Pixelated QwtPlotTextLabel and QwtPlotRenderer

[Seamus]

Come on: drawing to a QImage should also be done without using the cache for a painter with scaling ( see the code you have posted ! ) and all your code should be obsolete.

How can you tell if the QPainter is scaling when QPainter::transform().isScaling() always return false.

void PlotTextLabel::draw( QPainter *painter,
                            const QwtScaleMap &xMap, const QwtScaleMap &yMap,
                            const QRectF &canvasRect ) const
{
    qDebug() << __FUNCTION__ << painter->paintEngine()->type() << painter->transform().isScaling();
    QwtPlotTextLabel::draw(painter, xMap, yMap, canvasRect);
}

void PlotTextLabel::draw( QPainter *painter,
                            const QwtScaleMap &xMap, const QwtScaleMap &yMap,
                            const QRectF &canvasRect ) const
{
    qDebug() << __FUNCTION__ << painter->paintEngine()->type() << painter->transform().isScaling();
    QwtPlotTextLabel::draw(painter, xMap, yMap, canvasRect);
}

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[Uwe]

How can you tell if the QPainter is scaling when QPainter::transform().isScaling() always return false.

Then the painter is not scaling - but when you don't have a painter transformation, what is the reason of the "pixelation". Could you make a small compilable demo showing the issue ?

Uwe

PS: One thing I can see is that the pixmap cache should be created by QwtPainter::backingStore() for the Retina displays on the Mac. But this shouldn't be of importance for your situation.

[Seamus]

pixellation.tar.gz
To see the jagged edges just zoom in on the label (the text in the centre).

[Uwe]

O.k. now I see: QPrinter/QPrintPreviewDialog redirects to a QPicture - then the picture is replayed/scaled according to the size of the preview window.

But what is the correct solution for record/replay paint devices:

• not rounding while recording means that you run into issues when replaying ( Qt doesn't know how to round ).
• When rounding while recording you run into problems when replaying with a scaling factor

As there is no general solution to this problem it is something that should at least be decidable by the application, that knows more about the plot and the use case. With the current implementation this is possible with QwtPainter::setRoundingAlignment(), where you can disable rounding done by Qwt. When adding QPaintEngine::Picture to QwtPainter::isAligning() rounding would always be disabled and could not be enabled in application code anymore. That's why I don't like this fix.

But for your problem this all is of no imprtance. Texts are never 100% aligned to plot coordinates and the decision for using a cache in QwtPlotTextLabel is made because of performance considerations only. So I changed the implementation of QwtPlotTextLabel::draw() not using the cache for QPaintEngine::Picture and QPaintEngine::User ( usually QwtGraphic ). Fixed in SVN trunk and 6.1 branch.

When working with Qwt 6.1.0 this should be the workaround for your code ( accepting minor rounding errors when replaying images, rectangles or shapes ):

void PixellationMainWindow::renderPlot(QPrinter *printer)
{
    const bool doAlign = QwtPainter::roundingAlignment();
    QwtPainter::setRoundingAlignment( false );
 
    ...
 
    QwtPainter::setRoundingAlignment( doAlign );
}

void PixellationMainWindow::renderPlot(QPrinter *printer)
{
    const bool doAlign = QwtPainter::roundingAlignment();
    QwtPainter::setRoundingAlignment( false );

    ...

    QwtPainter::setRoundingAlignment( doAlign );
}

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Uwe