Creating a Plugin

These 4 sections will take you through the basics on building a Live CV plugin. A Live CV plugin is a QtQuick based plugin with the added difference that it may have external dependencies outside of Qt (Open CV, dlib, etc) This makes Qt based plugins compatible in Live CV, however to use Live CV plugins in a QtQuick application the user must make sure that the application can find the plugins dependencies (which is done automatically by Live CV). Before starting, it's important to know the structure and contents of plugins and plugin packages, which is detailed in the user section.

The full project we will be building is also available at this github repository, so if you want to just start hacking through an existing project or get stuck on something, you can use the repository as a reference. The repository is also configured as a fully deployable package, which will be described in detail within the project configuration section.

To start off, we can use the available template project in QtCreator, which will create our plugin template and our first item. So, in QtCreator, begin by creating a new project. Choose library, then {Qt Quick 2 Extension Plugin}.

Set up the location, and put in the name "tutorial" as the project name.

Next, select the compiler. The third step will ask you an object class-name, and an URI. The first item we will be building is an item that will count the number of pixels whithin a matrix. We will call it CountNonZeroPixels. So set up the object class-name to be "CountNonZeroPixels", and make sure to set the URI to "tutorial" (or <module_name> in case you named the module diferently).

This path will be required by Live CV when it loads the plugin. From here go to the last step and click Finish. You will notice a project with 2 c++ classes set up. The first one is the tutorial_plugin class, which contains a hook method to register all the plugin types or QML items once the plugin loads, and the actual QtQuick item that will do the pixel counting.

If we build this plugin, although it doesn't provide any functionality, we can already use it in Live CV. We can test this if we build the project, and from our build directory we copy the library files ( *.so on linux or *.dll on windows) together with your qmldir file into your livecv installation directory, in plugins/tutorial. So we should have the following file structure:

  • livecv/plugins/tutorial/
  • livecv/plugins/tutorial/qmldir

or on windows:

  • livecv/plugins/tutorial/tutorial.dll
  • livecv/plugins/tutorial/qmldir

Next, if we open Live CV and import the tutorial plugin, we can create a CountNonZeroPixels object:

import QtQuick 2.3
import tutorial 1.0


Although this object doesn't do anything for now, the fact that we are able to create it without any errors means that we are able to use our plugin succesfully.

If you want to build QtQuick based plugins without any dependencies, from here you can simply start adding your items and configure their properties.

In the following part we will look into linking our plugin to Open CV library and start working with Live CV types to do the actual pixel counting.