NtKinect: Kinect V2 C++ Programming with OpenCV on Windows10

How to extract human images using Depth and BodyIndex image with Kinect V2

Prerequisite knowledge

• NtKinect: How to get RGB camera image with Kinect V2 (Fundamental Settings)
• NtKinect: How to get BodyIndex image with Kinect V2

Extract human image from RGB image using BodyIndex image

Using the bodyIndex image, you can determin which pixel the person is in (the pixel whose value is not 255). Since the bodyIndex image is the coordinates of DepthSpace, it needs to be converted to the coordinates of ColorSpace to correspond the RGB image. We also get Depth image to use depth information in the conversion.

BodyIndex image

BodyIndex images can be acquired with a resolution of 512 x 424 as well as Depth images. Up to six people can be distinguished at the same time.

In NtKinect, the obtained BodyIndex image is represented by uchar or cv::Vector3b for each pixel.

NtKinect's functions for BodyIndex image

NtKinect's member variable for BodyIndex image

Getting Depth image

Depth (distance) images can be acquired with a resolution of 512 x 424. The measurable distance range is from 500 mm to 8000 mm, but the range to recognize human beings is from 500 mm to 4500 mm.

In NtKinect, the obtained Depth image is represented by UINT16 (16 bit unsigned integer) for each pixel.

NtKinect's function for Depth image

NtKinect's member variable for Depth iamge

3 types of coordinate system of Kinect V2

Since the position and resolution of each sensor is different, the data is obtained as a value expressed in the coordinate system of each sensor. When using data obtained from different sensors at the same time, it is necessary to convert the coordinates to match.

Kinect V2 has 3 coordinate systems, ColorSpace, DepthSpace, and CameraSpace. There are 3 data types ColorSpacePoint, DepthSpacePoint, and CameraSpacePoint representing coordinates in each coordinate system.

typedef struct _ColorSpacePoint {
    float X;
    float Y;
} ColorSpacePoint;

typedef struct _DepthSpacePoint {
    float X;
    float Y;
} DepthSpacePoint;

typedef struct _CameraSpacePoint {
    float X;
    float Y;
    float Z;
} CameraSpacePoint;

Coordinate systems and data types of Kinect V2

For the RGB image, Depth image, and skeleton information, the coordinate system is different. The coordinate system of the RGB image is ColorSpace, that of the Depth image is DepthSpace, and that of the skeleton information is CameraSpace.

Kinect V2's function for mapping coordinate systems

"Coordinate system conversion function" held by ICoordinateMapper class of Kinect V2 is as follows.

NtKinect's member variable for mapping coordinate system

An instance of ICoordinateMapper class used for mapping coordinate systems in Kinect V2 is held in NtKinect's member variable "coordinateMapper".

How to write program

1. Start using the Visual Studio's project KinectV2.zip of "NtKinect: How to get RGB camera image with Kinect V2 (Fundamental Settings)"

In the following explanation, it is assumed that the folder is renamed to "KinectV2_bodyIndex2" after expanding the above zip file.

2. Place the image file in the folder where the source files of the project is located.

There is a folder called "KinectV2" in the project, and there are "main.cpp" and "Kinect.h" there.

    KinectV2_bodyIndex2\KinectV2\main.cpp
    KinectV2_bodyIndex2\KinectV2\NtKinect.h

Please copy cat.jpg to that folder. this image file is downloaded from the free material site. I do not care about any image, but I think that it is easier to handle the same size as the RGB camera image.

