OpenKinect acquire raw depth image - kinect

Question

I am trying to use the example code from here.
I have made some changes in order to save the images to the computer. When I read the data in MATLAB it seems like values that should be 0 are set to 2047, and overall it does not seem to be correct when I reconstruct the 3D points using the default intrinsic camera parameters.
What I want to achieve is to save the images so that I can use
img = single(imread(depth.png'))/ 1000
and have the depth values in meters, and pixels with no measurements should be zero.
It is the Kinect V1 by the way.
Here is the code with comments where I have tried to change.
#include "libfreenect.hpp"
#include <iostream>
#include <vector>
#include <cmath>
#include <pthread.h>
#include <cv.h>
#include <cxcore.h>
#include <highgui.h>
using namespace cv;
using namespace std;
class myMutex {
public:
myMutex() {
pthread_mutex_init( &m_mutex, NULL );
}
void lock() {
pthread_mutex_lock( &m_mutex );
}
void unlock() {
pthread_mutex_unlock( &m_mutex );
}
private:
pthread_mutex_t m_mutex;
};
// Should one use FREENECT_DEPTH_REGISTERED instead of FREENECT_DEPTH_11BIT?
class MyFreenectDevice : public Freenect::FreenectDevice {
public:
MyFreenectDevice(freenect_context *_ctx, int _index)
: Freenect::FreenectDevice(_ctx, _index), m_buffer_depth(FREENECT_DEPTH_11BIT),
m_buffer_rgb(FREENECT_VIDEO_RGB), m_gamma(2048), m_new_rgb_frame(false),
m_new_depth_frame(false), depthMat(Size(640,480),CV_16UC1),
rgbMat(Size(640,480), CV_8UC3, Scalar(0)),
ownMat(Size(640,480),CV_8UC3,Scalar(0)) {
for( unsigned int i = 0 ; i < 2048 ; i++) {
float v = i/2048.0;
v = std::pow(v, 3)* 6;
m_gamma[i] = v*6*256;
}
}
// Do not call directly even in child
void VideoCallback(void* _rgb, uint32_t timestamp) {
std::cout << "RGB callback" << std::endl;
m_rgb_mutex.lock();
uint8_t* rgb = static_cast<uint8_t*>(_rgb);
rgbMat.data = rgb;
m_new_rgb_frame = true;
m_rgb_mutex.unlock();
};
// Do not call directly even in child
void DepthCallback(void* _depth, uint32_t timestamp) {
std::cout << "Depth callback" << std::endl;
m_depth_mutex.lock();
uint16_t* depth = static_cast<uint16_t*>(_depth);
// Here I use memcpy instead so I can use uint16
// memcpy(depthMat.data,depth,depthMat.rows*depthMat.cols*sizeof(uint16_t));
depthMat.data = (uchar*) depth;
m_new_depth_frame = true;
m_depth_mutex.unlock();
}
bool getVideo(Mat& output) {
m_rgb_mutex.lock();
if(m_new_rgb_frame) {
cv::cvtColor(rgbMat, output, CV_RGB2BGR);
m_new_rgb_frame = false;
m_rgb_mutex.unlock();
return true;
} else {
m_rgb_mutex.unlock();
return false;
}
}
bool getDepth(Mat& output) {
m_depth_mutex.lock();
if(m_new_depth_frame) {
depthMat.copyTo(output);
m_new_depth_frame = false;
m_depth_mutex.unlock();
return true;
} else {
m_depth_mutex.unlock();
return false;
}
}
private:
// Should it be uint16_t instead or even higher?
std::vector<uint8_t> m_buffer_depth;
std::vector<uint8_t> m_buffer_rgb;
std::vector<uint16_t> m_gamma;
Mat depthMat;
Mat rgbMat;
Mat ownMat;
myMutex m_rgb_mutex;
myMutex m_depth_mutex;
bool m_new_rgb_frame;
bool m_new_depth_frame;
};
int main(int argc, char **argv) {
bool die(false);
string filename("snapshot");
string suffix(".png");
int i_snap(0),iter(0);
Mat depthMat(Size(640,480),CV_16UC1);
Mat depthf (Size(640,480),CV_8UC1);
Mat rgbMat(Size(640,480),CV_8UC3,Scalar(0));
Mat ownMat(Size(640,480),CV_8UC3,Scalar(0));
// The next two lines must be changed as Freenect::Freenect
// isn't a template but the method createDevice:
// Freenect::Freenect<MyFreenectDevice> freenect;
// MyFreenectDevice& device = freenect.createDevice(0);
// by these two lines:
Freenect::Freenect freenect;
MyFreenectDevice& device = freenect.createDevice<MyFreenectDevice>(0);
namedWindow("rgb",CV_WINDOW_AUTOSIZE);
namedWindow("depth",CV_WINDOW_AUTOSIZE);
device.startVideo();
device.startDepth();
while (!die) {
device.getVideo(rgbMat);
device.getDepth(depthMat);
// Here I save the depth images
std::ostringstream file;
file << filename << i_snap << suffix;
cv::imwrite(file.str(),depthMat);
cv::imshow("rgb", rgbMat);
depthMat.convertTo(depthf, CV_8UC1, 255.0/2048.0);
cv::imshow("depth",depthf);
if(iter >= 1000) break;
iter++;
}
device.stopVideo();
device.stopDepth();
return 0;
}
Thanks in advance!
Erik

Answer 1

I dont have any experience with OpenKinect in particular; but should your depth buffer be uint16?
std::vector<uint8_t> m_buffer_depth;
Also; for Matlab, do check if the image that you are reading is a uint16 or uint8. If its the latter then convert it to uint16
uint16(imread('depth.png'));
Sorry couldn't help more. Hope this helps.

Answer 2

The values you have are the raw depth values. You need to remap those into MM for the numbers to make sense. Kinect 1 can see up to 10 meters. So I would go with raw_values/2407*10000.
If the values are saturated at 2047, you are probably using the FREENECT_DEPTH_11BIT_PACKED depth format.
For work in Matlab, it is always easier to use FREENECT_DEPTH_MM or FREENECT_DEPTH_REGISTERED.
Enjoy.