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The Curious Case of Benjamin Y-axis Button



  • I came across a strange behavior of QT5 on raspberry.
    I'm using a TFT display with resistive touchscreen on a raspberry pi 2.
    The external display is configured as a frame buffer /dev/fb1 while, as usual, monitor is /dev/fb0.
    The touchscreen is /dev/input/touchscreen but is a link to /dev/input/event3.
    I made a stupid application with a few sliders, button etc but does absolutely nothing except exiting the application when quit is pressed.
    There are also two QLCDNumber indicating the x, y coordinates of the pointer.
    I start the application with -platform linuxfb: fb=/dev/fb1 -style fusion -plugin tslib
    Tslib is calibrated and with environment set correctly.
    When I launch, the application is displayed correctly but if I interact via touchpanel you'll immediately notice that something is wrong.
    In framebuffer coordinates 0.0 are top, left and in fact the graphics are correct.
    Unfortunately QtLcd 2 show that the coordinates 0,0 for the touch panel are bottom, left.
    I recalibrated the tslib and tried various tslib tools and is entirely correct, but always in Qt the y-axis still reversed.
    After 1 week of attempts I decided to try to use the framebuffer /dev/fb0 to see if the same happens and magically the application works with the y-axis in the right direction using the calibrated touchplanel.

    After another week of trial, while looking at the sources in QTBase, I have found a variable QT_QPA_FB_TSLIB and after setting it to 1 the y-axis is back in the right direction using properly tslib and now everything works even using /dev/fb1 (I obviously omitted -plugin tslib from the command line)

    I have the impression that there is a bug in Qt5.5 that makes "-plugin tslb" functions only on the framebuffer 0 or that, I do not know why, goes wrong for /dev/fb1.

    Does anyone have experience about this oddity ??


  • Lifetime Qt Champion

    Hi and welcome to devnet,

    Something strange for sure. The implementation of the Linux framebuffer integration has changed quite a bit in 5.5. You should take a look at the bug report system to see if it's something known.


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