    KinectV2_bodyIndex2\KinectV2\cat.jpg





3. Change the contents of main.cpp.

main.cpp

#include <iostream> #include <sstream> #include "NtKinect.h" using namespace std; void copyRect(cv::Mat& src, cv::Mat& dst, int sx, int sy, int w, int h, int dx, int dy) { if (sx+w < 0 || sx >= src.cols || sy+h < 0 || sy >= src.rows) return; if (sx < 0) { w += sx; dx -= sx; sx=0; } if (sx+w > src.cols) w = src.cols - sx; if (sy < 0) { h += sy; dy -= sy; sy=0; } if (sy+h > src.rows) h = src.rows - sy; if (dx+w < 0 || dx >= dst.cols || dy+h < 0 || dy >= dst.rows) return; if (dx < 0) { w += dx; sx -= dx; dx = 0; } if (dx+w > dst.cols) w = dst.cols - dx; if (dy < 0) { h += dy; sy -= dy; dy = 0; } if (dy+h > dst.rows) h = dst.rows - dy; cv::Mat roiSrc(src,cv::Rect(sx,sy,w,h)); cv::Mat roiDst(dst,cv::Rect(dx,dy,w,h)); roiSrc.copyTo(roiDst); } void doJob() { NtKinect kinect; cv::Mat cat = cv::imread("cat.jpg"); cv::Mat bgImg; cv::Mat fgImg; while (1) { kinect.setRGB(); cv::cvtColor(kinect.rgbImage,fgImg,CV_BGRA2BGR); // cv::COLOR_BGRA2BGR (in case of opencv3 and later) bgImg = cat.clone(); kinect.setDepth(); kinect.setBodyIndex(); for (int y=0; y<kinect.bodyIndexImage.rows; y++) { for (int x=0; x<kinect.bodyIndexImage.cols; x++) { UINT16 d = kinect.depthImage.at<UINT16>(y,x); uchar bi = kinect.bodyIndexImage.at<uchar>(y,x); if (bi == 255) continue; ColorSpacePoint cp; DepthSpacePoint dp; dp.X = x; dp.Y = y; kinect.coordinateMapper->MapDepthPointToColorSpace(dp, d, &cp); int cx = (int) cp.X, cy = (int) cp.Y; copyRect(fgImg,bgImg,cx-2,cy-2,4,4,cx-2,cy-2); } } cv::imshow("cat", bgImg); auto key = cv::waitKey(1); if (key == 'q') break; } cv::destroyAllWindows(); } int main(int argc, char** argv) { try { doJob(); } catch (exception &ex) { cout << ex.what() << endl; string s; cin >> s; } return 0; }

First, we load the background image into the variable "cat".

Repeat the following process until 'q' is entered from the keyboard.

Get an RGB image from the camera. since the format of the RGB image is "BGRA" format, convert it to the "BGR" format according to the format of the jpeg image read into the variable "cat". Then, hold it in the variable "fgImg".

We holds the synthesized image in the variable "bgImg". Copy the contents of the variable "cat" as the background image.

Get the Depth image and the bodyIndexImage. When we find a pixel in which a human being appears in the bodyIndex image, we calculate the position of the corresponding ColorSpace, and paste the 4 x 4 area around the ColorSpace position from the RGB image "fgImg" on the composite image "bgImg". Since the coordinates of bodyIndex image are values in DepthSpace, they are converted to the values of ColorSpace using the depth information (the value of the Depth image). Since the resolution is different between the bodyIndex image (512 x 424) and the RGB image (1920 x 1080), a range of 4 x 4 RGB image is pasted per 1 pixel of bodyIndex image.

The copyRect(cv::Mat& src , cv::Mat& dst , int sx , int sy , int w , int h , int dx , int dy ) function defined in the program paste the partial image on src image to dst image. In the region on src, the upper left is (sx , sy ) and width and height are w and h respectively. In resion on dst/i>, the upper left is (dx , dy ). The size of the rectangular area is adjusted so that it does not extend beyond the range of image.

The programming code

copyRect(fgImg,bgImg,cx-2,cy-2,4,4,cx-2,cy-2);

which paste 4 x 4 pixels basically has the same meaning of the following codes.

for (int y=cy-2; y < cy+2; y++) { for (int x=cx-2; x < cx+2; x++) { if (x, y is contained in both image) { bgImg.at<cv::Vec3b>(y,x) = fgImg.at<cv::Vec3b>(y,x); // copy BGR pixel at (line:y, col:x) } } }

In OpenCV, the element in the cv::Mat is accessed by the "at<TYPE>(int line, int col)" member function. For the "group of 4 byte type data (ie. BGRA format pixels)", the TYPE is cv::Vec4b, and For the "group of 3 byte type data (ie. BGR format pixels)", the TYPE is cv::Vec3b.

4. When you run the program, RGB images are displayed. Exit with 'q' key.

Human images are extracted and displayed on the background image.





5. Please click here for this sample project KinectV2_bodyIndex2.zip。

Since the above zip file may not include the latest "NtKinect.h", Download the latest version from here and replace old one with it.