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119 lines
5.0 KiB
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Meanshift and Camshift {#tutorial_meanshift}
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======================
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Goal
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----
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In this chapter,
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- We will learn about the Meanshift and Camshift algorithms to track objects in videos.
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Meanshift
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---------
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The intuition behind the meanshift is simple. Consider you have a set of points. (It can be a pixel
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distribution like histogram backprojection). You are given a small window (may be a circle) and you
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have to move that window to the area of maximum pixel density (or maximum number of points). It is
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illustrated in the simple image given below:
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The initial window is shown in blue circle with the name "C1". Its original center is marked in blue
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rectangle, named "C1_o". But if you find the centroid of the points inside that window, you will
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get the point "C1_r" (marked in small blue circle) which is the real centroid of the window. Surely
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they don't match. So move your window such that the circle of the new window matches with the previous
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centroid. Again find the new centroid. Most probably, it won't match. So move it again, and continue
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the iterations such that the center of window and its centroid falls on the same location (or within a
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small desired error). So finally what you obtain is a window with maximum pixel distribution. It is
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marked with a green circle, named "C2". As you can see in the image, it has maximum number of points. The
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whole process is demonstrated on a static image below:
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So we normally pass the histogram backprojected image and initial target location. When the object
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moves, obviously the movement is reflected in the histogram backprojected image. As a result, the meanshift
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algorithm moves our window to the new location with maximum density.
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### Meanshift in OpenCV
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To use meanshift in OpenCV, first we need to setup the target, find its histogram so that we can
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backproject the target on each frame for calculation of meanshift. We also need to provide an initial
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location of window. For histogram, only Hue is considered here. Also, to avoid false values due to
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low light, low light values are discarded using **cv.inRange()** function.
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@add_toggle_cpp
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- **Downloadable code**: Click
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[here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/video/meanshift/meanshift.cpp)
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- **Code at glance:**
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@include samples/cpp/tutorial_code/video/meanshift/meanshift.cpp
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@end_toggle
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@add_toggle_python
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- **Downloadable code**: Click
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[here](https://github.com/opencv/opencv/tree/master/samples/python/tutorial_code/video/meanshift/meanshift.py)
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- **Code at glance:**
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@include samples/python/tutorial_code/video/meanshift/meanshift.py
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@end_toggle
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Three frames in a video I used is given below:
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Camshift
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--------
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Did you closely watch the last result? There is a problem. Our window always has the same size whether
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the car is very far or very close to the camera. That is not good. We need to adapt the window
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size with size and rotation of the target. Once again, the solution came from "OpenCV Labs" and it
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is called CAMshift (Continuously Adaptive Meanshift) published by Gary Bradsky in his paper
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"Computer Vision Face Tracking for Use in a Perceptual User Interface" in 1998 @cite Bradski98 .
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It applies meanshift first. Once meanshift converges, it updates the size of the window as,
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\f$s = 2 \times \sqrt{\frac{M_{00}}{256}}\f$. It also calculates the orientation of the best fitting ellipse
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to it. Again it applies the meanshift with new scaled search window and previous window location.
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The process continues until the required accuracy is met.
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### Camshift in OpenCV
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It is similar to meanshift, but returns a rotated rectangle (that is our result) and box
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parameters (used to be passed as search window in next iteration). See the code below:
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@add_toggle_cpp
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- **Downloadable code**: Click
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[here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/video/meanshift/camshift.cpp)
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- **Code at glance:**
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@include samples/cpp/tutorial_code/video/meanshift/camshift.cpp
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@end_toggle
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@add_toggle_python
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- **Downloadable code**: Click
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[here](https://github.com/opencv/opencv/tree/master/samples/python/tutorial_code/video/meanshift/camshift.py)
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- **Code at glance:**
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@include samples/python/tutorial_code/video/meanshift/camshift.py
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@end_toggle
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Three frames of the result is shown below:
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Additional Resources
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--------------------
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-# French Wikipedia page on [Camshift](http://fr.wikipedia.org/wiki/Camshift). (The two animations
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are taken from there)
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2. Bradski, G.R., "Real time face and object tracking as a component of a perceptual user
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interface," Applications of Computer Vision, 1998. WACV '98. Proceedings., Fourth IEEE Workshop
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on , vol., no., pp.214,219, 19-21 Oct 1998
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Exercises
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---------
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-# OpenCV comes with a Python [sample](https://github.com/opencv/opencv/blob/master/samples/python/camshift.py) for an interactive demo of camshift. Use it, hack it, understand
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it.
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