Detecting and Decoding QR Codes in Python with YOLO and Dynamsoft Barcode Reader

Nov 19, 2020

In the past two weeks, I trained a custom YOLOv3 model specifically for QR code detection and tested it using Darknet. In this article, I will demonstrate how to use OpenCV’s DNN (Deep Neural Network) module to load the YOLO model for detecting QR codes from static images and real-time camera video streams. Additionally, I will show how to use the Dynamsoft Barcode Reader to decode QR codes from the detected regions provided by YOLO.

This article is Part 1 in a 9-Part Series.

QR code detection with OpenCV DNN and YOLOv3

Prerequisites

OpenCV 4.x
```
  pip install opencv-python
```

Dynamsoft Barcode Reader

  pip install dynamsoft-capture-vision-bundle

Obtain a Dynamsoft Barcode Reader trial license and update your code with the provided license key:

  from dynamsoft_capture_vision_bundle import *

  errorCode, errorMsg = LicenseManager.init_license("LICENSE-KEY")
  if errorCode != EnumErrorCode.EC_OK and errorCode != EnumErrorCode.EC_LICENSE_CACHE_USED:
      print("License initialization failed: ErrorCode:",
              errorCode, ", ErrorString:", errorMsg)
    
  cvr_instance = CaptureVisionRouter() 

QR Code Detection with YOLO and OpenCV Python

To quickly get started with OpenCV’s DNN module, you can refer to the sample script object_detection.py from the OpenCV GitHub repository. Below, we’ll build on this sample to implement QR code detection using a YOLO model.

Step-by-Step Implementation of QR Detection

Load the Image: Use the imread() function to load an image into a Mat object. To manage large images, define maximum width and height values:

 import cv2 as cv
 import numpy as np
    
 frame = cv.imread("416x416.jpg")
 threshold = 0.6
 maxWidth = 1280
 maxHeight = 720
 imgHeight, imgWidth = frame.shape[:2]
 hScale = 1
 wScale = 1
 thickness = 1
    
 if imgHeight > maxHeight:
     hScale = imgHeight / maxHeight
     thickness = 6
 if imgWidth > maxWidth:
     wScale = imgWidth / maxWidth
     thickness = 6

Initialize the YOLO Model: Load the YOLO configuration (*.cfg), weights (*.weights), and class names (*.names) files:

 classes = open('qrcode.names').read().strip().split('\n')
 net = cv.dnn.readNetFromDarknet('qrcode-yolov3-tiny.cfg', 'qrcode-yolov3-tiny.weights')
 net.setPreferableBackend(cv.dnn.DNN_BACKEND_OPENCV)

Create a Blob: Convert the image into a blob object suitable for input into the network:

 blob = cv.dnn.blobFromImage(frame, 1/255, (416, 416), swapRB=True, crop=False)

Perform Inference: Feed the blob into the network and run forward pass to get the detections:

 ln = net.getLayerNames()
 ln = [ln[i - 1] for i in net.getUnconnectedOutLayers().flatten()]
 net.setInput(blob)
 outs = net.forward(ln)

Post-Processing: Extract class names, confidence scores, and bounding boxes from the network outputs, and draw them using OpenCV:

 def postprocess(frame, outs):
     frameHeight, frameWidth = frame.shape[:2]
    
     classIds = []
     confidences = []
     boxes = []
    
     for out in outs:
         for detection in out:
             scores = detection[5:]
             classId = np.argmax(scores)
             confidence = scores[classId]
             if confidence > threshold:
                 x, y, width, height = detection[:4] * np.array(
                     [frameWidth, frameHeight, frameWidth, frameHeight])
                 left = int(x - width / 2)
                 top = int(y - height / 2)
                 classIds.append(classId)
                 confidences.append(float(confidence))
                 boxes.append([left, top, int(width), int(height)])
    
     indices = cv.dnn.NMSBoxes(boxes, confidences, threshold, threshold - 0.1)
    
     if not isinstance(indices, tuple):
         for i in indices.flatten():  # Flatten the indices if they are in nested format
             box = boxes[i]
             left = box[0]
             top = box[1]
             width = box[2]
             height = box[3]
    
             # Crop the detected object from the frame
             cropped_image = frame[top:top + height, left:left + width]
             cv.imshow('cropped', cropped_image)
             cv.imwrite('cropped.jpg', cropped_image)
    
             # Draw bounding box around the detected object
             cv.rectangle(frame, (left, top), (left + width,
                                               top + height), (0, 0, 255), thickness)
    
             # Draw class name and confidence on the bounding box
             label = '%s:%.2f' % (classes[classIds[i]], confidences[i])
             cv.putText(frame, label, (left, top),
                        cv.FONT_HERSHEY_SIMPLEX, 0.5, (255, 255, 255))
    

Resize and Display the Image: Adjust the image size to fit your display:

 if hScale > wScale:
     frame = cv.resize(frame, (int(imgWidth / hScale), maxHeight))
 elif hScale < wScale:
     frame = cv.resize(frame, (maxWidth, int(imgHeight / wScale)))
 cv.imshow('QR Detection', frame)
 cv.waitKey()

Python QR Code detection

Decoding QR Codes with Dynamsoft Barcode Reader

After detecting QR codes, use the bounding boxes to decode the QR content. Setting the region parameters speeds up decoding compared to scanning the full image:

from dynamsoft_capture_vision_bundle import *

errorCode, errorMsg = LicenseManager.init_license("LICENSE-KEY")
if errorCode != EnumErrorCode.EC_OK and errorCode != EnumErrorCode.EC_LICENSE_CACHE_USED:
    print("License initialization failed: ErrorCode:",
            errorCode, ", ErrorString:", errorMsg)

cvr_instance = CaptureVisionRouter()  
 
def decodeframe(frame, left, top, right, bottom):
    error_code, error_msg, settings = cvr_instance.get_simplified_settings(EnumPresetTemplate.PT_READ_BARCODES.value)
    quad = Quadrilateral()
    quad.points = [Point(left, top), Point(right, top), Point(right, bottom), Point(left, bottom)]
    settings.roi = quad
    settings.roi_measured_in_percentage = False
    cvr_instance.update_settings(EnumPresetTemplate.PT_READ_BARCODES.value, settings)

    result = cvr_instance.capture(frame, EnumPresetTemplate.PT_READ_BARCODES.value)
    return result

Note: Replace LICENSE-KEY with your actual license key.

Python QR code scan

Source Code

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