Re: Any GIMP users (Linux)

davem@xxxxxxxxx (Dave Martindale) writes:

=?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= <mru@xxxxxxxxxxxxx> writes:
davem@xxxxxxxxx (Dave Martindale) writes:

What do you mean by "properly"? I don't think there is one absolute
best way to rescale an image, applicable to all images. (And I've
written some of my own interpolation software).

I mean without introducing aliasing artifacts when downscaling.

From what I can see, Photoshop *does* do low-pass filtering when
downsampling. (This basically amounts to increasing the size of the filter
kernel proportional to the reduction factor. So bicubic upsampling
always uses a 4x4 kernel no matter what the upsampling factor, but 2X
downsampling needs an 8x8 kernel, 3X downsampling needs 12x12, etc).

Of course, a cubic spline doesn't have a very sharp transition between
pass and cut, and some higher-frequency information that should get
removed might end up aliased in the result, but considerably attenuated.
That's likely to be true of any resampling kernel that is 4x4 in size.

If you want a more abrupt lowpass cutoff, you need to change to a more
expensive filter like Lanczos. But that has its problems too - sharp
edges can produce ringing artifacts.

What downsampling filter does ImageMagick use?

According to the docs, ImageMagick lets you choose from these:

Point
Box
Triangle
Hermite
Hanning
Hamming
Blackman
Gaussian
Quadratic
Cubic
Catrom
Mitchell
Lanczos
Bessel
Sinc

--
Måns Rullgård
mru@xxxxxxxxxxxxx
.

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