[gdal-dev] NITF CGM Parsing solution

[Chiou, Kevin]

Hi,
I would like to share how I was able to use "Uniconvertor" library to
parse NITF CGM data.

For those of you who worked with NITF images, you probably know NITF
specification supports graphics with CGM.  There are libraries out there
that will read out CGM data (GDAL being one of them), but none of them
translates raw CGM data into a useful form.  And it has being very
difficult to find any open source library that would actually parses
CGM.  

However, I have found the Uniconvertor, which is an open source
universal vector graphics translator in Python.  Since the program
supports CGM, we were able to use Uniconvertor to parse CGM into the
Uniconvertor format (they called it document).  At that point you can
read graphics data from the Uniconvertor "documents" and in each of the
"document" you can retrieve information for individual geometry as well
as properties.  

I was able to successfully read and render NITF official sample image
U_1092c.NTF, which contains around 100 different CGM shapes including
lines, texts, rectangles, polygons, circles and ellipses.

Here are some of the Pros and Cons of Uniconvertor:
Pros:
1)	It parse multiple vector formats: CDR, CDRX, CDT, CMX, AI, CGM,
WMF, XFIG, SVG, SK, SK1, AFF,etc.  You can use Uniconvertor to convert
other vector data into GDAL/OGR formats.
2)	It's open source (LGPL)

Cons:
1)	Written in Python.  You cannot use this in another languages.
2)	No interface documentation or tutorial.
3)	In the CGM case, Uniconvertor did some coordinate transformation
after it read in the data. I had to look through the source code and
worked out the inverse transformation to get the original coordinate.

Uniconvertor:
http://sk1project.org/modules.php?name=Products&product=uniconvertor

In conclusion, it's possible to use Uniconvertor to parse CGM data into
a usable form.  Moreover, since Uniconvertor supports multiple vector
formats, you can use the approach I described to convert other vector
graphics formats into an OGR dataset.

-Kevin

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