Thanks Martin and Michael, now I got it.
Here's some code for the archive (hints for optimization welcome):
// Some preparations
GridCoverage2D gridCoverage = ...;
RenderedImage renderedImage = gridCoverage.getRenderedImage();
Raster renderedGrid = renderedImage.getData();
int imgMinX = renderedImage.getMinX();
int imgMinY = renderedImage.getMinY();
int imgWidth = renderedImage.getWidth();
int imgHeight = renderedImage.getHeight();
Feature [] ROIs = ...
Geometry currROI = null;
MathTransform mt = gridCoverage.getGridGeometry().getGridToCRS();
Geometry pnt = null;
Geometry pntTrans = null;
RandomIter sourceIterator = RandomIterFactory.create( renderedGrid, null );
// Iterize over ROIs
for ( int ROI=0; ROI<ROIs.length; ROI++ ) {
// Example: count pixels and calculate sum of values in ROIs
int numPixels = 0;
int band = 0;
for( int h=imgMinY; h<imgMinY+imgHeight; h++ ) {
for( int w=imgMinX; w<imgMinX+imgWidth; w++ ) {
// Transform pixel coordinates into geographic
coordinates
pnt = new GeometryFactory().createPoint( new
Coordinate( (double) w,
(double) h ) );
try {
pntTrans =
org.geotools.geometry.jts.JTS.transform(pnt, mt);
} catch ( Exception e ) {
...
}
// Check whether ROI contains pixel and do calculation
if ( currROI.contains( pntTrans ) ) {
numPixels++;
sum += sourceIterator.getSampleFloat( w, h,
band );
}
}
}
System.out.println("Number of pixels in ROI " + (ROI+1) + ": " +
numPixels);
System.out.println("Sum of pixel values in ROI " + (ROI+1) + ": " +
sum);
}
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