Pointcloud2 to depth map

edit

I want to generate a 2D depth map _depth_map[x,y] = z from pointcloud2 where x,y,z are the dimension x,y,z

Currently my code is:

void callback(const sensor_msgs::PointCloud2ConstPtr& msg){

int width = msg->width;
int height = msg->height;
int v = 1;
int u = 1;
int arrayPosition = v * msg->row_step + u * msg->point_step;
int arrayPosX = arrayPosition + msg->fields[0].offset; // X has an offset of 0
int arrayPosY = arrayPosition + msg->fields[1].offset; // Y has an offset of 4
int arrayPosZ = arrayPosition + msg->fields[2].offset; // Z has an offset of 8

double X = 0.0;
double Y = 0.0;
double Z = 0.0;

memcpy(&X, &msg->data[arrayPosX], sizeof(double));
memcpy(&Y, &msg->data[arrayPosY], sizeof(double));
memcpy(&Z, &msg->data[arrayPosZ], sizeof(double));

ROS_INFO("X:%lf",X);
ROS_INFO("Y:%lf",Y);
ROS_INFO("Z:%lf",Z);

}

I am confused with the parameter u and v, how to obtain u and v from unordered point cloud. Cited from problem #191265 And why Z value = 0 all the time. Below is some sample result: