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 * Image processing operations for SciJava Ops.
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package org.scijava.ops.image.geom.geom3d;

import java.util.function.Function;

import net.imglib2.mesh.Mesh;
import net.imglib2.type.numeric.real.DoubleType;

import org.apache.commons.math3.linear.EigenDecomposition;
import org.apache.commons.math3.linear.RealMatrix;
import org.scijava.function.Computers;
import org.scijava.ops.spi.OpDependency;

/**
 * Generic implementation of {@code geom.mainElongation}. Adapted to 3D from
 * http://www.math.uci.edu/icamp/summer/research_11/park/shape_descriptors_survey.pdf
 *
 * @author Tim-Oliver Buchholz (University of Konstanz)
 * @implNote op names='geom.mainElongation', label='Geometric (3D): Main
 *           Elongation', priority='10000.'
 */
public class DefaultMainElongation implements
	Computers.Arity1<Mesh, DoubleType>
{

	@OpDependency(name = "geom.secondMoment")
	private Function<Mesh, RealMatrix> inertiaTensor;

	/**
	 * TODO
	 *
	 * @param input
	 * @param mainElongation
	 */
	@Override
	public void compute(final Mesh input, final DoubleType mainElongation) {
		final var it = inertiaTensor.apply(input);
		final var ed = new EigenDecomposition(it);

		final var l1 = ed.getRealEigenvalue(0) - ed.getRealEigenvalue(2) + ed
			.getRealEigenvalue(1);
		final var l2 = ed.getRealEigenvalue(0) - ed.getRealEigenvalue(1) + ed
			.getRealEigenvalue(2);
		final var l3 = ed.getRealEigenvalue(2) - ed.getRealEigenvalue(0) + ed
			.getRealEigenvalue(1);

		final var g = 1 / (8 * Math.PI / 15);

		final var a = Math.pow(g * l1 * l1 / Math.sqrt(l2 * l3), 1 / 5d);
		final var b = Math.pow(g * l2 * l2 / Math.sqrt(l1 * l3), 1 / 5d);
		mainElongation.set(1 - (b / a));
	}

}
