120 lines
2.8 KiB
C++
120 lines
2.8 KiB
C++
// This code is in the public domain -- Ignacio Castaño <castano@gmail.com>
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#include "ConvexHull.h"
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#include "Vector.inl"
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#include "nvcore/RadixSort.h"
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#include "nvcore/Array.inl"
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using namespace nv;
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inline static float triangleArea(Vector2::Arg v1, Vector2::Arg v2, Vector2::Arg v3)
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{
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return 0.5f * (v3.x * v1.y + v1.x * v2.y + v2.x * v3.y - v2.x * v1.y - v3.x * v2.y - v1.x * v3.y);
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}
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// Compute the convex hull using Graham Scan.
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void nv::convexHull(const Array<Vector2> & input, Array<Vector2> & output, float epsilon/*=0*/)
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{
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const uint inputCount = input.count();
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Array<float> coords;
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coords.resize(inputCount);
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for (uint i = 0; i < inputCount; i++) {
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coords[i] = input[i].x;
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}
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RadixSort radix;
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radix.sort(coords);
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const uint * ranks = radix.ranks();
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Array<Vector2> top(inputCount);
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Array<Vector2> bottom(inputCount);
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Vector2 P = input[ranks[0]];
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Vector2 Q = input[ranks[inputCount-1]];
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float topy = max(P.y, Q.y);
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float boty = min(P.y, Q.y);
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for (uint i = 0; i < inputCount; i++) {
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Vector2 p = input[ranks[i]];
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if (p.y >= boty) top.append(p);
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}
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for (uint i = 0; i < inputCount; i++) {
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Vector2 p = input[ranks[inputCount-1-i]];
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if (p.y <= topy) bottom.append(p);
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}
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// Filter top list.
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output.clear();
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output.append(top[0]);
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output.append(top[1]);
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for (uint i = 2; i < top.count(); ) {
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Vector2 a = output[output.count()-2];
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Vector2 b = output[output.count()-1];
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Vector2 c = top[i];
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float area = triangleArea(a, b, c);
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if (area >= -epsilon) {
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output.popBack();
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}
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if (area < -epsilon || output.count() == 1) {
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output.append(c);
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i++;
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}
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}
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uint top_count = output.count();
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output.append(bottom[1]);
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// Filter bottom list.
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for (uint i = 2; i < bottom.count(); ) {
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Vector2 a = output[output.count()-2];
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Vector2 b = output[output.count()-1];
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Vector2 c = bottom[i];
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float area = triangleArea(a, b, c);
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if (area >= -epsilon) {
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output.popBack();
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}
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if (area < -epsilon || output.count() == top_count) {
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output.append(c);
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i++;
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}
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}
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// Remove duplicate element.
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nvDebugCheck(output.front() == output.back());
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output.popBack();
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}
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/*
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void testConvexHull() {
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Array<Vector2> points;
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points.append(Vector2(1.00, 1.00));
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points.append(Vector2(0.00, 0.00));
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points.append(Vector2(1.00, 1.00));
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points.append(Vector2(1.00, -1.00));
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points.append(Vector2(2.00, 5.00));
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points.append(Vector2(-5.00, 3.00));
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points.append(Vector2(-4.00, -3.00));
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points.append(Vector2(7.00, -4.00));
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Array<Vector2> hull;
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convexHull(points, hull);
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}
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*/
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