ST_TriangleAspect.java

/**
 * H2GIS is a library that brings spatial support to the H2 Database Engine
 * <http://www.h2database.com>. H2GIS is developed by CNRS
 * <http://www.cnrs.fr/>.
 *
 * This code is part of the H2GIS project. H2GIS is free software; 
 * you can redistribute it and/or modify it under the terms of the GNU
 * Lesser General Public License as published by the Free Software Foundation;
 * version 3.0 of the License.
 *
 * H2GIS is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
 * for more details <http://www.gnu.org/licenses/>.
 *
 *
 * For more information, please consult: <http://www.h2gis.org/>
 * or contact directly: info_at_h2gis.org
 */

package org.h2gis.functions.spatial.topography;

import com.vividsolutions.jts.geom.Geometry;
import com.vividsolutions.jts.math.Vector2D;
import com.vividsolutions.jts.math.Vector3D;
import org.h2gis.api.DeterministicScalarFunction;
import org.h2gis.utilities.jts_utils.TriMarkers;

/**
 * This function is used to compute the aspect of a triangle. Aspect represents
 * the main slope direction angle compared to the north direction.
 *
 * @author Erwan Bocher
 */
public class ST_TriangleAspect extends DeterministicScalarFunction {

    public ST_TriangleAspect() {
        addProperty(PROP_REMARKS, "Compute the aspect of steepest downhill slope for a triangle\n. "
                + "The aspect value is expressed in degrees compared to the north direction.");
    }

    @Override
    public String getJavaStaticMethod() {
        return "computeAspect"; 
    }
    
    /**
     * Compute the aspect in degree. The geometry must be a triangle.
     * @param geometry Polygon triangle
     * @return aspect in degree
     * @throws IllegalArgumentException  ST_TriangleAspect accept only triangles
     */
    public static Double computeAspect(Geometry geometry) throws IllegalArgumentException {
        if (geometry == null) {
            return null;
        }
        Vector3D vector = TriMarkers.getSteepestVector(TriMarkers.getNormalVector(TINFeatureFactory.createTriangle(geometry)), TINFeatureFactory.EPSILON);
        if (vector.length() < TINFeatureFactory.EPSILON) {
            return 0d;
        } else {
            Vector2D v = new Vector2D(vector.getX(), vector.getY());
            return measureFromNorth(Math.toDegrees(v.angle()));
        }
    }

    /**
     * Transforms an angle measured in degrees counterclockwise from the x-axis
     * (mathematicians) to an angle measured in degrees clockwise from the
     * y-axis (geographers).
     *
     * @param angle Mathematician's angle
     * @return Geographer's angle
     */
    public static double measureFromNorth(double angle) {
        return (450 - angle) % 360;
    }
}