How to Read QR Codes from Screen Capture in Python with Qt and Dynamsoft Barcode SDK

Mar 24, 2026

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 3 in a 7-Part Series.

What you’ll build: A Python desktop application that lets you snip any area of your screen (or capture the full screen) and instantly decode all QR codes and barcodes in the captured region using PySide6, OpenCV, and the Dynamsoft Barcode Reader SDK.

Key Takeaways

Python can read QR codes directly from screen captures without a camera, eliminating Moiré pattern issues caused by photographing monitors.
The Dynamsoft Capture Vision SDK provides a cross-platform Python API for high-accuracy barcode and QR code decoding from in-memory images.
PySide6 (Qt for Python) combined with PIL’s ImageGrab enables pixel-perfect screen region selection and capture on Windows, macOS, and Linux.
This approach is ideal for desktop automation, QA testing, and any workflow where decoded QR data needs to stay on the PC.

Common Developer Questions

How do I read a QR code from a screen capture in Python?
Can I decode multiple QR codes from a screenshot using Python?
How do I build a screen snipping tool with PySide6 and decode barcodes from the captured area?

Prerequisites

OpenCV
```
  python3 -m pip install opencv-python
```
PySide6
```
  python3 -m pip install PySide6
```

Dynamsoft Barcode Reader

  python3 -m pip install dynamsoft-capture-vision-bundle

Get a 30-day free trial license for Dynamsoft Barcode Reader

How to Build a Screen QR Code Reader in 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.

Step 1: Add Buttons for Screen 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.

Step 3: Handle Mouse Events for Area Selection

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)

Step 4: Capture Screen Images with PIL

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()))

Step 5: Minimize and Restore 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() 

Step 6: Test the Screen QR Code 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.

Common Issues and Edge Cases

Moiré patterns on high-DPI screens: If you capture QR codes displayed on a monitor at non-native resolution, Moiré interference can reduce decode accuracy. Capture at native screen resolution for best results.
DPI scaling mismatch on Windows: On multi-monitor setups with different DPI scales, ImageGrab.grab() coordinates may not match the Qt widget coordinates. Set your Python process to DPI-aware mode (ctypes.windll.shcore.SetProcessDpiAwareness(2)) before creating the QApplication.
Empty or black captures on Wayland (Linux): PIL.ImageGrab.grab() relies on X11; on Wayland-based desktops it may return a black image. Use a Wayland-compatible screenshot tool (e.g., grim) as a fallback.

Source Code

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