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Thread: Create my own QGradient subclass

  1. #1
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    Default Create my own QGradient subclass

    I want to draw a polyline using QPainter. The line should change color dynamically according to point coordinates.

    In order to achieve this, I'd like to subclass QGradientin order to obtain a QFunctionGradient where I can set a std::function<QColor(QPointF)> that gives me the color of a point given its coordinates.

    So, at first, how It's possibile to draw a polyline with QPainter and color it according to a QGradient?

    And it's possible to subclass QPainter and use this subclass inside QBrush?

  2. #2
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    Default Re: Create my own QGradient subclass

    The API for the actual formulae of gradients (radial, linear etc.) is not exposed publicly.
    If it were, we would probably have something like a QAbstractGradient class with a set of protected virtual functions for us to reimplement with our own functionality.

    In your situation, what I would do is first use a QPainter to render a white polyline (polished, with anti-aliasing etc.) onto a QImage, then obtain the QImage.bits() pointer and go through every non-transparent pixel in the image, changing their colour based on a custom function, and preserving the alpha values.

    The resulting QImage can then be QPainter-ed anywhere else.

    Another technique: you can also generate a Qimage "by hand" (by setting each pixel), and then QPainting that coloured image on top of your polyline with the SourceIn composition mode.
    So you have a map of your gradient and you're using the antialiased polyline as a soft mask.

  3. #3
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    Default Re: Create my own QGradient subclass

    Ok so it's how I was thinking, I must do this by myself... I'll try, thanks for your advices.

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