Artifacts when trying to draw background grid without anti-aliasing in a QGraphicsScene

Question

I'm trying to draw a background grid in the drawBackground() function of my QGraphicsScene subclass:

void Scene::drawBackground(QPainter *painter, const QRectF &rect)
{
    const int gridSize = 50;

    const int realLeft = static_cast<int>(std::floor(rect.left()));
    const int realRight = static_cast<int>(std::ceil(rect.right()));
    const int realTop = static_cast<int>(std::floor(rect.top()));
    const int realBottom = static_cast<int>(std::ceil(rect.bottom()));

    // Draw grid.
    const int firstLeftGridLine = realLeft - (realLeft % gridSize);
    const int firstTopGridLine = realTop - (realTop % gridSize);

    QVarLengthArray<QLine, 100> lines;

    for (qreal x = firstLeftGridLine; x <= realRight; x += gridSize)
        lines.append(QLine(x, realTop, x, realBottom));
    for (qreal y = firstTopGridLine; y <= realBottom; y += gridSize)
        lines.append(QLine(realLeft, y, realRight, y));

    //painter->setRenderHint(QPainter::Antialiasing);
    painter->setPen(QPen(QColor(220, 220, 220), 0.0));
    painter->drawLines(lines.data(), lines.size());

    // Draw axes.
    painter->setPen(QPen(Qt::lightGray, 0.0));
    painter->drawLine(0, realTop, 0, realBottom);
    painter->drawLine(realLeft, 0, realRight, 0);
}

However, unless I turn on anti-aliasing, moving items around will sometimes leave artifacts in the grid (areas where it's not drawn). It seems it mostly happens at low zoom levels, when the view is zoomed out a bit. Any ideas what I might be doing wrong here?

I'd really don't want to turn anti-aliasing on since the lines are strictly horizontal and vertical, and I'd like them to be as crisp as possible.

Any help is much appriciated, Regards, Elvis

Have you tried explicitly erasing the contents of the scene before drawing? Try drawing a rectangle of white (or whatever your background colour is) before drawing your gridlines, and see if that helps. — Thomi, May 06 '10 at 07:48
Have you tried calling the base class function first? Maybe it is supposed to erase the background first: `QGraphicsScene::drawBackground(painter, rect)`, and then all your custom code. — ak., Dec 06 '10 at 16:05
have you tried calling setViewportUpdateMode(QGraphicsView::FullViewportUpdate) at the start of your program? — Tom, Mar 23 '11 at 10:05

score 2 · Answer 1 · edited Sep 25 '15 at 06:22

2

Sounds like you need to use a different viewport update method:

http://doc.qt.io/qt-5/qgraphicsview.html#viewportUpdateMode-prop

and perhaps the background caching:

http://doc.qt.io/qt-5/qgraphicsview.html#cacheMode-prop

edited Sep 25 '15 at 06:22

Christophe Weis

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answered Jun 14 '11 at 07:09

Phil Hannent

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I got the smiliar issue and it is fixed after I added "setViewportUpdateMode(QGraphicsView::FullViewportUpdate)", although not sure how is the performance impacted. – Vincent May 29 '12 at 07:49
Check my question which has a better solution than using full update: http://stackoverflow.com/questions/13933699/qgraphicslineitempaint-artifacts – paulm Dec 20 '12 at 13:26

Artifacts when trying to draw background grid without anti-aliasing in a QGraphicsScene

1 Answers1