I'm trying to use the primaryScreen().grabWindow(self.winId() to implement a screenshot, but when the background is not clear, the screenshot is good.
If the background is transparent, the stored image is saved as a black image. Why is this happening?

Qt Code: 
  1. import os , sys , time
  2. from PyQt5 import QtWidgets
  3. from PyQt5.QtWidgets import *
  4. from PyQt5.QtGui import *
  5. from PyQt5.QtCore import *
  6. import numpy as np
  7.  
  8. class MyApp(QWidget):
  9.  
  10.     def __init__(self):
  11.         super().__init__()                
  12.         self.setWindowFlags(Qt.WindowStaysOnTopHint | Qt.FramelessWindowHint)        
  13.         self.setAttribute(Qt.WA_TranslucentBackground)   
  14.         self.initUI()
  15.  
  16.     def initUI(self):  
  17.         self.setGeometry(30, 30, 300, 300)        
  18.         self.setWindowTitle('blRect')        
  19.         self.show()
  20.  
  21.     def screenshot(self):             
  22.         self.preview_screen = QApplication.primaryScreen().grabWindow(self.winId() )               
  23.         self.preview_screen.save('test.jpg', "jpg")
  24.  
  25.     def paintEvent(self, e):
  26.         qp = QPainter()
  27.         qp.begin(self)       
  28.         self.draw_rect(qp)        
  29.         self.screenshot()
  30.         qp.end()
  31.  
  32.     def draw_rect(self, qp):
  33.         qp.setBrush(QColor(255, 255, 0))
  34.         qp.setPen(QPen(QColor(0, 0, 0), -1))
  35.         qp.setBrush(QColor(0, 0, 0))    
  36.         # print(self.width() , self.height())    
  37.         for i in range(3):
  38.             rand_x = 100 * np.random.randn()
  39.             rand_y = 100 * np.random.randn()
  40.             qp.drawRect(self.width() /2 + rand_x, self.height() / 2 + rand_y, 100, 20)
  41.  
  42. def main():
  43.     app = QApplication(sys.argv)    
  44.     ex = MyApp()
  45.     sys.exit(app.exec_())
  46.  
  47. if __name__ == '__main__':
  48.     main()
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