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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