Java 3d API : Rotating a BoundingBox delivers a new Box with more volume

Question

我正在用 Java 3D API 编写一个小游戏，我使用 javax.media.j3d.BoundingBox 的对象来存储模型的边界。

BoundingBox 的构造函数如下:

public BoundingBox(Point3d lower, Point3d upper) {
    boundId = BOUNDING_BOX;
    this.lower = new Point3d(lower);
    this.upper = new Point3d(upper);
    updateBoundsStates();
}

现在我想平移和旋转 BoundingBox，因为模型本身会做，我使用 BoundingBox 的以下方法:

/** 
 * Transforms this bounding box by the given matrix.
 * @param matrix a transformation matrix
 */
public void transform(Transform3D matrix) {
    if(boundsIsInfinite)
        return;

    if (tmpP3d == null) 
        tmpP3d = new Point3d();

    double ux, uy, uz, lx, ly, lz;
    ux = upper.x; 
    uy = upper.y; 
    uz = upper.z;

    lx = lower.x; 
    ly = lower.y; 
    lz = lower.z;

    tmpP3d.set(ux, uy, uz);
    matrix.transform( tmpP3d );

    upper.x = tmpP3d.x;
    upper.y = tmpP3d.y;
    upper.z = tmpP3d.z;

    lower.x = tmpP3d.x;
    lower.y = tmpP3d.y;
    lower.z = tmpP3d.z;

    tmpP3d.set(lx, uy, uz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x  > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y  > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z  > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x  < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y  < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z  < lower.z ) lower.z = tmpP3d.z;

    tmpP3d.set(lx, ly, uz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x  > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y  > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z  > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x  < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y  < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z  < lower.z ) lower.z = tmpP3d.z;

    tmpP3d.set(ux, ly, uz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z < lower.z ) lower.z = tmpP3d.z;

    tmpP3d.set(lx, uy, lz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z < lower.z ) lower.z = tmpP3d.z;

    tmpP3d.set(ux, uy, lz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z < lower.z ) lower.z = tmpP3d.z;

    tmpP3d.set(lx, ly, lz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z < lower.z ) lower.z = tmpP3d.z;

    tmpP3d.set(ux, ly, lz);
    matrix.transform( tmpP3d );

    if ( tmpP3d.x > upper.x ) upper.x = tmpP3d.x;
    if ( tmpP3d.y > upper.y ) upper.y = tmpP3d.y;
    if ( tmpP3d.z > upper.z ) upper.z = tmpP3d.z;

    if ( tmpP3d.x < lower.x ) lower.x = tmpP3d.x;
    if ( tmpP3d.y < lower.y ) lower.y = tmpP3d.y;
    if ( tmpP3d.z < lower.z ) lower.z = tmpP3d.z;

    if (VirtualUniverse.mc.releaseBoundingBoxMemory) {
        // Free memory
        tmpP3d = null;
    }
}

但是:如果您使用此方法旋转 BoundingBox，它会生成一个新的 BoundingBox，其体积可能比旧的更大。如果您恰好旋转 90、180、270 或 360 度，它只会提供相同的体积。

附加信息:我必须手动计算 BoundingBox，因为我通过在模型上调用 setAutoComputeBounds(false) 和 setBounds(XYZ) 使用手动边界(而不是太大的自动计算边界)(在这种情况下它实际上是一个实例指向 javax.media.j3d.SharedGroup 的 javax.media.j3d) 并且 Java 3d 引擎(至少在这种情况下)似乎不会在平移/旋转的情况下自动重新计算您的手动边界对应型号。

我的问题是:我必须如何(重新)实现上面显示的方法 transform() 以获得始终具有相同体积的边界框？

Answer 1

BoundingBox 实际上始终与轴对齐，无论它的两个点在哪里。两个点不能定义任意立方体。无论你如何重写转换函数，它都不会起作用。

您可能会考虑 BoundingPolytope 类。它提供了更通用的边界，由平面构成，尽管它确实有一个接受另一个 Bounds 对象的构造函数，因此您实际上不需要自己定义平面。