Qt: Taking screenshot of EGLFS window - pixel-perfect identical result?

Question

We're using EGLFS to run a QML app on an embedded linux device without a X server. So I can't use the usual techniques for getting a screenshot (e.g. using a screenshot utility app). In this question I found the technique of using QQuickView::grabWindow().

My question: Is the result of this function guaranteed to be pixel-perfect identical to what the user sees on the screen? I'm worried that to grab the window contents, Qt might have to rerender the window using a different code path (e.g. render-to-texture), which may cause results to differ.

I'd like a reliable reference included in your answers, too.

Answer 1

As far as I can tell, the screenshot is not going to be identical. Follow the code from QQuickWindow::grabWindow: it ends up calling qt_gl_read_framebuffer, which is always going to read the image out as RGB(A)8. Your actual framebuffer might be different (for instance, it could be RGB565).

(I also seem to remember that one cannot use RGBA reading via glReadPixels out of a RGB565 framebuffer, but I am not 100% sure that's true in all OpenGL versions/variants...)

Depending on your drivers, a workaround could be reading /dev/fb0 contents. Anyhow, please file a bug report if you need this functionality.