java - Android NDK 将 3D float 组传递给 C 代码

Question

我想将 3d float 组(float[][][])从 java 传递给 c 编辑它并将数组返回给 java
Curretly 我只能用一维数组来做到这一点

jfloat* valuesjf = NULL;
float* valuesf = NULL;
void Java_Grids_Grid5_Update( JNIEnv* env,jobject thiz,jfloatArray values )
{
    valuesjf = (*env)->GetFloatArrayElements(env,values,NULL);
    valuesf = valuesjf;

    valuesf[0]=121+valuesf[0];//do some calculations

    (*env)->ReleaseFloatArrayElements(env, values, valuesjf, JNI_ABORT);
}

如何使用 3D 阵列来实现？

Answer 1

您可以将 Java 中的 3d float 组展平为 1d 数组。然后，您可以通过在 1d 数组中生成正确的索引来访问原始 3d 数组的每个元素。您还需要将数组每个维度的长度传递给 native 方法，以便 native 方法也可以为一维数组生成正确的索引

Java:

// The lengths of each dimension of the values array
int l, m, n;
// Instead of:
// float[][][] values = new float[l][m][n];
// Do this:
float[] values = new float[l * m * n];

// To access an entry at (x, y, z) in the array.
// Instead of:
// float v = values[z][y][x];
// Do this:
int i = (m * n * z) + (n * y) + x;
float v = values[i];

C++:

// You should not store a reference to the Java array as a global variable.
// The reference is only valid between the Get/ReleaseFloatArrayElements calls
// Since valuesf is assigned the same reference to the Java array,
// the same advice also applies to it
// jfloat* valuesjf = NULL;
// float* valuesf = NULL;

void 
Java_Grids_Grid5_Update(JNIEnv* env,
                         jobject thiz,
                         jfloatArray values,
                         jint l, jint, m, jint n) {

  jfloat* valuesjf = (*env)->GetFloatArrayElements(env,values,NULL);
  float* valuesf = valuesjf;

  int x, y, z;
  int i = (m * n * z) + (n * y) + x;
  valuesf[0]=121+valuesf[i]; //do some calculations

  (*env)->ReleaseFloatArrayElements(env, values, valuesjf, JNI_ABORT);

}