[gdal-dev] cubic / bilinear resampling with gdalwarp looks similar to nearest neighbour

[Frank Warmerdam]

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,
-- 
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