1. EEC-693/793 Applied Computer Vision with Depth Cameras
Lecture 7
Wenbing Zhao

2. Outline Depth data stream How depth measurement is done
Capture and process depth data
Classes related to depth data
Getting the Distance from a Particular Pixel

3. Depth Data Stream
Depth data stream consists of a sequence of depth image frames
Each depth image frame is in the form of a 16-bit grey scale image
Viewable range: 43 degrees vertical, 57 degrees horizontal
A depth pixel contains the following
(x,y) coordinates represent the location of the object in the depth sensor view
The value of the pixel at (x,y) contains the distance between the object and the Kinect sensor in millimeters

4. Depth Data Stream Depth data stream supports
640x480 at 30fps
320x240 at 30fps
80x60 at 30fps
Kinect depth sensing range
Up to 13.1 feet (4 meters)
As close as 40cm

5. How the Depth Data is Obtained
In the polling model, the application opens a channel for the stream, and whenever the application needs a frame, it sends a request to get the frame

6. Stereo Triangulation
Stereo triangulation is an analysis algorithm for computing the 3D position of points in an image frame
In general stereo triangulation, two images are used to obtain the two different views on a scene, in a similar fashion to human binocular vision.
By comparing these two images, the relative depth information is calculated
Kinect stereo triangulation uses one real (IR) image and a virtual image (more accurately, the line of each projected point)

7. Depth Measurement
When there is a pixel in any of the diagonal views of the sensor, it internally draws a line that is perpendicular to the sensor and then calculates the distance directly from there

8. Capturing and Processing Depth Data
Enable the depth stream channel with the type of depth image format
Attach the event handler to the stream channel
Process the incoming depth frames
Render the frames on UI
this.sensor = KinectSensor.KinectSensors[0];
this.sensor.DepthStream.Enable();
sensor.DepthFrameReady += depthFrameReady;
void depthFrameReady(object sender, DepthImageFrameReadyEventArgs e) { }

9. Processing Depth Data
void depthFrameReady(object sender, DepthImageFrameReadyEventArgs e) {
using (DepthImageFrame imageFrame = e.OpenDepthImageFrame()) {
if (null == imageFrame) {
return; }
imageFrame.CopyPixelDataTo(depthPixels);
// Write the pixel data into our bitmap
this.depthBitmap.WritePixels( new Int32Rect(0, 0, this.depthBitmap.PixelWidth, this.depthBitmap.PixelHeight),
this.depthPixels, stride, 0);
Very similar to the code for ColorImage Processing: color => depth

10. Classes Related to Depth Data

11. Classes Related to Depth Data

12. Depth Data and Distance
The DepthImageStream and DepthImageFrame classes have the MaxDepth and MinDepth properties, which return the maximum and minimum depth ranges for that particular stream or captured image frame, in millimeters
This range value returns the best range where distance can be measured properly
These values will change automatically based on the selection of DepthRange for the stream

13. Depth Range
The Range property sets the viewable range for the Kinect sensor, which is a type of DepthRange enumeration
A DepthRange enumeration has the following two values:
Default
Near
sensor.DepthStream.Range = DepthRange.Near;

14. Special Depth Range Values
DepthImageStream defines three additional read-only properties
TooFarDepth: It provides the depth point where the object is beyond the range of the sensor
TooNearDepth: It provides the depth point where the object is too close to the sensor
UnknownDepth: There are instances when a depth is completely indeterminate, and the value will be zero

15. Default Depth Range

16. Depth Data Representation
The Kinect sensor returns 16-bit raw depth frame data
The first three bits are used to represent the identified players and the remaining 13 bits give you the measured distance in millimeters
To retrieve the 13-bit depth data, perform a bitwise shift operation >> to move the bits to the beginning
int depth = depthFrame[depthIndex] >> DepthImageFrame.PlayerIndexBitmaskWidth;

17. Depth Data Representation

18. Getting the Distance from a Particular Pixel
Add feature to our DepthCam app
Very important: make the image control exactly the same size as the resolution chosen
Image should be 320x240 if Resolution320x240Fps30
Image should be 640x480 if Resolution640x480Fps30
Use mouse to select a pixel in the image => display the depth for that pixel
Need to add event handler for MouseDown
Point currentPoint = e.GetPosition(depthImageControl);
int pixelIndex = (int)(currentPoint.X + ((int)currentPoint.Y * this.frame.Width));
int distancemm = this.pixelData[pixelIndex] >> DepthImageFrame.
PlayerIndexBitmaskWidth;

19. Build the DepthCam App Create a new WFP project named DepthCam
Draw the GUI for the app
Modify xaml file to add WindowLoaded and WindowClosing as before
Add Microsoft.Kinect reference
Add member variables for displaying depth images
Start Kinect sensor, enable depth data stream, register depth stream handler, connect image control to the bitmap for display
Process each depth image
Add code for displaying depth data for pixel clicked

20. GUI Design Image control Group control for depth information
Two labels and two textboxes
Use meaningful names for the textboxes
Group control for pixel information
Four labels and four textboxes

21. Adding Code Add member variables:
WindowLoade method (WindowClosing() same as before):
KinectSensor sensor;
private WriteableBitmap depthBitmap;
private short[] depthPixels;
private int frameWidth; // to calculate depthIndex
private void WindowLoaded(object sender, RoutedEventArgs e) {
if (KinectSensor.KinectSensors.Count > 0) {
this.sensor = KinectSensor.KinectSensors[0];
if (this.sensor != null && !this.sensor.IsRunning) {
this.sensor.DepthStream.Enable(DepthImageFormat.Resolution320x240Fps30);
this.depthPixels = new short[this.sensor.DepthStream.FramePixelDataLength];
this.depthBitmap = new WriteableBitmap(this.sensor.DepthStream.FrameWidth, this.sensor.DepthStream.FrameHeight, 96.0, 96.0, PixelFormats.Gray16, null);
this.image1.Source = this.depthBitmap;
this.sensor.DepthFrameReady += this.depthFrameReady;
this.sensor.Start();
} else {
MessageBox.Show("No device is connected!"); this.Close();
} } }

22. Adding Code Event handler for depth frames:
void depthFrameReady(object sender, DepthImageFrameReadyEventArgs e) {
using (DepthImageFrame imageFrame = e.OpenDepthImageFrame()) {
if (null == imageFrame) {
return; }
this.frameWidth = imageFrame.Width;
this.maxDepthField.Text = "" + imageFrame.MaxDepth;
this.minDepthField.Text = "" + imageFrame.MinDepth;
imageFrame.CopyPixelDataTo(depthPixels);
int stride = imageFrame.Width * imageFrame.BytesPerPixel;
this.depthBitmap.WritePixels(
new Int32Rect(0, 0, this.depthBitmap.PixelWidth, this.depthBitmap.PixelHeight),
this.depthPixels, stride, 0);

23. Adding Code Add mouse down event on image control
Find the MouseDown line, double click it to get event handler template
Event handler for mouse click:
private void image1_MouseDown(object sender, MouseButtonEventArgs e) {
Point currentPoint = e.GetPosition(image1);
this.pixelXField.Text = currentPoint.X.ToString();
this.pixelYField.Text = currentPoint.Y.ToString();
int pixelIndex = (int)(currentPoint.X + ((int)currentPoint.Y * this.frameWidth));
this.depthIndexField.Text = "" + pixelIndex;
int distancemm = this.depthPixels[pixelIndex] >> DepthImageFrame.PlayerIndexBitmaskWidth;
this.depthField.Text = "" + distancemm; }