EarClippingTriangulator produces "inside-out" triangulation result

Issue details

Using the test case below, it appears the the EarClippingTriangulator is returning a result that is “inside-out” where it tries to connect points using the area outside the polygon. This would be also be seen in situations where the CW/CCW winding is incorrectly detected however I don’t believe that to be the case here and if the input points are reversed then the problem is still present. The test case is a bunch of squares merged together into a larger polygon and therefore there are a lot of colinear points which I would speculate is the trigger for this issue.

Reproduction steps/code

private void test()
{
	float[] points = new float[] {
			41.92309f, -78.590225f,
			41.92219f, -78.590225f,
			41.92219f, -78.591415f,
			41.9195f, -78.591415f,
			41.9195f, -78.592606f,
			41.9186f, -78.592606f,
			41.9186f, -78.589035f,
			41.920395f, -78.589035f,
			41.920395f, -78.58785f,
			41.9186f, -78.58785f,
			41.9186f, -78.58547f,
			41.9195f, -78.58547f,
			41.9195f, -78.58666f,
			41.92219f, -78.58666f,
			41.92219f, -78.589035f,
			41.92309f, -78.589035f,
			41.92309f, -78.590225f
	};
	EarClippingTriangulator triangulator = new EarClippingTriangulator();
	ShortArray shortArray = triangulator.computeTriangles(points);
	// Produces a result of [0, 16, 15, 15, 14, 13, 13, 12, 11, 9, 8, 7, 9, 7, 6, 9, 6, 5, 10, 9, 5, 4, 3, 2, 2, 1, 0, 0, 15, 13, 0, 13, 11, 0, 11, 10, 0, 10, 5, 0, 5, 4, 4, 2, 0]
	// POLYGON ((41.92309 -78.590225, 41.92219 -78.590225, 41.92219 -78.591415, 41.9195 -78.591415, 41.9195 -78.592606, 41.9186 -78.592606, 41.9186 -78.589035, 41.920395 -78.589035, 41.920395 -78.58785, 41.9186 -78.58785, 41.9186 -78.58547, 41.9195 -78.58547, 41.9195 -78.58666, 41.92219 -78.58666, 41.92219 -78.589035, 41.92309 -78.589035, 41.92309 -78.590225))
	// MULTIPOLYGON (((41.92309 -78.590225, 41.92309 -78.590225, 41.92309 -78.589035, 41.92309 -78.590225)), ((41.92309 -78.589035, 41.92219 -78.589035, 41.92219 -78.58666, 41.92309 -78.589035)), ((41.92219 -78.58666, 41.9195 -78.58666, 41.9195 -78.58547, 41.92219 -78.58666)), ((41.9186 -78.58785, 41.920395 -78.58785, 41.920395 -78.589035, 41.9186 -78.58785)), ((41.9186 -78.58785, 41.920395 -78.589035, 41.9186 -78.589035, 41.9186 -78.58785)), ((41.9186 -78.58785, 41.9186 -78.589035, 41.9186 -78.592606, 41.9186 -78.58785)), ((41.9186 -78.58547, 41.9186 -78.58785, 41.9186 -78.592606, 41.9186 -78.58547)), ((41.9195 -78.592606, 41.9195 -78.591415, 41.92219 -78.591415, 41.9195 -78.592606)), ((41.92219 -78.591415, 41.92219 -78.590225, 41.92309 -78.590225, 41.92219 -78.591415)), ((41.92309 -78.590225, 41.92309 -78.589035, 41.92219 -78.58666, 41.92309 -78.590225)), ((41.92309 -78.590225, 41.92219 -78.58666, 41.9195 -78.58547, 41.92309 -78.590225)), ((41.92309 -78.590225, 41.9195 -78.58547, 41.9186 -78.58547, 41.92309 -78.590225)), ((41.92309 -78.590225, 41.9186 -78.58547, 41.9186 -78.592606, 41.92309 -78.590225)), ((41.92309 -78.590225, 41.9186 -78.592606, 41.9195 -78.592606, 41.92309 -78.590225)), ((41.9195 -78.592606, 41.92219 -78.591415, 41.92309 -78.590225, 41.9195 -78.592606)))
}

Version of LibGDX and/or relevant dependencies

Tested on 1.9.3 Test run on Android

[NathanSweet]

The data set above has no duplicate or nearly duplicate points I would say many of the points are nearly duplicate.

