我试图在另一个填充的多边形上绘制一个较小的填充的多边形（在我的情况下，它们总是凸的而不是自相交的四边形），以给该填充的多边形周围一个边框。为此，我发现了如何计算四边形的质心-将围绕该点缩小较小的填充多边形。我已经从此处复制了算法：https://math.stackexchange.com/a/2878092
这是该类的完整代码：
public class BodyPartActor extends Actor {
private static TextureRegion textureRegion = Globals.getTextureRegion();
private static Color borderColor = Assets.getSkin().getColor("pink");
private static Color bodyPartColor = Assets.getSkin().getColor("green");
private static short[] triangles = new short[] {
0, 1, 2, // Two triangles using vertex indices.
0, 2, 3 // Take care of the counter-clockwise direction.
};
private PolygonRegion polygonRegion;
private BodyPart bodyPart;
private Vector2 centroid;
public BodyPartActor(BodyPart bodyPart) {
this.bodyPart = bodyPart;
polygonRegion = new PolygonRegion(textureRegion, this.bodyPart.getVertices(), triangles);
centroid = getCentroidOfQuadrangle();
}
private Vector2 getCentroidOfQuadrangle() {
float[] v = bodyPart.getVertices();
Vector2 centroidA = getCentroidOfTriangle(new float[]{v[0], v[1], v[2], v[3], v[4], v[5]});
Vector2 centroidB = getCentroidOfTriangle(new float[]{v[2], v[3], v[4], v[5], v[6], v[7]});
Vector2 centroidC = getCentroidOfTriangle(new float[]{v[4], v[5], v[6], v[7], v[0], v[1]});
Vector2 centroidD = getCentroidOfTriangle(new float[]{v[6], v[7], v[0], v[1], v[2], v[3]});
return getPointOfLinesIntersection(centroidA, centroidC, centroidB, centroidD);
}
private Vector2 getCentroidOfTriangle(float[] vertices) {
return new Vector2 (
(vertices[0] + vertices[2] + vertices[4]) / 3,
(vertices[1] + vertices[3] + vertices[5]) / 3
);
}
private Vector2 getPointOfLinesIntersection(Vector2 startLineA, Vector2 endLineA, Vector2 startLineB,
Vector2 endLineB) {
float detOfLineA = det2x2(startLineA.x, startLineA.y, endLineA.x, endLineA.y);
float detOfLineB = det2x2(startLineB.x, startLineB.y, endLineB.x, endLineB.y);
float xDeltaOfLineA = startLineA.x - endLineA.x;
float xDeltaOfLineB = startLineB.x - endLineB.x;
float yDeltaOfLineA = startLineA.y - endLineA.y;
float yDeltaOfLineB = startLineB.y - endLineB.y;
float xNom = det2x2(detOfLineA, xDeltaOfLineA, detOfLineB, xDeltaOfLineB);
float yNom = det2x2(detOfLineA, yDeltaOfLineA, detOfLineB, yDeltaOfLineB);
float deNom = det2x2(xDeltaOfLineA, yDeltaOfLineA, xDeltaOfLineB, yDeltaOfLineB);
return new Vector2(xNom / deNom, yNom / deNom);
}
/**
* Calculate determinant of a 2x2 matrix:
* |a b|
* |c d|
*/
private float det2x2(float a, float b, float c, float d) {
return (a * d) - (b * c);
}
@Override
public void draw(Batch batch, float parentAlpha) {
float x = bodyPart.getBody().getPosition().x;
float y = bodyPart.getBody().getPosition().y;
float width = polygonRegion.getRegion().getRegionWidth();
float height = polygonRegion.getRegion().getRegionHeight();
float originX = centroid.x;
float originY = centroid.y;
float rotation = calculateRotation(originX, originY);
PolygonSpriteBatch polygonSpriteBatch = (PolygonSpriteBatch) batch;
polygonSpriteBatch.setColor(borderColor.r, borderColor.g, borderColor.b, borderColor.a * parentAlpha);
polygonSpriteBatch.draw(polygonRegion, x, y, originX, originY, width, height, 1f, 1f, rotation);
polygonSpriteBatch.setColor(bodyPartColor.r, bodyPartColor.g, bodyPartColor.b, bodyPartColor.a * parentAlpha);
polygonSpriteBatch.draw(polygonRegion, x, y, originX, originY, width, height, 0.9f, 0.9f, rotation);
}
private float calculateRotation(float originX, float originY) {
// How to calculate when originX and originY are the center of quadrangle???
return bodyPart.getBody().getAngle() * MathUtils.radiansToDegrees; // Works only if originX and originY are 0.
