Flickable calculate position depending on scale

maitai_vw

Hello,
I am relatively new to QML so probably I am missing something obvious.

I am trying to "reproject" a flickable item (an image) to get its X, Y in original values in order to regenerate an image after a zoom or pan occurred. To make it short I need to know where is the topleft corner in the image source coordinates. It is very easy to do for a drag movement but I cannot find a solution when a pinch occurs.

At the beginning the flickable item is centered on the center of the image, and I save the initial X and Y values:

    flick.contentWidth = img.sourceSize.width
    flick.contentHeight = img.sourceSize.height
    flick.contentX = (flick.contentWidth - rect.width) / 2
    flick.contentY = (flick.contentHeight - rect.height) / 2
    initialX = flick.contentX
    initialY = flick.contentY
    initialW = flick.contentWidth'

Then when a drag occurs I get what I need with something like:

   backend.sendImageChange(flick.contentX - initialX, flick.contentY - initialY, 1.0)

But I cannot figure a way to do it when a pinch occurs. I have no problem calculating the current scale which is initialW/contentWidth, but I cannot find a way to convert the new contentX and contentY back in original image coordinates. The pinchArea is:

    PinchArea {
        id: pi
        width: Math.max(flick.contentWidth, flick.width)
        height: Math.max(flick.contentHeight, flick.height)

        property real initialWidth
        property real initialHeight
        //![0]
        onPinchStarted: {
            initialWidth = flick.contentWidth
            initialHeight = flick.contentHeight
            console.log("pinch start", flick.contentX, flick.contentY, flick.contentWidth, flick.contentHeight, flick.scale)
        }
        onPinchUpdated: {
            // adjust content pos due to drag
            flick.contentX += pinch.previousCenter.x - pinch.center.x
            flick.contentY += pinch.previousCenter.y - pinch.center.y
            // resize content
            flick.resizeContent(initialWidth * pinch.scale, initialHeight * pinch.scale, pinch.center)
        }

        onPinchFinished: {
            // Move its content within bounds.
            flick.returnToBounds()
            console.log("pinch end", flick.contentX, flick.contentY, flick.contentWidth, flick.contentHeight, flick.scale)
            var scale = initialW / flick.contentWidth
            backend.sendImageChange(flick.contentX - initialX, flick.contentY - initialY, scale)
        }

The flickable and the image behave as expected, but the calculated offset is wrong. I have tried to apply scale in various manners on contentX and contentY but couldn't find a working solution.

Thanks in advance for any help or explanations
Philippe.

maitai_vw

Finally got it: the offsets in original coordinates system can be calculated like that:

        var scale = flick.contentWidth / initialW
        var expectedX = ((flick.contentWidth - initialW) / 2.0) + initialX
        var expectedY = ((flick.contentHeight - initialH) / 2.0) + initialY
        var offsetX = (flick.contentX - expectedX) * scale
        var offsetY = (flick.contentY - expectedY) * scale