• Four of these six projects run on Windows, macOS, or Linux, and require a webcam. Optionally, these projects can use Raspberry Pi or another single-board computer that runs Linux.
• One project runs on Android 5.0 (Lollipop) or a later version, and requires a front-facing camera (which most Android devices have).
• One project runs on Android 4.1 (Jelly Bean) or a later version, and requires a rear-facing camera and gravity sensor (which most Android devices have). For development, it requires a Windows or macOS machine and approximately $95 worth of game development software.

You can download the example code files for this book from your account at www.packtpub.com. If you purchased this book elsewhere, you can visit www.packtpub.com/support and register to have the files emailed directly to you.

You can download the code files by following these steps:

1. Log in or register at www.packtpub.com.
2. Select the SUPPORT tab.
3. Click on Code Downloads & Errata.
4. Enter the name of the book in the Search box and follow the onscreen instructions.

Once the file is downloaded, please make sure that you unzip or extract the folder using the latest version of:

• WinRAR/7-Zip for Windows
• Zipeg/iZip/UnRarX for Mac
• 7-Zip/PeaZip for Linux

The code bundle for the book is also hosted on GitHub at https://github.com/PacktPublishing/OpenCV-4-for-Secret-Agents-Second-Edition. In case there's an update to the code, it will be updated on the existing GitHub repository.

We also have other code bundles from our rich catalog of books and videos available at https://github.com/PacktPublishing/. Check them out!

We also provide a PDF file that has color images of the screenshots/diagrams used in this book. You can download it here: http://www.packtpub.com/sites/default/files/downloads/9781789345360_ColorImages.pdf.

There are a number of text conventions used throughout this book.

CodeInText: Indicates code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles. Here is an example: "You can edit /etc/modules to check whether bcm2835-v4l2 is already listed there."

A block of code is set as follows:

set PYINSTALLER=pyinstaller

REM Remove any previous build of the app.
rmdir build /s /q
rmdir dist /s /q

REM Train the classifier.
python HistogramClassifier.py

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

        <activity
            android:name=".CameraActivity"
            android:screenOrientation="landscape"
            android:theme="@android:style/Theme.NoTitleBar.Fullscreen">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

Any command-line input or output is written as follows:

$ echo "bcm2835-v4l2" | sudo tee -a /etc/modules 

Bold: Indicates a new term, an important word, or words that you see onscreen. For example, words in menus or dialog boxes appear in the text like this. Here is an example: "Click the Android platform and then the Switch Platform button."