[gdal-dev] mosaicking files with partial overlap and binary mask

[Daniele Romagnoli]

Hi list,
I have some TIFF datasets with partially overlapping edges and embedded
binary masks to keep into account which parts along the edges should be
masked out.
When showing all the images on QGIS it looks good since the binary mask do
its job by making the "black/white" overlapping edge masked out.

Now I need to create a single TIF from all of these images.
I have tried with gdalbuildvrt but it doesn't produce good result.
Checking the doc I saw this:


*If there is some amount of spatial overlapping between files [...]
currently, alpha channel is not taken into account to do alpha compositing
(so a source with alpha=0 appearing on top of another source will override
is content). This might be changed in later versions.  *

So I think it's expected and I have switched to gdal_merge.py.
It works better since it does alpha compositing but it looks like the
result doesn't preserve the binary mask which I still need since I have M
groups of N files which I need to process in 2 passes:
- the inner group of N files get merged using the binary mask to a bigger
file.
- all the M bigger files get mosaicked again to a single huge file.

I will spend some more time in testing different attempts.
In the meantime, do you have any suggestion or past experience on how to
deal with that?
Side note: I have vague memory of gdal_merge being somehow memory bounded,
throwing memory allocation errors when trying to compose big images. This
is why in the past I was using gdalbuildvrt to do composition (without
overlapping).

Thanks in advance.
Regards,
Daniele




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Regards,
Daniele Romagnoli
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