curve with QLinearGradient

[yoshri92]

Hello,

I have a curve filled with a linear gradient which the colors depends on certain values.
The problem is : if the curve have a concave shape or local minimum, there is a shift in the position of the colors at the same Y position of the minimum and after its X position, please see the image.
Anyone could help me to find out why this happens

cheers,


helpme.jpg

[Uwe]

Assuming, that you fill the curve using QwtPlotCurve::setBrush(), the problem should be somewhere in the paint engine of your system. What type of graphic system is your backend ?

Uwe

[yoshri92]

Hi Uwe and thank you for your reply.

Yes I used QwtPlotCurve::setBrush(). The problem persists on both OS ubuntu 16.04 and Windows 10.

[Uwe]

Then we are talking about the raster paint engine and you should be able to reproduce the problem with a small standalone application drawing a QPolygon to a QImage.

Uwe

[yoshri92]

Indeed I was able to reproduce this in a simple polygon draw with qlineargradient :

QPainter p  ;
    p.begin(this) ;
    QPolygon poly ;
    QLinearGradient g(0,0,500,0) ;
    g.setColorAt(0, Qt::red);
    g.setColorAt(0.2, Qt::yellow);
    g.setColorAt(0.4, Qt::green);
    g.setColorAt(0.401, Qt::yellow);
    g.setColorAt(0.6, Qt::cyan);
    g.setColorAt(0.602,  Qt::black) ;
    g.setColorAt(0.8, Qt::blue);
    g.setColorAt(0.801, Qt::yellow);
    g.setColorAt(1, Qt::white);
    p.setBrush(g);
    poly << QPoint(0, 20) << QPoint(100, 300)  << QPoint(200,20) << QPoint(300, 20) << QPoint(400, 20) << QPoint(500, 20) << QPoint(500,400) << QPoint(0, 400);
    p.drawPolygon(poly);
    p.end() ;

QPainter p  ;
    p.begin(this) ;
    QPolygon poly ;
    QLinearGradient g(0,0,500,0) ;
    g.setColorAt(0, Qt::red);
    g.setColorAt(0.2, Qt::yellow);
    g.setColorAt(0.4, Qt::green);
    g.setColorAt(0.401, Qt::yellow);
    g.setColorAt(0.6, Qt::cyan);
    g.setColorAt(0.602,  Qt::black) ;
    g.setColorAt(0.8, Qt::blue);
    g.setColorAt(0.801, Qt::yellow);
    g.setColorAt(1, Qt::white);
    p.setBrush(g);
    poly << QPoint(0, 20) << QPoint(100, 300)  << QPoint(200,20) << QPoint(300, 20) << QPoint(400, 20) << QPoint(500, 20) << QPoint(500,400) << QPoint(0, 400);
    p.drawPolygon(poly);
    p.end() ;

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And this what it gave me test.png

Does this mean that this is a bug in the raster paint engine ? could it be changed to other paint engine ?

[Uwe]

Does this mean that this is a bug in the raster paint engine ?

Yes.

could it be changed to other paint engine ?

First of all you could try a different version of Qt.

With Qt4/X11 you could use the X11 paint engine, what is IMHO superior to all other options, when using Qwt: hardware accelerated + no bugs.
There are also 2 different types of OpenGL paint engines. When using Qwt from SVN trunk you could play with the QwtPlotCanvas::OpenGLBuffer mode.

But if I were in your shoes I would try:

1. fill a temporary QImage with your gradient
2. setting a clip path with the polygon
3. draw the image

Uwe

[yoshri92]

1. fill a temporary QImage with your gradient
2. setting a clip path with the polygon
3. draw the image

That worked out.
I overloaded the method QwtPlotCurve::fillCurve so it would do the same and it is acceptable.
Thank you very much