QML Canvas bug with getImageData with 5.0 and 5.1, or am I doing something wrong? [Fixed] Not really.

  • I'm not certain, but I think I've stumbled onto a bug with Canvas in Qt 5.0 and 5.1

    var ctx = canvas.getContext('2d');
    var imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);

    Now, as far as I understand imageData.data should now contain the pixel data. Using a C++ method I attempt to handle it (as a QVariantList) with:

    setFromData(imageData.data, imageData.width, imageData.height);

    This method creates a uchar* array of the pixels, creates a QImage from it, then returns.

    Except this code doesn't work. The array received is 0 bytes long.

    If I make a deep copy of the pixel array (in JS) into a new array and pass the copy it works perfectly:

    var copied = new Array();
    for( var x=0; x < imageData.data.length; x++ )
    copied[x] = imageData.data[x];

    printer.setFromData(copied, imageData.width, imageData.height);

    I have made a fairly minimal example which reproduces the effect:


    I have a suspicion that the offending code is here: http://qt.gitorious.org/qt/qtdeclarative/blobs/stable/src/qml/qml/v8/qv8engine.cpp#line241

    It looks like if it's a pixel array it returns an empty QVariant. Am I right?

    Any thoughts at all on this would be appreciated, thanks.

  • Hi,

    This does look like a bug. I do wonder which conversion codepath is taken for the second (manually deep-copied) array, though.
    In any event, please file a bug report with the minimal example attached. Very soon I think v8 is going away, to be replaced by v4vm, so it's possible that this is already fixed in what will become 5.2.


  • My bug report became a feature request since it "never has worked".

    It will most likely stay a stub which does nothing (not even a warning) and will instead return an empty QVariant to the native method.

    In short the correct way is to quite literally copy the contents into a new array object (in JS) and return that.

    Weak? I thought so too.


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