[Gdal-dev] Compositing warped images

[Robert Osfield]

Hi All,

I am currently working on adding to the OpenSceneGraph the ability to creating 
scene graph from multiple source images and DEM's.  Using GDAL I have 
implemented the ability to warp all the source data into the same projection, 
and then extract tiles from the warped source data into LOD'd geometry and 
texture map tiles.  

I am using the GDALWarpOperation::ChunkAndWarpImage(..) as per the GDAL Warp 
API Tutorial to do the warping into the same projection, and 
GDALRasterBand::RasterIO(..) method to extract the tiles.  I must say that I 
thoroughly impressed at the power that GDAL provides with such a compact set 
of API's :)

Now onto the problem in hand...  the warping itself creates black/null data 
areas at the edges which when the tile data is extracted the black areas are 
copied into the destination imagery/height field overwritting previous valid 
values.  What would be ideal for me would be for only valid values to be 
copied across to the destination, with null data ignored.  I havn't spotted a 
way of doing this with the RasterIO method so far, is there a way of doing 
this?

My fallback is to copy the extracted tile into a local tempory data area, then 
copy from this to the final destination, filtering the data as I go.  
However, this does mean allocating extra data.  Each of these tiles is not 
too big, typically around 256x256, and unlikey to get bigger than 1024x1024, 
and the tempory data can always be used.  So memory consumption isn't a real 
issue, but peformance is, and writing more code than I have to is :)

The other option might be to have an extended version of RasterIO which can do 
the filtering.

Thanks in advance for any advice that you can provide,
Robert.