[Gdal-dev] User Question: creating a geotiff from pnm

[Frank Warmerdam]

Michael Darweesh wrote:
> I'm new to the gdal lib and mapping and trying to come up to speed as 
> fast as I can, newbie help is appreciated.
> 
> I'm trying to do this.
> 
> I have an in-memory (pnm) raster projected in swiss oblique (EPSG:9814 I 
> believe) and I have the data which would be put into a world file in 
> memory too. What's the right way to package this image up into a 
> geotiff? I'd like to do this all in mempory since I'll be outputting the 
> geotiff as an http response. But, I could probably get away with dumping 
> it to disk if necessary.

Michael,

OK, doing this in memory is pretty trick.

On the output side, there is no application accessable support for output
GeoTIFF "in memory".  If you want to do this - as things stands you would
basically have to avoid using the GDAL GeoTIFF driver and call into the
libtiff API yourself.

There is some example code that does something like this in
gdal/frmts/gtiff/gt_wkt_srs.cpp (see GTIFMemBufFromWkt()).  This code is
used to produce degenerate GeoTIFFs with georeferencing info in them that
can be embedded in JP2 files without having to write them temporarily to disk.

We could also look at changing the code driver to support in memory output
under some circumstances but that could be fairly involved.  I would encourage
you to just write to a temporary file and then spool it to stdout.  That is
what I did in a similar situation in MapServer even though I would really like
to have avoided the extra trip to disk.

On the input side, once again, the PNM driver has no support for reading
from memory.  However, the PNM format is pretty simple, so you could manage
it yourself.

At this point we should take a step back and review.  If you are going to
write your own custom code to write to GeoTIFF using libtiff and libgeotiff
(so as to avoid writing to disk), then you don't really need to use GDAL at
all.

However, if you are going to write to disk, then you could parse the PNM
header "manually" and then create a "memory dataset" referencing the data
portion of the PNM file in memory.  This is a bit involved too, so I won't
try and describe it till you decide how you want to go.  If you do want to
use it, try reviewing the "In Memory Format" (MEM) on the web site first.

All in all, GDAL isn't not well organized to handle in-memory images of
files.  If this was needed "in general", then I think the best approach would
be a custom reimplementation of the VSI file io layer used to access files.
With approriate implementation it could support reading and writing memory
or disk.  This might be of use for other purposes as well.  I would certainly
like it for MapServer.

Also, a client is interested in making it possible to embed as the required
GDAL support files within the DLL and one approach to that would be to
virtualize access at the VSI layer.

However, if you just need a quick custom hack for this one situation, I
would suggest writing custom PNM-in-memory to GeoTIFF-in-memory code and
using the GTIFMemBufFromWKt() and related code as a bit of guide.

> Thanks,
> Mike
> ps. Is the gdal_translate.c sample code intentionally not showing or am 
> I browsing wrong?

It is called gdal_translate.cpp, perhaps that is why your not finding it?

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent