QPainter slow, how to write into a buffer instead of repainting again and again?

Hi all

I am writing a python application with PyQT for MAEMO 5 (N900).

I have to draw a graph with a lot of lines (see http://maemo.org/downloads/product/M...sleepanalyser/)

It works fine, how ever now I want to put the draw into a scroll area.
It works, however it is very slow when I scroll the area, as it is redrawing it all the time.
Is there a way I can store the drawing into a pixmap so it takes it from there?
It is a static drawing and does actually only have to be drawn once when I load the widget.
I searched the web all over but could not find a solution, mayby I search at the wrong corner.

A working example:

Code:

---

def paintEvent(self, event):
        paint = QtGui.QPainter()
        paint.begin(self)
 
        paint.setPen(QtGui.QColor("white"))
        paint.setBrush(QtGui.QColor("black"))    
 
        paint.drawRect(0,0,800,50)   
        paint.setPen(QtGui.QColor("yellow"))
        for i in range(0,  800):
            l=int(random.uniform(0,30))
            paint.drawLine(i, 48, i, l)
        paint.end()

---

I guess I have to use something like QImage or QBitmap, but I dont know how.
All I want is to draw it once and buffer it somewhere, so it can be taken from there every time paintEvent gets called.
Thank you for your advice!

You can use a QPainter to draw on a pixmap for example.
I don't know how to do this in python, but in C++ it's done like this:

Code:

---

QPixmap myPixmap(100, 100);
QPainter myPainter(&myPixmap);
myPainter.drawLine(...);

---

This might be interesting too:
http://techbase.kde.org/Development/...cs/Performance

...and with using QPixmap, QPixmapCache can be of interest too. See also a nice article here: http://doc.qt.nokia.com/qq/qq12-qpixmapcache.html

Hi all

Thank you very much for your tipps.
With the mentioned links, I now got it working.


For others, here the minimal code:

Code:

---

def CreateGraph()
    data.GraphBitMap = QtGui.QPixmap(500, 50) #create bitmap for graph
    GenerateGraph(data.GraphBitMap,  QtGui) # draw graph
 
 
 
def GenerateGraph(self,  QtGui):
    paint = QtGui.QPainter()
    paint.begin(self)
 
    #drawwing data here
 
    paint.end()
 
 
 
def paintEvent(self, event):
 
    paint = QtGui.QPainter()
    paint.begin(self)
 
    paint.drawPixmap(0, 0, data.GraphBitMap) #load graph from Bitmap
 
    paint.end()

---

I generate the pitmap, whenever it changes its content and store it in a cache.
Now, when ever paintEvent gets called, i just reload the bitmap, instead of creating it again.

It works very well and seems to use almost no CPU.