}
}
我似乎checked it here：http://eguruchela.com/math/Calculator/polygon-centroid-point正确地计算了质心 但the resulting borders doesn't look okay。我什至以为libGDX在计算顶点位置时存在一些浮点算术问题，但我找不到任何问题，所以我不知道问题出在哪里。旋转还有一个问题，因为现在我将原点绘制在四边形的质心处，所以它无法正确渲染（在尝试绘制边框之前，originX和originY为0，这使得正确渲染旋转的物体正确） ，现在与应有的方式不符。
您知道如何正确渲染此边框以及如何正确旋转多边形吗？也许解决此问题的整个方法是错误的，并且有更好的方法可以做到这一点？
答案 0 :(得分：0)
好的，我现在可以正常工作了。我使用了以下算法：https://math.stackexchange.com/a/2824263（方法2）。 Here's how it looks now。但是它不是100％稳定的。一些非常扁平的四边形是越野车，如果边框不粗则很少见，但如果边框it can look like this较粗，则将边框粗度设置得更高时更糟。但是对于我的薄边框来说，这已经足够了。这是该类的代码：
public class BodyPartActor extends Actor {
private static TextureRegion textureRegion = Globals.getTextureRegion();
private static Color borderColor = Assets.getSkin().getColor("bodyPartBorder");
private static Color fillColor = Assets.getSkin().getColor("bodyPartFill");
private static short[] triangles = new short[] {
0, 1, 2, // Two triangles using vertex indices.
0, 2, 3 // Take care of the counter-clockwise direction.
};
private static float borderThickness = 0.1f;
private PolygonRegion polygonRegionBorder;
private PolygonRegion polygonRegionFill;
private BodyPart bodyPart;
public BodyPartActor(BodyPart bodyPart) {
this.bodyPart = bodyPart;
float[] vertices = this.bodyPart.getVertices();
polygonRegionBorder = new PolygonRegion(textureRegion, vertices, triangles);
polygonRegionFill = new PolygonRegion(textureRegion, calculateInnerVertices(vertices), triangles);
}
/**
* @param v vertices of the outer quadrangle
*/
private float[] calculateInnerVertices(float[] v) {
Vector2 vA = calculateInnerVertex(v[6], v[7], v[0], v[1], v[2], v[3]);
Vector2 vB = calculateInnerVertex(v[0], v[1], v[2], v[3], v[4], v[5]);
Vector2 vC = calculateInnerVertex(v[2], v[3], v[4], v[5], v[6], v[7]);
Vector2 vD = calculateInnerVertex(v[4], v[5], v[6], v[7], v[0], v[1]);
return new float[]{vA.x, vA.y, vB.x, vB.y, vC.x, vC.y, vD.x, vD.y};
}
/**
* Positive borderThickness value will produce vertex that is offseted to the inner side of the provided quadrangle.
* Negative borderThickness value will produce vertex that is on the outer side of the quadrangle.
*/
private Vector2 calculateInnerVertex(float prevX, float prevY, float curX, float curY, float nextX, float nextY) {
Vector2 v = new Vector2(nextX - curX, nextY - curY);
Vector2 w = new Vector2(prevX - curX, prevY - curY);
Vector2 u = v.cpy().scl(w.len()).add(w.cpy().scl(v.len()));
float absDetVW = Math.abs((v.x * w.y) - (v.y * w.x));
return new Vector2(curX, curY).add(u.scl(borderThickness / absDetVW));
}
@Override
public void draw(Batch batch, float parentAlpha) {
float x = bodyPart.getBody().getPosition().x;
float y = bodyPart.getBody().getPosition().y;
float rotation = bodyPart.getBody().getAngle() * MathUtils.radiansToDegrees;
PolygonSpriteBatch polygonSpriteBatch = (PolygonSpriteBatch) batch;
polygonSpriteBatch.setColor(borderColor.r, borderColor.g, borderColor.b, borderColor.a * parentAlpha);
polygonSpriteBatch.draw(polygonRegionBorder, x, y, 0f, 0f, 1f, 1f, 1f, 1f, rotation);
polygonSpriteBatch.setColor(fillColor.r, fillColor.g, fillColor.b, fillColor.a * parentAlpha);
polygonSpriteBatch.draw(polygonRegionFill, x, y, 0f, 0f, 1f, 1f, 1f, 1f, rotation);
}
}