The OpenCV functions that allow us to interact with the operating system, the file system, and hardware such as cameras are collected into a library called HighGUI (which stands for "high-level graphical user interface"). HighGUI allows us to open windows, to display images, to read and write graphics-related files (both images and video), and to handle simple mouse, pointer, and keyboard events. We can also use it to create other useful doodads like sliders and then add them to our windows. If you are a GUI guru in your window environment of choice, then you might find that much of what HighGUI offers is redundant. Yet even so you might find that the benefit of cross-platform portability is itself a tempting morsel.
From our initial perspective, the HighGUI library in OpenCV can be divided into three parts: the hardware part, the file system part, and the GUI part. [38] We will take a moment to overview what is in each part before we really dive in.
The hardware part is primarily concerned with the operation of cameras. In most operating systems, interaction with a camera is a tedious and painful task. HighGUI allows an easy way to query a camera and retrieve the latest image from the camera. It hides all of the nasty stuff, and that keeps us happy.
The file system part is concerned primarily with loading and saving images. One nice feature of the library is that it allows us to read video using the same methods we would use to read a camera. We can therefore abstract ourselves away from the particular device we're using and get on with writing interesting code. In a similar spirit, HighGUI provides us with a (relatively) universal pair of functions to load and save still images. These functions simply rely on the filename extension and automatically handle all of the decoding or encoding that is necessary.
The third part of HighGUI is the window system (or GUI). The library provides some simple functions that will allow us to open a window and throw an image into that window. It also allows us to register and respond to mouse and keyboard events on that window. These features are most useful when trying to get off of the ground with a simple application. Tossing in some slider bars, which we can also use as switches,[39] we find ourselves able to prototype a surprising variety of applications using only the HighGUI library.
As we proceed in this chapter, we will not treat these three segments separately; rather, we will start with some functions of highest immediate utility and work our way to the subtler points thereafter. In this way you will learn what you need to get going as soon as possible.
[38] Under the hood, the architectural organization is a bit different from what we described, but the breakdown into hardware, file system, and GUI is an easier way to organize things conceptually. The actual HighGUI functions are divided into "video IO", "image IO", and "GUI tools". These categories are represented by the cvcap*, grfmt*, and window* source files, respectively.
[39] OpenCV HighGUI does not provide anything like a button. The common trick is to use a two-position slider to achieve this functionality (more on this later).
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