4. Direct drawing on UI

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Direct drawing on UI


  • Hello,

    I am making QT app with need to display images (from camera or HDD) in real time.
    I am looking for fastest way to do this.
    Ideal is to copy RGB data directly into video memory.

    I am new in QT.

    In .NET I can make object to draw from any Control (analog of Widget) like this:
    Graphics g = control.CreateGraphics();
    using this object draw anything on Control like this:
    g.DrawRect
    g.DrawImage
    g.DrawPixel
    ...
    and even have direct access to the memory and make memcopy(...);

    Can i do the same in QT?
    Is any other way is exist?

    Thank for the answer.

    THE SOLUTION

    I need to use paint event

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  • Hi, welcome to the Qt forum! Have a look at QOpenGLWidget Class for hardware accelerated graphics and see QPainter Class to learn about 2D drawing operations.

    1

  • Hello.
    Thank you for welcoming. Hope i can find my way in QT with your help.

    I had a short a look on it and have some questions

    -Is QOpenGLWidget hardware/software depended?
    -Can i attach QPainter to widget outside of paint event. For example like this:
    QPainter* DrawTool = new QPainter(DrawWidgetName);

    May be you can point me to some examples?

    Thank you in advance.

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  • Lifetime Qt Champion

    hi
    No, you are not allowed to paint outside paintEvent of the widget.
    You can cheat and draw on image and show image though.

    here is small sample of that
    https://www.dropbox.com/s/qwde8lb0fkjrrh3/mycheapgraph.zip?dl=0

    1

  • Hi

    And it is fast enough?

    I mean it will not take my RGB, convert it to Image, when to Pixmap, when use slow OS methods to display,....

    The reason i asking, similar way in .NET is terribly slow.

    Update:
    Some more questions:
    -Can i pass my image (RGB data in a memory) to paintEvent(QPaintEvent *)?
    -Can it be inside of QPaintEvent?
    -How to pass it? Something like this: videoFrame->update(RGBPointer)?
    -Pass image using global variable?

    0 1 Reply
  • Lifetime Qt Champion

    @Mikl
    hi
    sorry just saw the question about painting outside of event.
    You should use opengl :) The sample to draw on pixmap will have way to much overhead.

    -Can i pass my image (RGB data in a memory) to paintEvent(QPaintEvent *)?
    No, it has fixed signature so u cant change it. (adding the param)

    -Can it be inside of QPaintEvent?
    yes. should not be an issue

    -How to pass it? Something like this: videoFrame->update(RGBPointer)?
    Really depends on what format you get from camera.

    -Pass image using global variable?
    nope. never really a reason to use globals.
    since you might create your own widget , you can just add a function
    to assign/share the data from the source.

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  • Hi

    -I like the way to draw on opengl. Should be 'fast'. But will i meet capability problem? it will run on Win AND Linux?
    Can you point me to the example, please?

    -Pass image using global variable?
    never was thinking about it. just looking for solution

    -Can i pass my image (RGB data in a memory) to paintEvent(QPaintEvent *)?
    -Can it be inside of QPaintEvent?
    -How to pass it? Something like this: videoFrame->update(RGBPointer)?
    From camera i have raw 24bit RGB data.
    If it is not possible to pass my data to paintEvent(QPaintEvent *) how can i get it? Even if it is inside of QPaintEvent. Not really see how to do it. I will start something. Correct/add me please.

    main()
{
char* RGBData = new char[frame size];//data from camera

videoFrame = new VideoFrameWidget();
videoFrame->update(RGBData);
}

class VideoFrameWidget : public QWidget
{
	Q_OBJECT
public:
	VideoFrameWidget(QWidget *parent);
	virtual void paintEvent(QPaintEvent *);
};

void VideoFrameWidget::paintEvent(QPaintEvent *data)
{
//use opengl to draw image from data
}
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  • Lifetime Qt Champion

    hi
    why cant RGBData just live in VideoFrameWidget ?
    so you dont need to give it from main. ?

    but else u just use functions to give it to the widget.

    0 1 Reply

  • @mrjj

    It can, but i found it not logic. it is coming from the camera.
    It was simplification of main. In a reality it will be a loop.

    If i understood you correctly, you propose to do it like this:

    main()
{
 char* RGBData;//data from camera
 VideoFrameWidget videoFrame = new VideoFrameWidget();
 CameraDevice videoSource = new CameraDevice ();

 for(;;)
 {
  RGBData = videoSource->getFrame();
  videoFrame->setFrame(RGBData);
  videoFrame->update();
 }
}

class VideoFrameWidget : public QWidget
{
    Q_OBJECT
public:
    VideoFrameWidget(QWidget *parent);
    virtual void paintEvent(QPaintEvent *);
    void setFrame(char* newFrameData);
private:
 char* frameData;
};

void VideoFrameWidget::paintEvent(QPaintEvent*)
{
//use opengl to draw image from frameData 
}

 void VideoFrameWidget::setFrame(char* newFrameData)
{
 frameData = newFrameData;
}
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  • Lifetime Qt Champion

    yes exactly and then u can use
    char* frameData;
    in paintEvent

    0 1 Reply

  • @mrjj

    But i still have a question of realization of paintEvent :)

    Can you give me a hand, please

    0 1 Reply
  • Lifetime Qt Champion

    @Mikl
    hii . super :)
    im not good with opengl so better ask @Wieland
    or anyone using it often. :)

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  • @mrjj
    Thank you for helping

    @Wieland
    Can i ask for help with my problem please?
    I need to display raw RGB on a screen in a fastest way.

    0 1 Reply
  • Lifetime Qt Champion

    @Mikl
    note, this is user run forum.
    so he might first be on in the evening.
    :)

    0 1 Reply

  • @mrjj
    Ok. Thanks.

    For me it is looks strange, but reading this example
    http://doc.qt.io/qt-5/qtopengl-2dpainting-example.html
    i found, what the only different is using GLWidget as base class.
    All painting is done by the same QPainter.

    Am i right?

    If it is so, task can be really simple.
    Maybe you know the fastest way to paint from RGB raw data on widget: draw line by line, pixel by pixel, memcopy,....

    0 2 Replies
  • Lifetime Qt Champion

    @Mikl
    Yes as far as i know its VERY similar to
    normal widget drawing even when opengl.
    (when 2d)

    well, the sample i have seen , used something like
    uchar* data = getDataFromSomewhere();
    QImage img(data, width, height, QImage::Format_ARGB32);
    and then convert ot pixmap and draw.
    or directly into pixmap with
    http://doc.qt.io/qt-4.8/qpixmap.html#loadFromData

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  • @Mikl said:

    For me it is looks strange, but reading this example
    http://doc.qt.io/qt-5/qtopengl-2dpainting-example.html
    i found, what the only different is using GLWidget as base class.
    All painting is done by the same QPainter.

    Am i right?

    Hi! Yes, getting 2D hardware acceleration for QPainter is really easy :)

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