Hi,
I'm implementing a service using GeoServer to carry out change detection. 
The hypothetical service would allow a user to select an area of interest,
display available datasets, allow the user to select the "before" and 
"after" datasets then compute the result.  For now, I just need to subtract
the two images.

At the moment I:
1) Crop the selected section of both images creating two new
GridCoverage2D's (Operations.DEFAULT.crop)
2) Get the RenderedImage objects from
3) Make them the same size so I can subtract pixel-for-pixel
4) Subtract using SubtractDescriptor
5) Select required band using BandSelectDescriptor
6) Binarize using a given threshold using BinarizeDescriptor

However, this results in a PlanarImage not a coverage. I've tried to create
a new coverage from this using the CRS and envelope info but the result
shifts to the NW corner of the original image.  For now I'm just returning a
jpeg image showing the result.  Ideally, I would like to return the data in
vector format but can't seem to find a suitable tool for this.

It would be nice if there was a Java library somewhere with similar
operations, but GeoTools doesn't seem to support many.

I realise this is a very general request, but does anybody know how I can
simplify this process using exising tools? And does a anyone know of an
easy(ish) way of converting from raster to vector within my service?

Thankyou,
Jon

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