Scanning QR Code from Desktop Screen with Qt and Python Barcode SDK

Oct 07, 2021

In everyday life, we often use our smartphones to scan QR codes. However, when working on a computer, using a smartphone isn’t always the most convenient option for scanning QR codes that appear on your screen. Photos taken from monitors can have Moiré patterns, which can interfere with QR code recognition. Additionally, you might want to use the decoded information directly on your PC, such as a URL to open a website. This article will guide you through creating a simple Python barcode reader that makes QR code scanning directly from your desktop screen easy and efficient.

This article is Part 5 in a 11-Part Series.

Prerequisites

OpenCV
```
  python3 -m pip install opencv-python
```
PySide6
```
  python3 -m pip install PySide6
```
Dynamsoft Barcode Reader
```
  python3 -m pip install dbr
```
Dynamsoft Barcode SDK License

Steps to Read QR Codes from Desktop Screens with Python

To enhance the GUI Python barcode reader built with Qt for Python, OpenCV, and Dynamsoft Barcode Reader, we’ll add screen snipping functionality. Drawing inspiration from this snipping tool implementation, we’ll follow these steps to implement screenshot functionality:

Create a custom Qt widget that overlays the screen.
Draw the selected area in the paintEvent() function as the mouse moves.
Capture the image of the selected area using PIL.ImageGrab.grab() when the mouse is released.

Adding Buttons for Snipping Events

Open the design.ui file in Qt Designer and add two buttons to trigger snipping events.

After saving the updated layout, compile the UI file to a Python file:

PySide6-uic design.ui -o design.py

In app_advanced.py, connect the new buttons to their respective slot functions:

self.ui.pushButton_area.clicked.connect(self.snipArea)
self.ui.pushButton_full.clicked.connect(self.snipFull)

Create a new file SnippingTool.py and define a custom Qt widget for screen snipping:

import numpy as np
import cv2
from PIL import ImageGrab
from PySide6 import QtWidgets, QtCore, QtGui
from PySide6.QtCore import Qt

class SnippingWidget(QtWidgets.QWidget):
    is_snipping = False

    def __init__(self, parent=None, app=None):
        super(SnippingWidget, self).__init__()
        self.parent = parent
        self.setWindowFlags(Qt.WindowStaysOnTopHint)

        self.screen = app.primaryScreen()
        self.setGeometry(0, 0, self.screen.size().width(), self.screen.size().height())
        self.begin = QtCore.QPoint()
        self.end = QtCore.QPoint()
        self.onSnippingCompleted = None

    def start(self):
        SnippingWidget.is_snipping = True
        self.setWindowOpacity(0.3)
        QtWidgets.QApplication.setOverrideCursor(QtGui.QCursor(QtCore.Qt.CrossCursor))
        self.show()

Ensure the widget’s size matches the screen resolution obtained from the primaryScreen() function.

Handling Mouse Events

Implement the mouse event handlers to define the selected area:

def mousePressEvent(self, event):
    self.begin = event.pos()
    self.end = self.begin
    self.update()

def mouseMoveEvent(self, event):
    self.end = event.pos()
    self.update()

def mouseReleaseEvent(self, event):
    SnippingWidget.is_snipping = False
    QtWidgets.QApplication.restoreOverrideCursor()
    x1 = min(self.begin.x(), self.end.x())
    y1 = min(self.begin.y(), self.end.y())
    x2 = max(self.begin.x(), self.end.x())
    y2 = max(self.begin.y(), self.end.y())

    self.repaint()
    QtWidgets.QApplication.processEvents()
    self.close()

Draw a rectangle to indicate the selected area in the paintEvent() function:

def paintEvent(self, event):
    if SnippingWidget.is_snipping:
        brush_color = (128, 128, 255, 100)
        lw = 3
        opacity = 0.3
    else:
        self.begin = QtCore.QPoint()
        self.end = QtCore.QPoint()
        brush_color = (0, 0, 0, 0)
        lw = 0
        opacity = 0

    self.setWindowOpacity(opacity)
    qp = QtGui.QPainter(self)
    qp.setPen(QtGui.QPen(QtGui.QColor('black'), lw))
    qp.setBrush(QtGui.QColor(*brush_color))
    rect = QtCore.QRectF(self.begin, self.end)
    qp.drawRect(rect)

Capturing Screen Images

Use the PIL library to capture images from the screen after releasing the mouse:

Capture a selected area:

  img = ImageGrab.grab(bbox=(x1, y1, x2, y2))
    
  try:
      img = cv2.cvtColor(np.array(img), cv2.COLOR_RGB2BGR)
  except:
      img = None

Capture the full screen:

  img = ImageGrab.grab(bbox=(0, 0, self.screen.size().width(), self.screen.size().height()))

Minimizing and Restoring the Application Window

To avoid blocking the snipping area, minimize the application window before starting the snipping widget and restore it afterward:

def onSnippingCompleted(self, frame):
    self.setWindowState(Qt.WindowMaximized)
    if frame is None:
        return 

    frame, self._results = self._barcodeManager.decode_frame(frame)
    self.showResults(frame, self._results)

def snipArea(self):
    self.setWindowState(Qt.WindowMinimized)
    self.snippingWidget.start()    

def snipFull(self):
    self.setWindowState(Qt.WindowMinimized)
    self.snippingWidget.fullscreen() 

Testing the Screen QR Reader

Run the barcode recognition program:
```
 python3 app_advanced.py
```
Search Google for QR code.
Click the Select Area button to scan one or multiple QR codes displayed in the search results.

Alternatively, you can perform a full-screen barcode scan with a single click.

Source Code

https://github.com/yushulx/python-barcode-qrcode-sdk/tree/main/examples/official/9.x/qt_gui

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