Intersections2D.js

define([
        './Cartesian3',
        './defined',
        './DeveloperError'
    ], function(
        Cartesian3,
        defined,
        DeveloperError) {
    'use strict';

    /**
     * Contains functions for operating on 2D triangles.
     *
     * @exports Intersections2D
     */
    var Intersections2D = {};

    /**
     * Splits a 2D triangle at given axis-aligned threshold value and returns the resulting
     * polygon on a given side of the threshold.  The resulting polygon may have 0, 1, 2,
     * 3, or 4 vertices.
     *
     * @param {Number} threshold The threshold coordinate value at which to clip the triangle.
     * @param {Boolean} keepAbove true to keep the portion of the triangle above the threshold, or false
     *                            to keep the portion below.
     * @param {Number} u0 The coordinate of the first vertex in the triangle, in counter-clockwise order.
     * @param {Number} u1 The coordinate of the second vertex in the triangle, in counter-clockwise order.
     * @param {Number} u2 The coordinate of the third vertex in the triangle, in counter-clockwise order.
     * @param {Number[]} [result] The array into which to copy the result.  If this parameter is not supplied,
     *                            a new array is constructed and returned.
     * @returns {Number[]} The polygon that results after the clip, specified as a list of
     *                     vertices.  The vertices are specified in counter-clockwise order.
     *                     Each vertex is either an index from the existing list (identified as
     *                     a 0, 1, or 2) or -1 indicating a new vertex not in the original triangle.
     *                     For new vertices, the -1 is followed by three additional numbers: the
     *                     index of each of the two original vertices forming the line segment that
     *                     the new vertex lies on, and the fraction of the distance from the first
     *                     vertex to the second one.
     *
     * @example
     * var result = Cesium.Intersections2D.clipTriangleAtAxisAlignedThreshold(0.5, false, 0.2, 0.6, 0.4);
     * // result === [2, 0, -1, 1, 0, 0.25, -1, 1, 2, 0.5]
     */
    Intersections2D.clipTriangleAtAxisAlignedThreshold = function(threshold, keepAbove, u0, u1, u2, result) {
        //>>includeStart('debug', pragmas.debug);
        if (!defined(threshold)) {
            throw new DeveloperError('threshold is required.');
        }
        if (!defined(keepAbove)) {
            throw new DeveloperError('keepAbove is required.');
        }
        if (!defined(u0)) {
            throw new DeveloperError('u0 is required.');
        }
        if (!defined(u1)) {
            throw new DeveloperError('u1 is required.');
        }
        if (!defined(u2)) {
            throw new DeveloperError('u2 is required.');
        }
        //>>includeEnd('debug');

        if (!defined(result)) {
            result = [];
        } else {
            result.length = 0;
        }

        var u0Behind;
        var u1Behind;
        var u2Behind;
        if (keepAbove) {
            u0Behind = u0 < threshold;
            u1Behind = u1 < threshold;
            u2Behind = u2 < threshold;
        } else {
            u0Behind = u0 > threshold;
            u1Behind = u1 > threshold;
            u2Behind = u2 > threshold;
        }

        var numBehind = u0Behind + u1Behind + u2Behind;

        var u01Ratio;
        var u02Ratio;
        var u12Ratio;
        var u10Ratio;
        var u20Ratio;
        var u21Ratio;

        if (numBehind === 1) {
            if (u0Behind) {
                u01Ratio = (threshold - u0) / (u1 - u0);
                u02Ratio = (threshold - u0) / (u2 - u0);

                result.push(1);

                result.push(2);

                if (u02Ratio !== 1.0) {
                    result.push(-1);
                    result.push(0);
                    result.push(2);
                    result.push(u02Ratio);
                }

                if (u01Ratio !== 1.0) {
                    result.push(-1);
                    result.push(0);
                    result.push(1);
                    result.push(u01Ratio);
                }
            } else if (u1Behind) {
                u12Ratio = (threshold - u1) / (u2 - u1);
                u10Ratio = (threshold - u1) / (u0 - u1);

                result.push(2);

                result.push(0);

                if (u10Ratio !== 1.0) {
                    result.push(-1);
                    result.push(1);
                    result.push(0);
                    result.push(u10Ratio);
                }

                if (u12Ratio !== 1.0) {
                    result.push(-1);
                    result.push(1);
                    result.push(2);
                    result.push(u12Ratio);
                }
            } else if (u2Behind) {
                u20Ratio = (threshold - u2) / (u0 - u2);
                u21Ratio = (threshold - u2) / (u1 - u2);

                result.push(0);

                result.push(1);

