[Solved] GraphicsView: Fixed line width regardless of transformations?

Asperamanca

I need to draw a line that is always exactly n pixels wide, regardless of any transformations applied.

Cosmetic pens are not supported in GraphicsView (as can be seen in the comments to "QTBUG-5294":https://bugreports.qt-project.org/browse/QTBUG-5294 )

How else to do this? I could calculate the necessary width once I get the transformation in the paint() method, but that would mean changing the boundingRect. Not something I'd like to do while painting.

Asperamanca

This is how I do it now:

I calculate my effective non-cosmetic pen from the given pen plus the scene transform

@ const qreal effectiveSourcePenWidth = qMax(sourcePen.widthF(),static_cast<qreal>(1.0));

if ( ! sourcePen.isCosmetic())
{
    QPen finalPen = sourcePen;
    finalPen.setWidthF(effectiveSourcePenWidth);
    return sourcePen;
}

QPointF scenePos = mapToScene(QPointF());
const qreal halfPenWidth = effectiveSourcePenWidth / 2.0;
QPointF edgeRight = mapFromScene(scenePos.x() + halfPenWidth,scenePos.y());

QPen finalPen = sourcePen;
finalPen.setCosmetic(false);
finalPen.setWidthF(qAbs(edgeRight.x()));

return finalPen;@

Based on that pen, I calculate my boundingRect

@ if (m_eOrientation == Qt::Vertical)
{
m_CachedBoundingRect.setLeft(-effectiveHalfWidth);
m_CachedBoundingRect.setRight(effectiveHalfWidth);
m_CachedBoundingRect.setTop(-1000000.0);
m_CachedBoundingRect.setBottom(1000000.0);
}
else
{
m_CachedBoundingRect.setTop(-effectiveHalfWidth);
m_CachedBoundingRect.setBottom(effectiveHalfWidth);
m_CachedBoundingRect.setLeft(-1000000.0);
m_CachedBoundingRect.setRight(1000000.0);
}

m_CachedBoundingRectSceneTransform = sceneTransform();
m_bCachedBoundingRectValid = true;@

I remember the sceneTransform I used, because whenever it changes, I need to recalculate pen and boundingRect. Now this is the tricky part, because I simply can't know about all cases where the sceneTransform changes. One thing I can recognize is if the transform of my own item changes:

@ if (change == QGraphicsItem::ItemTransformHasChanged)
{
invalidateBoundingRectIfTransformChanged();
}@

Otherwise I assume that any change to the sceneTransform must be followed by a paint(), so within paint() I do
@ invalidateBoundingRectIfTransformChanged();
updateBoundingRectAndPen();
@

Of course, this might trigger another paint. But after that, the sceneTransform should be stable.