BuildAnAppToTakeAPhoto.md

Build an app to take a photo

In this step, you will build an app in Python that will launch the camera and take a photo.

Create folders for the project

• Create a folder called AttendeeDetector somewhere on your machine.

Open this folder in Visual Studio Code

• Launch Visual Studio Code.

• Open the newly created folder

  • On MacOS select File->Open...
  • On Windows select File->Open Folder...

• Navigate to the new AttendeeDetector folder and select Open.

You will see the empty folder appear in the Explorer.

Install the required Python packages

This app will need the OpenCV Python package. This can be installed from Pip.

• From Visual Studio Code, select Terminal->New Terminal

• In the terminal run the following command:

  pip install opencv-python

Create a file for the code

• From Visual Studio Code, select the New File button in the Explorer bar in the AttendeeDetector section.

  

• Name the file picturetaker.py.

  You may be prompted to install a Linter. This is a tool that can inspect your code for error as you write it. If a popup appears asking if you want to install this, select Install.

Write the code

• Add the following code to this file:

  import cv2
  
  cam = cv2.VideoCapture(0)
  cv2.namedWindow('Press space to take a photo')
  
  while True:
    ret, frame = cam.read()
    cv2.imshow('Press space to take a photo', frame)
  
    k = cv2.waitKey(1)
    if k%256 == 32:
      break
  
  cam.release()
  cv2.destroyAllWindows()

• Save the file.

  If you don't to have to remember to always save files, you can turn on Auto Save by selecting File -> Auto Save.

Run the code

There are two ways to run this code:

• Start debugging by either:

  • Select Debug -> Start Debugging
  • Press F5
  • Select the Debug pane from the toolbar on the left and select the green Start Debugging button.

  If you use one of these methods you will be able to set breakpoints and debug your code.

• Run this directly from the Visual Studio Code terminal using the command

  python picturetaker.py

  If you use this method you will not be able to set breakpoints and debug your code.

When the code runs, a window will appear showing the view from your camera. Press Space to end the app.

What does this code do

The overall flow of this code is:

1. Create the camera and a window
2. Loop forever, capturing a frame from the camera and displaying it on screen
3. Every millisecond check for a key press. If no key is pressed, or a key other than space is pressed, continue the loop capturing the next frame from the camera and display it on screen.
4. If the space key is pressed, break out of the loop and clean up the app before exiting

Lets look in more detail at the actual code.

import cv2

This tells the Python compiler that we want to use code in the cv2 module. This module was installed as part of the opencv-python package.

cam = cv2.VideoCapture(0)
...
cam.release()

This creates an instance of the camera that can be used to capture an image. Once the code has run, this is released cleaning up any resources needed. The value of 0 passed to the VideoCapture function will use the only camera if your device has one, or the first if your device has more than one. When you run this code, if the wrong camera is being used on a device with multiple cameras (for example the rear one instead of a front-facing one), then you can change this value to 1 or higher to select the right camera.

cv2.namedWindow('Press space to take a photo')
...
cv2.destroyAllWindows()

This will create a window with the title Press space to take a photo, and show it on the screen. Once the app has finished running, the window will be removed. This window will be named the same as its title, and this name can be used later to reference this window in other code.

while True:
  ...

This code will loop the suite continuously until it is broken out of. The while True: loop means run all this code until something causes control to break out.

ret, frame = cam.read()
cv2.imshow('Press space to take a photo', frame)

This reads the current image from the camera into the frame variable. This is then shown inside the window named Press space to take a photo. This causes the image from the camera to appear inside the window that was created earlier.

k = cv2.waitKey(1)

This reads a key from the board, waiting 1 millisecond for a key to be pressed. If a key is pressed, k will be a value indicating which key was pressed, otherwise it will be -1.

if k%256 == 32:
  break

This checks if the value of the key is the same as the space key (32), and if so, it breaks out of the while loop allowing the program to end.

Next step

In this step you created the first Python file for this project, adding code to launch the camera in a window and capture an image. In the next step you will create a Flask Web App that this photo can be sent to.