Doxygen tutorials: python basic

doc/py_tutorials/py_gui/py_trackbar/py_trackbar.markdown

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+Trackbar as the Color Palette {#tutorial_py_trackbar}
+=============================
+
+Goal
+----
+
+-   Learn to bind trackbar to OpenCV windows
+-   You will learn these functions : **cv2.getTrackbarPos()**, **cv2.createTrackbar()** etc.
+
+Code Demo
+---------
+
+Here we will create a simple application which shows the color you specify. You have a window which
+shows the color and three trackbars to specify each of B,G,R colors. You slide the trackbar and
+correspondingly window color changes. By default, initial color will be set to Black.
+
+For cv2.getTrackbarPos() function, first argument is the trackbar name, second one is the window
+name to which it is attached, third argument is the default value, fourth one is the maximum value
+and fifth one is the callback function which is executed everytime trackbar value changes. The
+callback function always has a default argument which is the trackbar position. In our case,
+function does nothing, so we simply pass.
+
+Another important application of trackbar is to use it as a button or switch. OpenCV, by default,
+doesn't have button functionality. So you can use trackbar to get such functionality. In our
+application, we have created one switch in which application works only if switch is ON, otherwise
+screen is always black.
+@code{.py}
+import cv2
+import numpy as np
+
+def nothing(x):
+    pass
+
+# Create a black image, a window
+img = np.zeros((300,512,3), np.uint8)
+cv2.namedWindow('image')
+
+# create trackbars for color change
+cv2.createTrackbar('R','image',0,255,nothing)
+cv2.createTrackbar('G','image',0,255,nothing)
+cv2.createTrackbar('B','image',0,255,nothing)
+
+# create switch for ON/OFF functionality
+switch = '0 : OFF \n1 : ON'
+cv2.createTrackbar(switch, 'image',0,1,nothing)
+
+while(1):
+    cv2.imshow('image',img)
+    k = cv2.waitKey(1) & 0xFF
+    if k == 27:
+        break
+
+    # get current positions of four trackbars
+    r = cv2.getTrackbarPos('R','image')
+    g = cv2.getTrackbarPos('G','image')
+    b = cv2.getTrackbarPos('B','image')
+    s = cv2.getTrackbarPos(switch,'image')
+
+    if s == 0:
+        img[:] = 0
+    else:
+        img[:] = [b,g,r]
+
+cv2.destroyAllWindows()
+@endcode
+The screenshot of the application looks like below :
+
+![image](images/trackbar_screenshot.jpg)
+
+Exercises
+---------
+
+-#  Create a Paint application with adjustable colors and brush radius using trackbars. For drawing,
+    refer previous tutorial on mouse handling.
+