Working with UI elements, such as buttons and trackbars, in an OpenCV window

In this recipe, we will learn how to add UI elements, such as buttons and trackbars, into OpenCV windows and work with them. Trackbars are useful UI elements that:

• Show the value of an integer variable, assuming the value is within a predefined range
• Allow us to change the value interactively through changing the trackbar position

Let's create a program that allows users to specify the fill color for an image by interactively changing each Red, Green, Blue (RGB) channel value.