                if (u21Ratio !== 1.0) {
                    result.push(-1);
                    result.push(2);
                    result.push(1);
                    result.push(u21Ratio);
                }

                if (u20Ratio !== 1.0) {
                    result.push(-1);
                    result.push(2);
                    result.push(0);
                    result.push(u20Ratio);
                }
            }
        } else if (numBehind === 2) {
            if (!u0Behind && u0 !== threshold) {
                u10Ratio = (threshold - u1) / (u0 - u1);
                u20Ratio = (threshold - u2) / (u0 - u2);

                result.push(0);

                result.push(-1);
                result.push(1);
                result.push(0);
                result.push(u10Ratio);

                result.push(-1);
                result.push(2);
                result.push(0);
                result.push(u20Ratio);
            } else if (!u1Behind && u1 !== threshold) {
                u21Ratio = (threshold - u2) / (u1 - u2);
                u01Ratio = (threshold - u0) / (u1 - u0);

                result.push(1);

                result.push(-1);
                result.push(2);
                result.push(1);
                result.push(u21Ratio);

                result.push(-1);
                result.push(0);
                result.push(1);
                result.push(u01Ratio);
            } else if (!u2Behind && u2 !== threshold) {
                u02Ratio = (threshold - u0) / (u2 - u0);
                u12Ratio = (threshold - u1) / (u2 - u1);

                result.push(2);

                result.push(-1);
                result.push(0);
                result.push(2);
                result.push(u02Ratio);

                result.push(-1);
                result.push(1);
                result.push(2);
                result.push(u12Ratio);
            }
        } else if (numBehind !== 3) {
            // Completely in front of threshold
            result.push(0);
            result.push(1);
            result.push(2);
        }
        // else Completely behind threshold

        return result;
    };

    /**
     * Compute the barycentric coordinates of a 2D position within a 2D triangle.
     *
     * @param {Number} x The x coordinate of the position for which to find the barycentric coordinates.
     * @param {Number} y The y coordinate of the position for which to find the barycentric coordinates.
     * @param {Number} x1 The x coordinate of the triangle's first vertex.
     * @param {Number} y1 The y coordinate of the triangle's first vertex.
     * @param {Number} x2 The x coordinate of the triangle's second vertex.
     * @param {Number} y2 The y coordinate of the triangle's second vertex.
     * @param {Number} x3 The x coordinate of the triangle's third vertex.
     * @param {Number} y3 The y coordinate of the triangle's third vertex.
     * @param {Cartesian3} [result] The instance into to which to copy the result.  If this parameter
     *                     is undefined, a new instance is created and returned.
     * @returns {Cartesian3} The barycentric coordinates of the position within the triangle.
     *
     * @example
     * var result = Cesium.Intersections2D.computeBarycentricCoordinates(0.0, 0.0, 0.0, 1.0, -1, -0.5, 1, -0.5);
     * // result === new Cesium.Cartesian3(1.0 / 3.0, 1.0 / 3.0, 1.0 / 3.0);
     */
    Intersections2D.computeBarycentricCoordinates = function(x, y, x1, y1, x2, y2, x3, y3, result) {
        //>>includeStart('debug', pragmas.debug);
        if (!defined(x)) {
            throw new DeveloperError('x is required.');
        }
        if (!defined(y)) {
            throw new DeveloperError('y is required.');
        }
        if (!defined(x1)) {
            throw new DeveloperError('x1 is required.');
        }
        if (!defined(y1)) {
            throw new DeveloperError('y1 is required.');
        }
        if (!defined(x2)) {
            throw new DeveloperError('x2 is required.');
        }
        if (!defined(y2)) {
            throw new DeveloperError('y2 is required.');
        }
        if (!defined(x3)) {
            throw new DeveloperError('x3 is required.');
        }
        if (!defined(y3)) {
            throw new DeveloperError('y3 is required.');
        }
        //>>includeEnd('debug');

        var x1mx3 = x1 - x3;
        var x3mx2 = x3 - x2;
        var y2my3 = y2 - y3;
        var y1my3 = y1 - y3;
        var inverseDeterminant = 1.0 / (y2my3 * x1mx3 + x3mx2 * y1my3);
        var ymy3 = y - y3;
        var xmx3 = x - x3;
        var l1 = (y2my3 * xmx3 + x3mx2 * ymy3) * inverseDeterminant;
        var l2 = (-y1my3 * xmx3 + x1mx3 * ymy3) * inverseDeterminant;
        var l3 = 1.0 - l1 - l2;

        if (defined(result)) {
            result.x = l1;
            result.y = l2;
            result.z = l3;
            return result;
        }
        return new Cartesian3(l1, l2, l3);
    };

    return Intersections2D;
});