Draw rectangle on image inside canvas using mousemove event in javascript
I am trying to draw a rectangle on an image that is inside the canvas using the mousemove event. But I am getting a rectangle in the image with a rectangle filled color due to clearRect. can anyone understand me. how to draw a rectangle with a border on the image. Below is the code I followed to achieve it.
var canvas = document.getElementById('canvas'),
ctx = canvas.getContext('2d'),
rect = {},
drag = false;
function init() {
var imageObj = new Image();
imageObj.onload = function() {
ctx.drawImage(imageObj, 0, 0);
};
imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
canvas.addEventListener('mousedown', mouseDown, false);
canvas.addEventListener('mouseup', mouseUp, false);
canvas.addEventListener('mousemove', mouseMove, false);
}
function mouseDown(e) {
rect.startX = e.pageX - this.offsetLeft;
rect.startY = e.pageY - this.offsetTop;
drag = true;
}
function mouseUp() {
drag = false;
}
function mouseMove(e) {
if (drag) {
rect.w = (e.pageX - this.offsetLeft) - rect.startX;
rect.h = (e.pageY - this.offsetTop) - rect.startY ;
ctx.clearRect(rect.startX, rect.startY,rect.w,rect.h);
draw();
}
}
function draw() {
ctx.clearRect(rect.startX, rect.startY, rect.w, rect.h);
ctx.strokeRect(rect.startX, rect.startY, rect.w, rect.h);
}
<head>
<meta charset="utf-8" />
<title>Draw a rectangle!</title>
</head>
<body onload="init();">
<canvas id="canvas" width="500" height="500"></canvas>
</body>
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You probably need to first clear the whole canvas using clearRect
, then paint right away imageObj
and finally paint the stroke. This all happens inside the function mouseMove
, so it basically keeps on drawing these elements permanently.
Try this snippet below:
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var rect = {};
var drag = false;
var imageObj = null;
function init() {
imageObj = new Image();
imageObj.onload = function () { ctx.drawImage(imageObj, 0, 0); };
imageObj.src = 'http://www.html5canvastutorials.com/demos/assets/darth-vader.jpg';
canvas.addEventListener('mousedown', mouseDown, false);
canvas.addEventListener('mouseup', mouseUp, false);
canvas.addEventListener('mousemove', mouseMove, false);
}
function mouseDown(e) {
rect.startX = e.pageX - this.offsetLeft;
rect.startY = e.pageY - this.offsetTop;
drag = true;
}
function mouseUp() { drag = false; }
function mouseMove(e) {
if (drag) {
ctx.clearRect(0, 0, 500, 500);
ctx.drawImage(imageObj, 0, 0);
rect.w = (e.pageX - this.offsetLeft) - rect.startX;
rect.h = (e.pageY - this.offsetTop) - rect.startY;
ctx.strokeStyle = 'red';
ctx.strokeRect(rect.startX, rect.startY, rect.w, rect.h);
}
}
//
init();
<canvas id="canvas" width="500" height="500"></canvas>
Hope this is what you were looking for and helped you in some way.
PS I have intentionally applied color red
to the stroke. You can delete it, of course.
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