Need Help: Wander affector - some particles never wiggle, and xVariance seems to do nothing

4w8c2c

Hi all,
I'm playing with the Wander affector in QtQuick.Particles and I'm stuck on something weird.
My setup: a simple rocket emitter shooting particles straight up from the bottom of the window. I added a Wander on the rocket group to make them drift left and right a bit, like they're wobbling in the wind. I also added a TrailEmitter so I can see the actual path each particle takes.
Here's the code.

import QtQuick 2.5
import QtQuick.Particles 2.0

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Rocket Wander Test")

    Rectangle {
        id: root
        width: parent.width
        height: parent.height
        color: "#1F1F1F"

        ParticleSystem {
            id: particleSystem
        }

        ImageParticle {
            id: trailPainter
            system: particleSystem
            groups: ['trail']
            source: "assets/particle.png"
            color: "cyan"
            alpha: 0.6
        }

        ImageParticle {
            id: rocketPainter
            system: particleSystem
            groups: ['rocket']
            source: "assets/rocket.png"
            entryEffect: ImageParticle.Fade
        }

        Emitter {
            id: rocketEmitter
            anchors.bottom: parent.bottom
            width: parent.width; height: 40
            system: particleSystem
            group: 'rocket'
            emitRate: 2
            maximumEmitted: 4
            lifeSpan: 5200
            lifeSpanVariation: 400
            size: 64
            velocity: AngleDirection { angle: 270; magnitude: 150; magnitudeVariation: 10 }
        }

        TrailEmitter {
            id: pathTrail
            system: particleSystem
            group: 'trail'
            follow: 'rocket'
            emitRatePerParticle: 30
            lifeSpan: 6000
            size: 6
            sizeVariation: 1
            velocity: PointDirection { x: 0; y: 0 }
        }

        Wander {
            groups: ['rocket']
            anchors.fill: parent
            system: particleSystem
            affectedParameter: Wander.Position
            pace: 150
            xVariance: 1200
            yVariance: 0
        }
    }
}

I expected that, every rocket wiggles left and right by some amount, and turning xVariance up makes the wiggle wider. But instead this is what I'm seeing:

Some rockets wiggle, some come out as a perfectly straight line, even though they all go through the same Wander block. I attached a screenshot showing this (some straight trails, some wavy trails, side by side).

I ran the same test four times, only changing xVariance each time (500, then 700, then 900, then 1200). The wiggle width looked exactly the same every single time. Changing xVariance did not seem to make the wiggle any wider or narrower.
The only thing that changing xVariance from 500 to 700 to 900 to 1200 affected was how much of the x axis range it covered, not the wiggle amplitude itself.

I tried making the Wander region smaller and moving it (using anchors.horizontalCenter and a fixed width instead of anchors.fill: parent). With this setup, nothing wiggled at all, no matter what I set xVariance to (I tried 30 and 500, both flat lines). I also attached a screenshot of this test.

So right now it looks like, xVariance does not seem to control how wide the wiggle is.
Whether a rocket wiggles at all seems to depend on something else, maybe where it spawns, or where the Wander box sits on screen, but I have not been able to pin down what exactly.

Has anyone run into this before? Is there something about how Wander picks which particles to affect, or how xVariance is supposed to work, that I am missing? Any pointers to the actual source or a working example would help a lot.
Thanks in advance.