman', and 'Parzen'.
And a untappered 'lobe' type windowing function which quickly falls to zero,
before being 'cut off' by the windows support, such as 'Lanczos', 'Welch', and 'Cosine'.
If you study the above window function graph you will see how the two styles of filter differ.
The tappering 'bell' shaped windowing functions will use a Lobes Support over 4 lobes of the Sinc Weighting function (or Jinc function in cylindrical (distort) resampling). The untappering 'lobe' type of windowing filters will default to using a 3 lobe support. This results in a rough equivelence between the two types of windowing filter, due to differnece in the windowing function roll off. This also means that the untappering windowed filters will be by default slightly faster that the tappering ones.
As a result of this 'support' difference, there is not a lot of difference visible in the graphs of the filter functions.
Of course if you want you can change the Lobes Support or even just the general support of any of the resize filters. They are all about the same only with very slight variations in ringing that will be generated.
I myself have not found a great deal of difference in results between these various windowing functions. From my reading of research papers the results seems to be more of a qualitative opinion of their suitability, rather than anything concrete. In summery, my feeling is that just about any windowing function can be used, but if I was to pick one you are better sticking to the most popular 'Lanczos' windowing filter, as it has good reasons (frequency spectrum response) for being a good choice as a resampling filter.