docs: public double[][] diff_y(double maty[][]) {
          mat3 = new double[width][height];
          double mat1, mat2;
          for (int i = 0; i < width; i++) {
              for (int j = 0; j < height; j++) {
                  if (i == 0) {
                      mat1 = maty[i][j];
                      mat2 = maty[i + 1][j];
                  } else if (i == width - 1) {
                      mat1 = maty[i - 1][j];
                      mat2 = maty[i][j];
                  } else {
                      mat1 = maty[i - 1][j];
                      mat2 = maty[i + 1][j];
                  }
                  mat3[i][j] = (mat2 - mat1) / 2;
              }
          }
          // maty= subMatrix(mat2,mat1,width,height);
  
          return mat3;
      }