QGraphicsView - background grid - shifts/moves when pan/zooom...

Dariusz

Hey

I've reimplemented scene drawBackground to generate a custom looking grid like :

    qreal left = int(rect.left()) - (int(rect.left()) % mGridSize);
    qreal top = int(rect.top()) - (int(rect.top()) % mGridSize);

    QVarLengthArray<QLineF, 100> lines;

    for (qreal x = left; x < rect.right(); x += mGridSize)
        lines.append(QLineF(x, rect.top(), x, rect.bottom()));
    for (qreal y = top; y < rect.bottom(); y += mGridSize)
        lines.append(QLineF(rect.left(), y, rect.right(), y));

    QVarLengthArray<QLineF, 100> thickLines;

    for (qreal x = left; x < rect.right(); x += mGridSize * 5)
        thickLines.append(QLineF(x, rect.top(), x, rect.bottom()));
    for (qreal y = top; y < rect.bottom(); y += mGridSize * 5)
        thickLines.append(QLineF(rect.left(), y, rect.right(), y));

    QPen myPen(Qt::NoPen);
    painter->setBrush(QBrush(QColor(55, 55, 55, 255)));
    painter->setPen(myPen);
    painter->drawRect(rect);

    QPen penHLines(QColor(75, 75, 75), 1, Qt::SolidLine, Qt::FlatCap, Qt::RoundJoin);
    painter->setPen(penHLines);
    painter->drawLines(lines.data(), lines.size());

    painter->setPen(QPen(QColor(100, 100, 100), 2, Qt::SolidLine, Qt::FlatCap, Qt::RoundJoin));
    painter->drawLines(thickLines.data(), thickLines.size());


    painter->setPen(Qt::blue);

    QVector<QPointF> points;
    for (qreal x = left; x < rect.right(); x += mGridSize) {
        for (qreal y = top; y < rect.bottom(); y += mGridSize) {
            points.append(QPointF(x, y));
        }
    }
    painter->drawPoints(points.data(), points.size());

But when I place item close to gird lines/snap it and then move/zoom the view. The grid miss-aligns and changes its position...

How can I correct it?

TIA

EDIT I've move the function to view to capture pan/zoom events to see if I can somehow control the offset for the "thick lines" drawing...

Here is the gif showing issue :
(

Dariusz

I've updated the question with gif/some updates... Still fighting with it o.o

I think by using int vertOffset = horizontalScrollBar()->sliderPosition();
I can somehow calculate the "jump" value offset and offset position of line by that. Not sure how yet tho...

WSTaiYi

可以参考这个网站进行进行背景图的绘制，缩放的原理等https://www.desmos.com/calculator?lang=zh-CN
同时参考这这个链接，进行背景绘制https://stackoverflow.com/questions/24728477/infinite-grid-in-qt

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QGraphicsView - background grid - shifts/moves when pan/zooom...