ahmad dar - Page 5 - Developer IT

[Qt/C++] Need help in optimizing a drawing code ...

- by Ahmad

Hello all ... I needed some help in trying to optimize this code portion ... Basically here's the thing .. I'm making this 'calligraphy pen' which gives the calligraphy effect by simply drawing a lot of adjacent slanted lines ... The problem is this: When I update the draw region using update() after every single draw of a slanted line, the output is correct, in the sense that updates are done in a timely manner, so that everything 'drawn' using the pen is immediately 'seen' the drawing.. however, because a lot (100s of them) of updates are done, the program slows down a little when run on the N900 ... When I try to do a little optimization by running update after drawing all the slanted lines (so that all lines are updated onto the drawing board through a single update() ), the output is ... odd .... That is, immediately after drawing the lines, they lines seem broken (they have vacant patches where the drawing should have happened as well) ... however, if I trigger a redrawing of the form window (say, by changing the size of the form), the broken patches are immediately fixed !! When I run this program on my N900, it gets the initial broken output and stays like that, since I don't know how to enforce a redraw in this case ... Here is the first 'optimized' code and output (partially correct/incorrect) void Canvas::drawLineTo(const QPoint &endPoint) { QPainter painter(&image); painter.setPen(QPen(Qt::black,1,Qt::SolidLine,Qt::RoundCap,Qt::RoundJoin)); int fx=0,fy=0,k=0; qPoints.clear(); connectingPointsCalculator2(qPoints,lastPoint.x(),lastPoint.y(),endPoint.x(),endPoint.y()); int i=0; int x,y; for(i=0;i<qPoints.size();i++) { x=qPoints.at(i).x(); y=qPoints.at(i).y(); painter.setPen(Qt::black); painter.drawLine(x-5,y-5,x+5,y+5); **// Drawing slanted lines** } **//Updating only once after many draws:** update (QRect(QPoint(lastPoint.x()-5,lastPoint.y()-5), QPoint(endPoint.x()+5,endPoint.y()+5)).normalized()); modified = true; lastPoint = endPoint; } Image right after scribbling on screen: http://img823.imageshack.us/img823/8755/59943912.png After re-adjusting the window size, all the broken links above are fixed like they should be .. Here is the second un-optimized code (its output is correct right after drawing, just like in the second picture above): void Canvas::drawLineTo(const QPoint &endPoint) { QPainter painter(&image); painter.setPen(QPen(Qt::black,1,Qt::SolidLine,Qt::RoundCap,Qt::RoundJoin)); int fx=0,fy=0,k=0; qPoints.clear(); connectingPointsCalculator2(qPoints,lastPoint.x(),lastPoint.y(),endPoint.x(),endPoint.y()); int i=0; int x,y; for(i=0;i<qPoints.size();i++) { x=qPoints.at(i).x(); y=qPoints.at(i).y(); painter.setPen(Qt::black); painter.drawLine(x-5,y-5,x+5,y+5); **// Drawing slanted lines** **//Updating repeatedly during the for loop:** update(QRect(QPoint(x-5,y-5), QPoint(x+5,y+5)).normalized());//.adjusted(-rad,-rad,rad,rad)); } modified = true; int rad = (myPenWidth / 2) + 2; lastPoint = endPoint; } Can anyone see what the issue might be ?

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