Android OpenGL ES拾取（使用Java）

第一步：建立拾取射线

核心函数是GLU.gluUnProject，原型如下

static int gluUnProject(float winX, float winY, float winZ, float[] model, int modelOffset, float[] project, int projectOffset, int[] view, int viewOffset, float[] obj, int objOffset)

winZ取0得到射线近点，winZ取1得到远点。模型矩阵（model）和投影矩阵（project）比较麻烦，由于ES版本不提供获取模型和投影矩阵的方法，因此通过自行跟踪矩阵变换的方式来解决。具体实现由MatrixTrackingGL和MatrixGrabber以及MatrixStack三个类协作提供，这三个类可以在

<android SDK PATH>\samples\android-<version>\ApiDemos\src\com\example\android\apis\graphics\spritetext\

文件夹下找到。
视图矩阵是在render的onSurfaceChanged时提供的长宽构建的

@Overridepublic void onSurfaceChanged(GL10 gl, int width, int height)｛    viewPort = new int[]{0,0,width,height}｝

最后是获取结果，obj参数被赋值为4元素数组，要用前三个除以第四个得到最终结果。下面是获取射线的代码

public class Ray {    public Ray(MatrixGrabber matrixGrabber,int[] viewPort,float xTouch, float yTouch){        float[] temp = new float[4];        float winy =(float)viewPort[3] - yTouch;        int result = GLU.gluUnProject(xTouch, winy, 1f, matrixGrabber.mModelView, 0, matrixGrabber.mProjection, 0, viewPort, 0, temp, 0);        if(result == GL10.GL_TRUE){            farCoords[0] = temp[0] / temp[3] * Cube3.one;            farCoords[1] = temp[1] / temp[3] * Cube3.one;            farCoords[2] = temp[2] / temp[3] * Cube3.one;        }        result = GLU.gluUnProject(xTouch, winy, 0, matrixGrabber.mModelView, 0, matrixGrabber.mProjection, 0, viewPort, 0, temp, 0);        if(result == GL10.GL_TRUE){            nearCoords[0] = temp[0] / temp[3] * Cube3.one;            nearCoords[1] = temp[1] / temp[3] * Cube3.one;            nearCoords[2] = temp[2] / temp[3] * Cube3.one;        }    }    public float[] farCoords = new float[3];    public float[] nearCoords = new float[3];}

其中Cube3.one=0x10000，因为我的数据是整数的，乘以0x10000将float转化为int。

第二部获取焦点

判断一个三角形和射线是否有交点，这个过程用到内积、外积等向量运算，是网上的人用原c代码转Java

/**     * 检测射线与三角形相交     * @param R 射线     * @param T 三角形     * @param I 焦点     * @return -1:无法构造三角形（三点一线，三点重合）     *         0:无交点     *         1:相交于I     *         2:射线与三角形共面     */    public static int intersectRayAndTriangle(Ray R, Triangle T, float[] I){        float[]    u, v, n;             // triangle vectors        float[]    dir, w0, w;          // ray vectors        float     r, a, b;             // params to calc ray-plane intersect        // get triangle edge vectors and plane normal        u =  Vector.minus(T.V1, T.V0);        v =  Vector.minus(T.V2, T.V0);        n =  Vector.crossProduct(u, v);             // cross product        if (Arrays.equals(n, new float[]{0.0f, 0.0f, 0.0f})){           // triangle is degenerate            return -1;                 // do not deal with this case        }        dir =  Vector.minus(R.farCoords, R.nearCoords);             // ray direction vector        w0 = Vector.minus(R.nearCoords, T.V0);        a = - Vector.dot(n,w0);        b =  Vector.dot(n,dir);        if (Math.abs(b) < SMALL_NUM) {     // ray is parallel to triangle plane            if (a == 0){                // ray lies in triangle plane                return 2;            }else{                return 0;             // ray disjoint from plane            }        }        // get intersect point of ray with triangle plane        r = a / b;        if (r < 0.0f){                   // ray goes away from triangle            return 0;                  // => no intersect        }        float[] tempI =  Vector.addition(R.nearCoords,  Vector.scalarProduct(r, dir));           // intersect point of ray and plane        I[0] = tempI[0];        I[1] = tempI[1];        I[2] = tempI[2];        // is I inside T?        float    uu, uv, vv, wu, wv, D;        uu =  Vector.dot(u,u);        uv =  Vector.dot(u,v);        vv =  Vector.dot(v,v);        w =  Vector.minus(I, T.V0);        wu =  Vector.dot(w,u);        wv = Vector.dot(w,v);        D = (uv * uv) - (uu * vv);        // get and test parametric coords        float s, t;        s = ((uv * wv) - (vv * wu)) / D;        if (s < 0.0f || s > 1.0f)        // I is outside T            return 0;        t = (uv * wu - uu * wv) / D;        if (t < 0.0f || (s + t) > 1.0f)  // I is outside T            return 0;        return 1;                      // I is in T    }

如果返回1，那么I就是交点。用到的向量工具类：

public class Vector {    // dot product 内积    public static float dot(float[] u,float[] v) {        return ((u[X] * v[X]) + (u[Y] * v[Y]) + (u[Z] * v[Z]));    }    public static float[] minus(float[] u, float[] v){        return new float[]{u[X]-v[X],u[Y]-v[Y],u[Z]-v[Z]};    }    public static float[] addition(float[] u, float[] v){        return new float[]{u[X]+v[X],u[Y]+v[Y],u[Z]+v[Z]};    }    //scalar product 缩放    public static float[] scalarProduct(float r, float[] u){        return new float[]{u[X]*r,u[Y]*r,u[Z]*r};    }    // cross product 外积    public static float[] crossProduct(float[] u, float[] v){        return new float[]{(u[Y]*v[Z]) - (u[Z]*v[Y]),(u[Z]*v[X]) - (u[X]*v[Z]),(u[X]*v[Y]) - (u[Y]*v[X])};    }    // length 长度    public static float length(float[] u){        return (float) Math.abs(Math.sqrt((u[X] *u[X]) + (u[Y] *u[Y]) + (u[Z] *u[Z])));    }    public static final int X = 0;    public static final int Y = 1;    public static final int Z = 2;}

全部源代码放在github上面，是一个魔方程序，可以用手指拾取魔方块进行旋转