Project 9: OpenCV Video Template

This project opens a video, plays it, and exposes an OpenCV Mat instance and simple API to control the video feed and render the current Mat in real time on a GUI. It uses the GUI setup from Project 6 on the tools from Project 5.

Table of Contents

• Usage
  • Video Viewer
  • Window Settings
  • Style Editor
  • Demo Window
• Framework
• Resources
  • Code samples
  • Documentation

Usage

Four windows allow the user to control many aspects of the application:

Video Viewer

This window shows the rendered image on the screen and some useful information about the image and its underlying OpenGL texture. These are its controls:

1. Video Position allows users to view the index if the current frame and to scrub the video feed.
2. Loop starts the video at the beginning if the next frame couldn’t be displayed.
3. Play advances to the next frame every time the screen refreshes.
4. Next frame advances to the next frame.
5. Display Scale allows users to zoom in to and out of the image.

Hovering over the image creates a widget that allows the user to see a 4x magnified slice of the image.

Window Settings

This window controls the graphics settings:

1. Display Scale sets the magnification of the GUI, which may increase the viewing experience of users with high-DPI displays. This defaults to the operating system’s internal scale, but it can be set to any number.
2. Swap Interval sets the number of monitor-refreshes to wait until the GUI redraws.

Style Editor

This allows the user to customize the aesthetics of the GUI. It can be exported to C++ code.

It is a window provided by and included in the ImGui source code.

Demo Window

This is a demo window provided by and included in the ImGui source code.

Framework

Four main file pairs control the user interface

• main.cpp/main.h: a shell for starting the application. Only contains gui::begin(); in a main() function.
• app.cpp/app.h: responsible for creating a window with GLFW, loading OpenGL through GLAD, maintaining a render loop, initializing ImGui before rendering, and rendering the GUI at the end of the render loop. It also contains some application-specific helper functions in the namespace gui:
  • float estimateSystemScale(): uses GLFW to estimate the operating system’s native scale.
  • GLFWwindow* getCurrentWindow(): gets the current GLFWwindow instance.
  • void setSwapInterval(int newSwapInterval): sets the window’s swap interval, i.e. how many frame refreshes before the GUI refreshes. This also sets the UI’s framerate. Higher swap interval values yield a lower framerate, but reduce the CPU consumption and increase battery life of machines running the software. Note that this is specific to the operating system and may not work effectively.
  • void setClearColor(int red, int green, int blue, int alpha): sets the color of the background.
  • void setGuiScale(float guiScale): sets the scale of the GUI.
  • Most importantly gui::begin() starts the GUI and is called from main.
• ui.cpp/ui.h: responsible for the GUI layout, controls, and behavior of the application. Without this files, only a blank window will be displayed.
  • void setStyle(): sets the look-and-feel of the UI. The settings I provide in this code create a gray background with black windows, white text, and orange highlights, but all the preferences I set can be changed in the style editor built into the application.
  • void populateUI(): responsible for the layout of the entire user interface.
  • void initUI(): initializes the variables and settings used for the user interface prior to the first time void populateUI() is called.

Therefore, populateUI()is not at all responsible for rendering the UI, yet it controls everything about the struture, layout, and appearance of the UI. Likewise, render() controls the window and all tools used to display the GUI, but doesn’t need to worry about how the GUI is set up or what it does.

Resources

Code samples

• Project 05: OpenCV and ImGui
• Project 06: Raytracing in One Weekend

  Documentation