Can not draw rectangle at exact points on a canvas using mouse when the page is scroll able

Question

Although the code work just fine if we draw the rectangle without scrolling the canvas or page. But in case we do scrolling, the rectangle do not appear on the screen from the point where we start to drag the mouse.

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;

//Mousedown
$(canvas).on('mousedown', function(e) {
    last_mousex = parseInt(e.clientX-canvasx);
    last_mousey = parseInt(e.clientY-canvasy);
    mousedown = true;
});

//Mouseup
$(canvas).on('mouseup', function(e) {
    mousedown = false;
});

//Mousemove
$(canvas).on('mousemove', function(e) {
    mousex = parseInt(e.clientX-canvasx);
    mousey = parseInt(e.clientY-canvasy);
    if(mousedown) {
        ctx.clearRect(0,0,canvas.width,canvas.height); //clear canvas
        ctx.beginPath();
        var width = mousex-last_mousex;
        var height = mousey-last_mousey;
        ctx.rect(last_mousex,last_mousey,width,height);
        ctx.strokeStyle = 'black';
        ctx.lineWidth = 10;
        ctx.stroke();
    }
    //Output
    $('#output').html('current: '+mousex+', '+mousey+'<br/>last: '+last_mousex+', '+last_mousey+'<br/>mousedown: '+mousedown);
});

canvas {
    cursor: crosshair;
    border: 1px solid #000000;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas" width="800" height="500"></canvas>
<div id="output"></div>

score 2 · Accepted Answer · answered Aug 22 '20 at 10:42

Using this answer to compute the mouse position, it works great. It takes the scrolling into consideration.

function getCursorPosition(canvas, event) {
    const rect = canvas.getBoundingClientRect();
    const x = event.clientX - rect.left;
    const y = event.clientY - rect.top;
    return { x, y };
}

//Canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
//Variables
var canvasx = $(canvas).offset().left;
var canvasy = $(canvas).offset().top;
var last_mousex = last_mousey = 0;
var mousex = mousey = 0;
var mousedown = false;

//Mousedown
$(canvas).on('mousedown', function(e) {
    var pos = getCursorPosition(canvas, e);
    last_mousex = pos.x;
    last_mousey = pos.y;
    mousedown = true;
});

//Mouseup
$(canvas).on('mouseup', function(e) {
    mousedown = false;
});

//Mousemove
$(canvas).on('mousemove', function(e) {
    var pos = getCursorPosition(canvas, e);
    mousex = pos.x;
    mousey = pos.y;
    if(mousedown) {
        ctx.clearRect(0,0,canvas.width,canvas.height); //clear canvas
        ctx.beginPath();
        var width = mousex-last_mousex;
        var height = mousey-last_mousey;
        ctx.rect(last_mousex,last_mousey,width,height);
        ctx.strokeStyle = 'black';
        ctx.lineWidth = 10;
        ctx.stroke();
    }
    //Output
    $('#output').html('current: '+mousex+', '+mousey+'<br/>last: '+last_mousex+', '+last_mousey+'<br/>mousedown: '+mousedown);
});

canvas {
    cursor: crosshair;
    border: 1px solid #000000;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<canvas id="canvas" width="800" height="500"></canvas>
<div id="output"></div>

Can not draw rectangle at exact points on a canvas using mouse when the page is scroll able

1 Answers1