[Gdal-dev] gdalwarp to merge several images

[Nicolas Ribot]

> Nicolas,
>
> Looking in the code it seems there is an undocumented warp option called
> SKIP_NOSOURCE that will cause gdalwarp to not attempt to actually warp
> chunks where there is no input data.
>
> eg
> gdalwarp -wo SKIP_NOSOURCE=yes -te 400000 3720000 410000 3730000
>       openev/utm.tif out.tif

Hello Franck.
This undocumented option works like a charm !
It effectively skips input images outside target area.
And it is much clearer in my script to integrate this option in the
command line instead of writing some lines of code to exclude input
images based on bbox comparison... ;-)

>
> I can't remember exactly why this isn't the default.  I think the issue
> was that there was a risk of the output file not getting properly initialized
> if blocks where skipped that shouldn't be skipped.   But I *think* it would
> be safe for me to modify gdalwarp to set this option by default for the
> second and subsequent files being processed.
>
> Note the input files outside the target area were not actually having imagery
> read.  But the warper was doing a great deal of work processing the whole
> region only to discover none of the output pixels had any corresponding input
> data.
>
> Arg.  This does make gdalwarp a bit questionable for some mosaicing purposes.
>

Yes, work processing done by the warper is heavy.
I noticed that it takes about the same time for gdalwarp to process an
image that would fit outsite the target area than to process an image
with data in target area.
(for small images, about 5 MB each)

Thank you for the support

Nicolas