GDAL is coded in C (or maybe C++, I'm not sure). What I meant was I just used the output of the command line program gdalinfo to get at the coordinates (using the `` operator).
Doing that means incurring some overhead, but since you can cache the results you really only have to do /whatever you need to do/ once per image, so usually you don't care*.
Besides, if you need to, it's probably relatively easy to write XS extensions using the GDAL library. I just didn't have to. I wrote some pretty extensive interfaces to geos though.
* or at least, not if you've got about two hundred images, all about 10Mb+ in size. The overhead of starting a separate process and filtering its output will be dwarfed by the disk IO.
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