Bilinear uses the nearest four pixels, bicubic uses the nearest 16
pixels. That's what makes them linear and cubic. Do you have a source
for another way to do it?
It would be possible to do averaging, *then* resample, but I'm not
sure that's worth it here. (Weighted averaging would be better anyway.)
~Seth
On Oct 13, 2010, at 9:48 PM, Frank Warmerdam wrote:
Craig de Stigter wrote:
Apologies for the long delay. I've finally had opportunity to look
into this again. I'm still unsure why exactly the bilinear and
bicubic resampling produces such second-rate output in GDAL. I have
a few questions:
1. Is there a fundamental reason why the bilinear resampling
couldn't use all the pixels from the source region, rather than
just the 4 corner pixels? A destination pixel would then be the
weighted average of all the source pixels for each band.
Craig,
Well, it would no longer be bilinear. It would be averaged.
2. Suppose a 1/6 sampling ratio, i.e. a 6x6 region (0, 0)-->(6, 6)
in the source image corresponds to a 1x1 region in the output.
What source pixels are used by the bilinear resampling in GDAL?
Wikipedia <http://en.wikipedia.org/wiki/Bilinear_interpolation>
suggests it should be the 'nearest' four pixels to the desired
destination pixel, i.e. (2, 2), (2, 3), (3, 2), (3, 3). Is that
what GDAL is doing, or is it using (0, 0), (0, 5), (5, 0), (5, 5) ?
It will use (2, 2), (2, 3), (3, 2), (3, 3).
3. Assuming a 'no' answer to (1), if I were to contribute a patch
to make bilinear/bicubic resampling take all the source pixels into
account, would it meet much opposition? It would probably make
large-scale downsampling using these methods much slower, though
its questionable whether anyone would complain given that the
quality of current output is so poor...
I'm not adverse to your preparing a patch that implements an
"averaged" resampling kernel, but it doesn't replace bilinear.
Generally speaking the warper is not intended for dramatic changes
of resolution - it is intended for changes of geometry at roughly the
same resolution - for instance reprojection.
Average was already implemented as a resampling kernel in the
"build overviews" tool because it is aimed at dramatic changes of
resolution.
Best regards,
--
---------------------------------------
+--------------------------------------
I set the clouds in motion - turn up | Frank Warmerdam, warmer...@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush | Geospatial Programmer for
Rent
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