Question

I’m trying to make a very basic image creator using PySide. But I’m having some troubles to draw things.

This is the portion of my script responsible for the draw area:

class Area(QWidget):

   global x1
   global y1
   global x2
   global y2
   try:
      x1
   except NameError:
      x1=0
   try:
      y1
   except NameError:
      y1=0

   try:
      x2
   except NameError:
      x2=100
   try:
      y2
   except NameError:
      y2=100

   def mousePressEvent(self, me):
      global x1     
      global y1
      x1 = me.x()
      y1 = me.y()

   def mouseMoveEvent(self, mo):
      global x2
      global y2
      x2 = mo.x()
      y2 = mo.y()
      self.update()

   def paintEvent(self, pe):
      global paint
      paint = QPainter()
      paint.begin(self)
      paint.drawLine(x1,y1,x2,y2)
      paint.save()
      paint.restore()

blankarea = Area()
blankarea.show()

Basicaly, what the script does is just draw a line based on mouse events. However, every time the mouse click event occurs (responsible for start a line) it erases the line previously made, even after using QPainter.save() and QPainter.restore(). What could be causing this problem?

Answer 1

The paintEvent() method is repainting the entire widget every time it is called. The drawing from the previous calls is completely replaced with the results of each future paintEvent() call.

If you want the effect you are describing, you will need to accumulate the points and draw the entire sequence of them each time.

From the docs:
http://pyside.github.io/docs/pyside/PySide/QtGui/QWidget.html#PySide.QtGui.PySide.QtGui.QWidget.paintEvent

When the paint event occurs, the update region has normally been
erased, so you are painting on the widget’s background.