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233
ViewsHow to draw a selection rectangle with drag and drop on a HTML canvas?
I'd like to draw a selection rectangle (with mouse drag and drop, like with MS Paint) on a HTML canvas.
The following snippet works, but with two different canvas.
Question: How to do this on a single canvas, such that the image photo of the canvas can later be changed without removing the selection rectangle?
Should we keep 2 different canvas (if so, how to place the #2 exactly on top of the first one, without absolute positioning for both)?
Or is it possible in a simpler way with a single canvas? (and something like ctx.globalCompositeOperation = "source-over"; or a similar method)
var c1 = document.getElementById("c1"), c2 = document.getElementById("c2");
var ctx1 = c1.getContext("2d"), ctx2 = c2.getContext("2d");
ctx2.setLineDash([5, 5]);
var origin = null;
window.onload = () => { ctx1.drawImage(document.getElementById("img"), 0, 0); }
c1.onmousedown = e => { origin = {x: e.offsetX, y: e.offsetY}; };
window.onmouseup = e => { origin = null; };
c1.onmousemove = e => {
if (!!origin) {
ctx2.strokeStyle = "#ff0000";
ctx2.clearRect(0, 0, c2.width, c2.height);
ctx2.beginPath();
ctx2.rect(origin.x, origin.y, e.offsetX - origin.x, e.offsetY - origin.y);
ctx2.stroke();
}
};#img { display: none; }
#c1 { border: 1px solid green; }
#c2 { border: 1px solid blue; }<img id="img" src="https://i.imgur.com/okSIKkW.jpg">
<canvas id="c1" height="200" width="200"></canvas>
<canvas id="c2" height="200" width="200"></canvas>2 answers
Answer question0
There might be a better solution with one single <canvas> and multiple layers (if so, feel free to post another answer), but I finally used a method similar to html5 - canvas element - Multiple layers with an overlay of 2 canvas:
var c1 = document.getElementById("c1"), c2 = document.getElementById("c2");
var ctx1 = c1.getContext("2d"), ctx2 = c2.getContext("2d");
ctx2.setLineDash([5, 5]);
var origin = null;
window.onload = () => { ctx1.drawImage(document.getElementById("img"), 0, 0); }
c2.onmousedown = e => { origin = {x: e.offsetX, y: e.offsetY}; };
window.onmouseup = e => { origin = null; };
c2.onmousemove = e => {
if (!!origin) {
ctx2.strokeStyle = "#ff0000";
ctx2.clearRect(0, 0, c2.width, c2.height);
ctx2.beginPath();
ctx2.rect(origin.x, origin.y, e.offsetX - origin.x, e.offsetY - origin.y);
ctx2.stroke();
}
};#img { display: none; }
#canvas-container { position: relative; }
canvas { position: absolute; left: 0; top: 0; }
#c1 { z-index: 0; }
#c2 { z-index: 1; }<img id="img" src="https://i.imgur.com/okSIKkW.jpg">
<div id="canvas-container">
<canvas id="c1" height="200" width="200"></canvas>
<canvas id="c2" height="200" width="200"></canvas>
</div>0
The following solution uses a single canvas and rerenders on mouse movement.
https://jsfiddle.net/uhtdv0mj/3/
const c1 = document.getElementById("c1");
const ctx1 = c1.getContext("2d");
const drawImage = () => {
ctx1.drawImage(document.getElementById("img"), 0, 0);
};
const drawSelection = (e) => {
ctx1.strokeStyle = "#000";
ctx1.beginPath();
ctx1.rect(origin.x, origin.y, e.offsetX - origin.x, e.offsetY - origin.y);
ctx1.stroke();
};
const clear = () => {
ctx1.clearRect(0, 0, c1.width, c1.height);
};
const render = (e) => {
clear();
drawImage();
if (origin) drawSelection(e);
};
window.onload = drawImage;
let origin = null;
c1.onmousedown = e => { origin = {x: e.offsetX, y: e.offsetY}; };
c1.onmouseup = e => { origin = null; render(e); };
c1.onmousemove = render;