#73: r.out.gdal tiff output does not work
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Reporter: helena | Owner: grass-dev@lists.osgeo.org
Type: defect | Status: new
Priority: critical | Milestone: 6.4.0
Component: Raster | Version: svn-trunk
Resolution: | Keywords: r.out.gdal, tiff
Platform: Unspecified | Cpu: Unspecified
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Comment (by mmetz):
Replying to [comment:38 hamish]:
> Pretend you are r.out.gdal iterating through the raster cells to output.
You come across a NULL cell. You have to write down some value, and are
completely constrained to options available in the output type. What do
you write out? Overflow/wrap the type? type min? type max? well (as things
were) whatever you do is going to be arbirtary, and thus could lead to
data loss, which is grounds for a most serious blocker bug.
That would require an additional check that the nodata value is not
present in the raster band to be exported. This is not too difficult.
What would be more difficult is to select a nodata value that is not
present in the raster band to be exported if type min and type max are
present. Offhand, I can only think of a helper data structure (sorted
heap/binary tree/hash table) where all raster values without duplicates
are kept which can consume a lot of memory for large floating point
rasters. This would also require two passes over each raster band, first
investigating the values and selecting a proper nodata value, second
replacing NULLs with this nodata value. Then, what to do if a user-
specified nodata value is within the range of the selected GDAL data type
but this value is present in the raster? Give a warning but let the user
override the check, causing data loss?
Sorry, I haven't thought about this problem of nodata value causing data
loss, somehow left this to the user.
Markus M
--
Ticket URL: <http://trac.osgeo.org/grass/ticket/73#comment:39>
GRASS GIS <http://grass.osgeo.org>
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