[Gdal-dev] Can GDAL GDALDataset return compressed image data

[Alison Luo]

Never mind. I finally got the compressed data!!!
nSegmentStart is the offset from the file handler, not
from the image handler. Once I changed my code to
compute the offset from the file handler, I got the
compressed data correctly.

Alison

--- Alison Luo <alison_luo2001 at yahoo.com> wrote:

> Hi Frank,
> 
> I still can not get the correct compressed data
> stream
> using NITFSegmentInfo. I add the offset
> nSegmentStart
> to the pointer hAccess, which should bring me to the
> beginning of the compressed data stream. Then I read
> nSegmentSize bytes of data from there. But from what
> I
> read in memory, the returned block of data is not
> correct. I expect to see the exact jpeg compressed
> data returned.
> 
> I don't know what's wrong with my workflow. I'm
> attaching the sample file I've been using.
> 
> By the way, is there any free NITF file viewer for
> Windows platform to download?
> 
> Thanks,
> Alison
> 
> --- Frank Warmerdam <warmerdam at pobox.com> wrote:
> 
> > Alison Luo wrote:
> > > Hi Ivan,
> > > 
> > > Thank you so much for your help. I have more
> > questions
> > > regarding the block of code you referred to in
> > > nitifdataset.cpp. I also took a look of class
> > > NITFSegmentInfo in nitflilb.h:
> > > 
> > > NITFSegmentInfo has a member hAccess, which is
> the
> > > NITF image stream.
> > 
> > Alison,
> > 
> > It is a pointer to the NITFImage structure for
> image
> > segemnts.
> > 
> >  >  I suppose other pasSegmentInfo
> > > member     
> > >     GUInt32 nSegmentHeaderStart;
> > >     GUInt32 nSegmentHeaderSize;
> > >     GUInt32 nSegmentStart;
> > >     GUInt32 nSegmentSize;
> > > would somehow indicate how to get the encoded
> jpeg
> > > stream from hAccess? Could you please explain
> how
> > the
> > > jpeg buffer is filled given this information?
> > 
> > They are information on where the image header,
> and
> > image
> > data chunks are in the file.  So compressed data
> > stream
> > would consist of nSegmentSize bytes started at
> byte
> > nSegmentStart
> > in the file.
> > 
> > > Why doesn't GDAL provide APIs to access encoded
> > image
> > > stream, but only provide API to get raw image
> > stream?
> > 
> > GDAL does not provide direct access to compressed
> > datastreams
> > because the purpose of GDAL is to provide a
> > "normalized" view to
> > different datasets, not to provide low level
> access.
> > 
> > Best regards,
> > -- 
> >
>
---------------------------------------+--------------------------------------
> > I set the clouds in motion - turn up   | Frank
> > Warmerdam, warmerdam at pobox.com
> > light and sound - activate the windows |
> > http://pobox.com/~warmerdam
> > and watch the world go round - Rush    | President
> > OSGeo, http://osgeo.org
> > 
> > 
> 
> 
> 
>      
>
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