canvas.step dought

saivineeth

Hi i have created a sine wave by using canvas.step
why the screen is cleaning compeltly when u r canvas.step set to zero ??

import QtQuick 2.9
import QtQuick.Window 2.15

Window {
    id: screen
    visible: true
    height: 1080
    width: 1920
    title: qsTr("SineWave")

    Rectangle {
        height: parent.height
        width: parent.width

        Timer {
            id: timer
            interval: 1
            running: true
            repeat: true
            onTriggered: {
                if (canvas.step > 1921)
                    canvas.step =0
                    //timer.stop()
                canvas.step++
                canvas.requestPaint()
            }
        }

        Canvas {
            id: canvas
            property int step: 0
            anchors.fill: parent
            onPaint: {
                var ctx1 = getContext("2d")
                var cw = parent.width
                var ch = parent.height
                var cx = cw
                var cy = ch/2
                var w = width
                var h = height
                ctx1.lineWidth = 4
                ctx1.clearRect(0, 0, cw, ch)
                ctx1.beginPath()
                ctx1.moveTo(0, cy)
                for (var x = 0; x < canvas.step; x++) {
                   ctx1.lineTo(x, cy + (Math.sin(x / 305) * 400));
                    ctx1.fillStyle = Qt.rgba(0,255,0,255);
                    ctx1.fillRect(x+100, cy+100, 10, 1000);
                }

                ctx1.stroke();
            }
        }


    }
}

UlrichS

Hi, that's expected outcome. In your onPaint you clear the whole canvas area with:

ctx1.clearRect(0, 0, cw, ch)

on each paint event. If you remove that then the canvas is not cleared.

Regards,
Ulrich