How to paint cross filling?

[lwz]

Fill curve with different brushes, while two curves' cross. Like the picture
选区_001.png


My code like this.

void FillAreaCurve::fillCurve(QPainter *painter, const QwtScaleMap &xMap, const QwtScaleMap &yMap, const QRectF &canvasRect, QPolygonF &polygon) const
{
.........
// calculate crossing points
for ( int i = 0; i < corssingNum; i++ ) {
......
// according to different terms, get the clipRect and assign different brush to painter.
painter->save();
painter->setClipRect( clipRect, Qt::IntersectClip );
closePolyline( painter, xMap, yMap, polygon );
polygon = QwtClipper::clipPolygonF( canvasRect, polygon, true );
painter->setPen( Qt::NoPen );
painter->setBrush( brush );
QwtPainter::drawPolygon( painter, polygon );
painter->restore();
}
}

Apparently, how much time it would consume is determined by the crossing points' number, which is out of my control.
Is there another way to fill my curves quicker ?

[Uwe]

If you don't want to implement the filling algo yourself ( setting pixels on a QImage ) using clip rects is not a bad idea.

Assuming you already have created a polygon ( clipped to the canvas rectangle ) with the outline of both points in paint device coordinates and your list of crossing points is also clipped, so that all points outside the canvas rectangle are not included:
Then all that needs to be done in your loop is:

double y1 = canvasRect.y();
 
for ( int i = 0; i < crossingNum; i++ )
{
    const double y2 = yMap.transform( crossingPoint[i].y() );
 
    painter->setClipRect( canvasRect.x(), y1, canvasRect.width(), y2 - y1 );
    painter->setBrush( ... ); 
    painter->drawPolygon( ... );
 
    y1 = y2;
}
 
painter->setClipRect( canvasRect.x(), y1, canvasRect.width(), canvasRect.bottom() - y1 );
painter->setBrush( ... ); 
painter->drawPolygon( ... );

double y1 = canvasRect.y();

for ( int i = 0; i < crossingNum; i++ )
{
    const double y2 = yMap.transform( crossingPoint[i].y() );

    painter->setClipRect( canvasRect.x(), y1, canvasRect.width(), y2 - y1 );
    painter->setBrush( ... ); 
    painter->drawPolygon( ... );

    y1 = y2;
}

painter->setClipRect( canvasRect.x(), y1, canvasRect.width(), canvasRect.bottom() - y1 );
painter->setBrush( ... ); 
painter->drawPolygon( ... );

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Qt is known for rendering before clipping, so it's worth to try to clip the polygon with QwtClipper against the current clip rectangle, before calling QPainter::drawPolygon. If this results in a significant better performance you could consider to implement a faster clipping algo instead of Sutherland-Hodgman using the fact, that your points are ordered in vertical direction.

Uwe

[lwz]

Thanks you. I'll try some other clipping algo.