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I have a simple GridGeometry2D in Geotools 13.1 that is created like this:

GridEnvelope grid = new GridEnvelope2D(0, 0, 100, 100);
Envelope2D envelope = new Envelope2D(CRS.decode("EPSG:3857"), 10.0, 10.0, 1.0, 1.0);
GridGeometry2D geom = new GridGeometry2D(grid, envelope);

For a quick test, I'm trying to find the grid coordinates for point (10.0, 10.0):

DirectPosition out = geom.getCRSToGrid2D().transform(
                (DirectPosition) new DirectPosition2D(10.0, 10.0), 
                (DirectPosition) new DirectPosition2D());

I expected the result for the x field to be 0.5 (since the default pixel anchoring mode is center), but instead this happened:

DirectPosition2D[-0.5000000000001137, 99.5]

I understand that the 99.5 is probably because the GridGeometry2D considers that y coordinates increase towards the north, but the GridEnvelope2D ycoordinates apparently increase towards the south (as it is common in image processing).

What I don't understand is why the x coordinate is negative if the minimal x for my grid is 0. Why is this happening?

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