why canvas toDataURL/save method NOT WORK correctly in android?

tuduweb

CANVAS base64data must less than 1018 ?

My qml code:

Canvas {
    id: canvas
    width: 150
    height: 150

    onPaint: {
        var ctx = getContext("2d");

        ctx.fillStyle = Qt.rgba(1, 0, 0, 1);
        ctx.fillRect(0, 0, width, height);
    }
}

Button{
    text: "save"
    onClicked: {
        console.log(canvas.toDataURL())
        var res = canvas.save(localUrl + "/savedImage.png")
        //....
    }
}

the param localUrlset android getExternalFilesDir in android or QDir::currentPath() in win10

when i click the BUTTON in win10, my program run normally, image is saved to my computer, and the file is correct. file size is 1.05kb.

Application output tab output:

base64 length: 1475

qml: data:image/png;base64,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

BUT , when my program run in android, something go wrong, image file is not correct. file size is 0kb

Application output tab output:
base64 length: 1018

D libOnlineBoard_arm64-v8a.so: qml: data:image/png;base64,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

In this case in android, I change my code many times, i find program run well when the CANVAS base64data less than 1018, and the max length is 1018.
when the length over than 1018, go wrong.

how can i fix this problem?
thx!

tuduweb

qt 5.15.2
android 10 arm64
android 8.1 x86