[Gdal-dev] Indexing with non-square boundary

[Avi A Blackmore]

Hi there,

	I've run into a thorny issue with gdaltindex, when indexing our satellite 
imagery set.  The imagery taken by the satellites is, of course, shot on 
an angle: the satellites' paths around the Earth are not vertical, but 
skewed on an angle of about 12-13 degrees.  The actual image files, of 
course, are rectangular, which results in large black "null" areas in the 
image.  We use UMN Mapserver to display the imagery, and make the black 
null areas transparent, so that they don't cause a display problem.

	The problem is, these null areas, while invisible when the image is 
displayed, are still considered "part" of the image as far as gdaltindex 
is concerned.  The bounding box is calculated from the corners of the 
image file, not from the actual corners of the "non-null" parts of the 
image.  

	The result is that, when our site does a query on the image index layer, 
we get a lot of spurious "hits" in our imagery list: images whose bounding 
box intersects the current extents of the map even though the actual 
"live" image data is well outside those extents.

	So my question is, using GDAL/OGR, how would I create a tile index whose 
bounding boxes are polygons that reflect the actual image path, and omit 
the black null areas?  I haven't found a way to do this using gdaltindex.
-- 
Avi Blackmore
System Administrator/Programmer
Agri ImaGIS Technologies, Inc.