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Thread: Render canvas only to QImage

  1. #1
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    Question Render canvas only to QImage

    Hello,

    I have a regular plot with a spectrogram (with title, axis ticks, etc...).

    I need to render the plot canvas (without borders, axes, or anything else) to a QImage of a specific size (e.g. 512x512), for using it in QOpenGLTexture.
    The goal is to map the spectrogram image directly to a rectangle in OpenGL, with no empty spaces.

    I'm having trouble using QwtPlotRenderer for this task. The standard render() renders too much and there are empty spaces in canvas around the spectrogram as well. renderCanvas() requires a scale map which is rather confusing to me.

    Is there some easier way I'm missing?

    Thanks,
    Alexander

  2. #2
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    Default Re: Render canvas only to QImage

    Quote Originally Posted by alex_sh View Post
    renderCanvas() requires a scale map which is rather confusing to me.
    If the data of your spectrogram is in the range 100-2000 on the bottom axis and 10-20 on the left axis and you want to render to a 500x500 image:

    Qt Code:
    1. QwtScaleMap maps[4];
    2.  
    3. maps[QwtPlot::xBottom].setPaintInterval( 0, 500 );
    4. maps[QwtPlot::xBottom].setScaleInterval( 100, 200 );
    5.  
    6. maps[QwtPlot::yLeft].setPaintInterval( 0, 500 );
    7. maps[QwtPlot::yLeft].setScaleInterval( 20, 10 );
    To copy to clipboard, switch view to plain text mode 
    Note that paint device coordinates are increasing from top to bottom, while y-scales usually have the smaller value at the bottom.

    HTH,
    Uwe

  3. The following user says thank you to Uwe for this useful post:

    alex_sh (9th February 2023)

  4. #3
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    Default Re: Render canvas only to QImage

    This works like a charm.

    Thank you very much, Uwe!

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