Strange Painter behaviour

[cwomeijer]

Hi Everyone,

I seem to have some odd behaviour I can't explain.
Here is my code:

void ScribbleArea::drawGrid()
{
	QPainter painter(&image);
    painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
								Qt::RoundJoin));
 
	for(int y=0; y<image.height(); y+=gridSize)
		for(int x=0; x<image.width(); x+=gridSize)
			painter.drawPoint(x,y);
}
 
void ScribbleArea::drawPolygon()
{
	QPainter painter(&image);
    painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
								Qt::RoundJoin));
 
#if 0
	drawGrid();
#else
	for(int y=0; y<image.height(); y+=gridSize)
		for(int x=0; x<image.width(); x+=gridSize)
			painter.drawPoint(x,y);
#endif
 
	painter.drawLine(myPolygon);
    modified = true;
}
 
void ScribbleArea::paintEvent(QPaintEvent * /* event */)
{
    QPainter painter(this);
    painter.drawImage(QPoint(0, 0), image);
}

void ScribbleArea::drawGrid()
{
	QPainter painter(&image);
    painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
								Qt::RoundJoin));

	for(int y=0; y<image.height(); y+=gridSize)
		for(int x=0; x<image.width(); x+=gridSize)
			painter.drawPoint(x,y);
}

void ScribbleArea::drawPolygon()
{
	QPainter painter(&image);
    painter.setPen(QPen(myPenColor, myPenWidth, Qt::SolidLine, Qt::RoundCap,
								Qt::RoundJoin));

#if 0
	drawGrid();
#else
	for(int y=0; y<image.height(); y+=gridSize)
		for(int x=0; x<image.width(); x+=gridSize)
			painter.drawPoint(x,y);
#endif
	
	painter.drawLine(myPolygon);
    modified = true;
}

void ScribbleArea::paintEvent(QPaintEvent * /* event */)
{
    QPainter painter(this);
    painter.drawImage(QPoint(0, 0), image);
}

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Please note that this code copied from the scribble example!
Anyway I wanted to add my own grid drawing routine
Which seems to work just fine when called seperatly.

However if drawPolygon() calls drawGrid() the grid never shows
up. I do get to the routine but I simply don't see the grid drawn.
If I copy the routine into the drawPolygon() (see the #if 0)
then it does work.
Can someone explain this?

Regards

Otto

[jpn]

I suspect it has something to do with that you initialize 2 parallel painters on the image.
Does it work if you use the same painter, like this?

void ScribbleArea::drawGrid(QPainter* painter)
{
    for(int y=0; y<image.height(); y+=gridSize)
        for(int x=0; x<image.width(); x+=gridSize)
            painter->drawPoint(x,y);
}
 
void ScribbleArea::drawPolygon()
{
    QPainter painter(&image);
    ...
    drawGrid(&painter);
    ...
}

void ScribbleArea::drawGrid(QPainter* painter)
{
    for(int y=0; y<image.height(); y+=gridSize)
        for(int x=0; x<image.width(); x+=gridSize)
            painter->drawPoint(x,y);
}

void ScribbleArea::drawPolygon()
{
    QPainter painter(&image);
    ...
    drawGrid(&painter);
    ...
}

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[cwomeijer]

Yes, that was what I also suspected.
However after the modifications the behaviour was
identical as before.

It really looks like it doesn't like to call any other methods
while being in drawPolygon().