QML Canvas animation breaks QModbus communication

[Rcc21]

I'm working on a custom Qt QML application that needs to communicate with a QModbus client. In this particular case, I'm testing with the Qt Modbus Slave example.

At the application startup, the communication is initialized and it starts writing five registers every second. Everything works flawlessly until, in the main application, a QML canvas animation starts; after some milliseconds QModbus warnings are printed in the application output console:

[    13.260 W qt.modbus] unknown:0 - (RTU client) Cannot match response with open request, ignoring
[    15.261 W qt.modbus] unknown:0 - (RTU client) Cannot match response with open request, ignoring

and it keeps popping out until the animation stops.

This is the animation code:

onModeChanged: canvas.requestPaint()

    onAnimationActiveChanged: {
        if (animationActive) {
            animation.start()
        } else {
            animation.stop()
        }
    }

    Canvas {
        id: canvas
        anchors.fill: parent
        visible: animationActive

        property int lineWidth: 24
        property double position: 0
        property string indicatorColor: cleaning ? palette.cleaningIndicator : palette.white
        property real xScale: panelScale
        onPaint: {
            var ctx = getContext("2d")
            var x = width / 2
            var y = height / 2
            var radius = (width / 2) - lineWidth
            var start = Math.PI * (position - 1.5)
            var end = Math.PI * position
            ctx.reset()
            ctx.scale(xScale, 1)

            ctx.beginPath()
            ctx.moveTo(0, height / 2)
            ctx.lineTo(0, height)
            ctx.lineTo(width, height)
            ctx.lineTo(width, height / 2)
            ctx.closePath()
            ctx.clip()

            ctx.beginPath()
            ctx.arc(x, y, radius, start, end, true)
            var grd = ctx.createLinearGradient(x + radius * Math.cos(start),
                                               y + radius * Math.sin(start),
                                               x + radius * Math.cos(end),
                                               y + radius * Math.sin(end))
            grd.addColorStop(0.00, "transparent")
            grd.addColorStop(1.00, indicatorColor)
            ctx.lineWidth = lineWidth
            ctx.strokeStyle = grd
            ctx.stroke()

            ctx.beginPath()
            ctx.arc(x + radius * Math.cos(end), y + radius * Math.sin(end),
                    lineWidth / 2, 0, Math.PI * 2, false)
            ctx.fillStyle = indicatorColor
            ctx.fill()

            start = Math.PI * (position - 0.5)
            end = Math.PI * (position + 1)

            ctx.beginPath()
            ctx.arc(x, y, radius, start, end, true)
            grd = ctx.createLinearGradient(x + radius * Math.cos(
                                               start), y + radius * Math.sin(
                                               start), x + radius * Math.cos(
                                               end), y + radius * Math.sin(end))
            grd.addColorStop(0.00, "transparent")
            grd.addColorStop(1.00, indicatorColor)
            ctx.lineWidth = lineWidth
            ctx.strokeStyle = grd
            ctx.stroke()

            ctx.beginPath()
            ctx.arc(x + radius * Math.cos(end), y + radius * Math.sin(end),
                    lineWidth / 2, 0, Math.PI * 2, false)
            ctx.fillStyle = indicatorColor
            ctx.fill()
        }
        onPositionChanged: canvas.requestPaint()
    }

    RotationAnimation {
        id: animation
        running: animationActive
        target: canvas
        property: "position"
        duration: 10000
        from: 2
        to: 0
        easing.type: Easing.Linear
        onStopped: {
            if (animationActive) {
                animation.start()
            } else {
                animation.stop()
            }
        }
    }

I'm using Qt 5.12.6 Linux desktop. I noticed that using Qt 5.14.0 the problem doesn't appear, but since the application needs to run on a custom Linux board changes Qt is really expansive now.

Side note: I had to make this custom canvas/animation to create a sort of spinning circular crown and this is the only way that I'm achieving what I want.

Any suggestions about this problem?

[J.Hilk]

@Rcc21
doesn't surprise me, the QModbus implementation in 5.12 is buggy af

Try setting the reties to 0 and/or move your Modbus communication into its own thread

[Rcc21]

@J-Hilk
Thanks for the answer.
The communication is already in a different thread. Setting retries to 0 didn't help.

[J.Hilk]

@Rcc21

2 questions:

• Are you sure your threading is done right ? Have to ask, all to often it's actually not threaded at all
• Have you set a priority to the Modbus-thread?

[Rcc21]

@J-Hilk

• I'm pretty sure the threading is correct; I've followed the worker threads pattern.
• I didn't set the priority, but after your answer, I tried both QThread::HighestPriority and QThread::TimeCriticalPriority, but still not working.

[J.Hilk]

@Rcc21
well, I'm close to out of ideas 🙈

Have you tried 12.5 ? The private headers changed a lot between 12.5 & 12.6

I would suggest maybe opening a bug report otherwise and while that is running Subclassing QQuickPaintedItem and doing the drawing in there may result in a significant performance boost, that could also solve your issue

[Rcc21]

@J-Hilk

A colleague of mine tried the 5.12.4, but the error is still there. I'll try with the QQuickPaintedItem or I'll switch to the 5.14.

Thanks for your time.