'Change the composition mode of the item (see QPainter::CompositionMode
in the Qt documentation). This is useful when overlaying multiple ImageItems.
**Most common arguments:**
QtGui.QPainter.CompositionMode_SourceOver     Default; image replaces the background if it
is opaque. Otherwise, it uses the alpha channel to blend
the image with the background.
QtGui.QPainter.CompositionMode_Overlay        The image color is mixed with the background color to
reflect the lightness or darkness of the background.
QtGui.QPainter.CompositionMode_Plus           Both the alpha and color of the image and background pixels
are added together.
QtGui.QPainter.CompositionMode_Multiply       The output is the image color multiplied by the background.'

def setCompositionMode(self, mode):

    self.paintMode = mode
    self.update()
