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Thread: Rendering a QQuickItem into a QImage (Qt6)

  1. #1
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    Default Rendering a QQuickItem into a QImage (Qt6)

    Hi Qt Specialist,

    I'm really desperate to make it work, I try different approaches but nothing works like expected. And most of the examples found by Googling concern Qt5.

    First the Use Case: I have to do a color picker into a complex 2D scene, this scene is constructed with QSGNodes tree inside a QQuickItem. Of course, to make it more difficult, I have to pick the color without the drawing overlays and I should be able to pick outside of the item bounds.

    I try this:
    1. I create a simplified QQuickItem
    2. try to bind it to an offscreen window (setParentItem & QQuickRenderControl)
    3. add a render target (fromPaintDevice, I have no clue how to transform any 'kind' of textures into a QImage)
    3. resize
    4. render with polishItems, beginFrame, ....
    5. check the image

    I get this error:
    QQuickWindow: No render target (neither swapchain nor custom target was provided)

    Here is the code using Vulkan:
    Qt Code:
    1. QVulkanInstance* inst = new QVulkanInstance;
    2. inst->create();
    4. _render_ctrl = new QQuickRenderControl;
    6. // this window will never be shown on-screen
    7. _off_screen_window = new QQuickWindow(_render_ctrl);
    8. _off_screen_window->setVulkanInstance(inst);
    10. if (!_render_ctrl->initialize())
    11. qWarning() << "failed to init";
    13. _image = new QImage(300, 200, QImage::Format_RGBA8888_Premultiplied);
    14. _image->fill(qRgb(255,255,0));
    16. _off_screen_window->setRenderTarget(QQuickRenderTarget::fromPaintDevice(_image));
    18. _off_screen_item->setParentItem(_off_screen_window->contentItem());
    19. _off_screen_item->setSize(QSize(300,200));
    20. _off_screen_item->setVisible(true);
    22. _off_screen_window->resize(300, 200);
    23. //_off_screen_window->show();
    25. _render_ctrl->polishItems();
    26. _render_ctrl->beginFrame();
    27. _render_ctrl->sync();
    28. _render_ctrl->render();
    29. _render_ctrl->endFrame();
    31. _image->save("xxx.png"); // nothing draw just the original image
    To copy to clipboard, switch view to plain text mode 

    Could someone explain to me how in Qt6 I could render offscreen a QQuickItem in a QImage? Or at least into a texture and how to obtain an average color from the extracted texture?

    Is the call QQuickWindow::setGraphicsApi(QSGRendererInterface: :Vulkan); compatible with the offscreen QQuickWindow? The Qt examples and documentation do not really cover this case.

    The only solution, for now, I'm using (slow - no shaders to help - and need to duplicate the rendering) is the QPainter over a QImage.


  2. #2
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    Default Re: Rendering a QQuickItem into a QImage (Qt6)

    The docs claim you could redirect rendering in Vulkan to VkImage:

    Qt Code:
    1. QQuickRenderTarget rt = QQuickRenderTarget::fromVulkanImage(vulkanImage, VK_IMAGE_LAYOUT_PREINITIALIZED, pixelSize);
    2. quickWindow->setRenderTarget(rt);
    To copy to clipboard, switch view to plain text mode 

    Then you can either sample that image or try to convert it to some other image API (e.g. QImage)
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.

  3. #3

    Default Re: Rendering a QQuickItem into a QImage (Qt6)

    Rendering a QQuickItem into a QImage in Qt6 can be achieved by redirecting rendering in Vulkan to a VkImage using QQuickRenderTarget. Have you tried using the suggested approach with QQuickRenderTarget::fromVulkanImage, and if so, what issues are you encountering in your implementation?

    Additionally, have you considered other methods or libraries to achieve your color picking in a complex 2D scene, or are you focused on solving this within the Qt framework?

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