anthony wrote:For upsampling hermite, and triangle is probably good.
ASIDE: A Resized upscale produce the same as there equivelent interpolations as the resize filter becomes 'unscaled'.
For downsampling a Sinc filter works better.
I have not compared Resize Sinc-based filters against Distort Jinc-based filters for downsampling.
Only the upsampling test with the rose which clearly shows the removal of 'jaggies' (block artifacts)
I have compared Jinc VS Sinc for downsampling, and the preliminary results are that Jinc is (slightly) better.
So you know what I'm after w.r.t. using filters without negative lobes for downsampling:
I have a nonlinear ("adaptive") scheme, named Nohalo, which is good for upsampling and which, as the name indicate, does not produce any ringing. It is better than Hermite, for example. Nohalo is already implemented in VIPS/NIP2; also in a GEGL branch, in an outdated prototype version (I need to crack the whip on one of my grad students
For GEGL and VIPS/NIP2 ("competitors" of ImageMagick, kind of), I want to blend Nohalo with a scheme good for downsampling, with blending weights that depend on the transformation being applied (locally: when warping or doing perspective, one can switch from upsampling to downsampling within the same image).
So: I agree that a windowed Sinc (Jinc) is better when downsampling in general, but there are situations (certain types of art, for example: think old-style serigraphs or Matisse scissor art; apparently, some scientific-medical situations as well) in which ringing is a no-no. I'm hoping to set up Nohalo with the chosen downsampler as the preferred option in such situations. For this to be the case across the board I need to figure out what the best downsampling scheme with no (or almost no) ringing is.
At this point, my hunch is that it's going to be resize or distort triangle, quadratic or cubic (not Hermite), possibly but probably not with some blur < 1, or some version of Gaussian blur.
I'll keep you posted on what I find, and will read your comments with interest.