Like #5631, working with such scales is not ideal. This works correctly:

public class EarClip extends ApplicationAdapter {
	private ShapeRenderer renderer;
	EarClippingTriangulator triangulator = new EarClippingTriangulator();

	public void create () {
		renderer = new ShapeRenderer();
	}

	public void render () {
		float[] points = new float[] { //
			41.92309f, -78.590225f, 41.92219f, -78.590225f, 41.92219f, -78.591415f, 41.9195f, -78.591415f, 41.9195f, -78.592606f,
			41.9186f, -78.592606f, 41.9186f, -78.589035f, 41.920395f, -78.589035f, 41.920395f, -78.58785f, 41.9186f, -78.58785f,
			41.9186f, -78.58547f, 41.9195f, -78.58547f, 41.9195f, -78.58666f, 41.92219f, -78.58666f, 41.92219f, -78.589035f,
			41.92309f, -78.589035f, 41.92309f, -78.590225f};

		for (int i = 0, n = points.length; i < n; i += 2) {
			points[i] = (points[i] - 41.9f) * 20000;
			points[i + 1] = (points[i + 1] + 78.5f) * 20000 + 2000;
		}

		ShortArray triangles = triangulator.computeTriangles(points);

		Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
		renderer.begin(ShapeType.Line);

		renderer.setColor(Color.RED);
		for (int i = 0; i < triangles.size; i += 3) {
			int p1 = triangles.get(i) * 2;
			int p2 = triangles.get(i + 1) * 2;
			int p3 = triangles.get(i + 2) * 2;
			renderer.triangle( //
				points[p1], points[p1 + 1], //
				points[p2], points[p2 + 1], //
				points[p3], points[p3 + 1] //
			);
		}

		renderer.setColor(Color.GREEN);
		renderer.polygon(points);
		renderer.end();
	}

	public void resize (int width, int height) {
		renderer.getProjectionMatrix().setToOrtho2D(0, 0, width, height);
	}

	public static void main (String[] args) throws Exception {
		new LwjglApplication(new EarClip());
	}
}

Probably you could do a general use massaging, like:

public class EarClip extends ApplicationAdapter {
	private ShapeRenderer renderer;
	EarClippingTriangulator triangulator = new EarClippingTriangulator();

	public void create () {
		renderer = new ShapeRenderer();
	}

	public void render () {
		float[] points = new float[] { //
			41.92309f, -78.590225f, 41.92219f, -78.590225f, 41.92219f, -78.591415f, 41.9195f, -78.591415f, 41.9195f, -78.592606f,
			41.9186f, -78.592606f, 41.9186f, -78.589035f, 41.920395f, -78.589035f, 41.920395f, -78.58785f, 41.9186f, -78.58785f,
			41.9186f, -78.58547f, 41.9195f, -78.58547f, 41.9195f, -78.58666f, 41.92219f, -78.58666f, 41.92219f, -78.589035f,
			41.92309f, -78.589035f, 41.92309f, -78.590225f};

		// Adjust for triangulation.
		float[] forTris = new float[points.length];
		System.arraycopy(points, 0, forTris, 0, points.length);
		float minX = Float.MAX_VALUE, minY = Float.MAX_VALUE;
		for (int i = 0, n = forTris.length; i < n; i += 2) {
			minX = Math.min(minX, forTris[i]);
			minY = Math.min(minY, forTris[i + 1]);
		}
		for (int i = 0, n = forTris.length; i < n; i += 2) {
			forTris[i] = forTris[i] - minX;
			forTris[i + 1] = forTris[i + 1] - minY;
		}
		ShortArray triangles = triangulator.computeTriangles(forTris);

		// Adjust for rendering.
		for (int i = 0, n = points.length; i < n; i += 2) {
			points[i] = (points[i] - 41.9f) * 20000;
			points[i + 1] = (points[i + 1] + 78.5f) * 20000 + 2000;
		}

		Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
		renderer.begin(ShapeType.Line);

		renderer.setColor(Color.RED);
		for (int i = 0; i < triangles.size; i += 3) {
			int p1 = triangles.get(i) * 2;
			int p2 = triangles.get(i + 1) * 2;
			int p3 = triangles.get(i + 2) * 2;
			renderer.triangle( //
				points[p1], points[p1 + 1], //
				points[p2], points[p2 + 1], //
				points[p3], points[p3 + 1] //
			);
		}

		renderer.setColor(Color.GREEN);
		renderer.polygon(points);
		renderer.end();
	}

	public void resize (int width, int height) {
		renderer.getProjectionMatrix().setToOrtho2D(0, 0, width, height);
	}

	public static void main (String[] args) throws Exception {
		new LwjglApplication(new EarClip());
	}
}

[kurtzmarc]

areVerticesClockwise is returning false. If we force it to return true, then the triangulation is correct.