Javascript 当画布尺寸事先未知时,无法在画布上的正确位置绘制
我们需要在视频元素上使用鼠标进行绘制。我正在用画布渲染这幅画。当我们事先不知道视频的宽度和高度时,我无法在正确的位置画线。它是在不同的位置绘制的。下面是代码 HTMLJavascript 当画布尺寸事先未知时,无法在画布上的正确位置绘制,javascript,html,css,html5-canvas,html5-video,Javascript,Html,Css,Html5 Canvas,Html5 Video,我们需要在视频元素上使用鼠标进行绘制。我正在用画布渲染这幅画。当我们事先不知道视频的宽度和高度时,我无法在正确的位置画线。它是在不同的位置绘制的。下面是代码 HTML <!DOCTYPE html> <html> <head> <link rel="stylesheet" href="/css/main.css" /> </head> <body> <h1>Realtime communicatio
<!DOCTYPE html>
<html>
<head>
<link rel="stylesheet" href="/css/main.css" />
</head>
<body>
<h1>Realtime communication with WebRTC</h1>
<div id="videos">
<video id="localVideo" autoplay muted></video>
<video id="remoteVideo" autoplay></video>
</div>
<canvas id="localCanvas"></canvas>
<script type="text/javascript" src="http://code.jquery.com/jquery-1.7.1.min.js"></script>
<script src="js/main.js"></script>
</body>
</html>
main.js
body {
font-family: sans-serif;
}
video {
max-width: 100%;
width: 320px;
}
canvas {
position: absolute;
}
'use strict';
var localVideo = document.querySelector('#localVideo');
navigator.mediaDevices.getUserMedia({
audio: false,
video: true
})
.then(gotStream)
.catch(function(e) {
alert('getUserMedia() error: ' + e.name);
});
function gotStream(stream) {
// Checking Whether video is ready to set canvas width and height
var b = setInterval(() => {
if(localVideo.readyState >= 3){
document.getElementById('localCanvas').style.width = document.querySelector('#localVideo').offsetWidth + "px";
document.getElementById('localCanvas').style.height = document.querySelector('#localVideo').offsetHeight + "px";
clearInterval(b);
}
},500);
localVideo.srcObject = stream;
}
document.addEventListener("DOMContentLoaded", function(){
let xPos = window.scrollX + document.querySelector('#localVideo').getBoundingClientRect().left;
let yPos = window.scrollY + document.querySelector('#localVideo').getBoundingClientRect().top;
document.getElementById('localCanvas').style.left = xPos + "px";
document.getElementById('localCanvas').style.top = yPos + "px";
document.getElementById('localCanvas').style.width = document.querySelector('#localVideo').offsetWidth + "px";
document.getElementById('localCanvas').style.height = document.querySelector('#localVideo').offsetHeight + "px";
InitThis();
});
var mousePressed = false;
var lastX, lastY;
var ctx;
function InitThis() {
ctx = document.getElementById('localCanvas').getContext("2d");
const canvas = document.querySelector('#localCanvas');
$('#localCanvas').mousedown(function (e) {
mousePressed = true;
Draw(e.pageX - canvas.offsetLeft, e.pageY - canvas.offsetTop, false);
});
$('#localCanvas').mousemove(function (e) {
if (mousePressed) {
Draw(e.pageX - canvas.offsetLeft, e.pageY - canvas.offsetTop, true);
}
});
$('#localCanvas').mouseup(function (e) {
mousePressed = false;
});
$('#localCanvas').mouseleave(function (e) {
mousePressed = false;
});
}
function Draw(x, y, isDown) {
if (isDown) {
ctx.beginPath();
ctx.strokeStyle = "blue";
ctx.lineWidth = 9;
ctx.lineJoin = "round";
ctx.moveTo(lastX, lastY);
ctx.lineTo(x, y);
ctx.closePath();
ctx.stroke();
}
lastX = x; lastY = y;
}
这里我们只是在CSS中设置max width和width属性,我们事先不知道高度。因此,当onload事件侦听器运行时,我将基于localVideo设置画布的宽度和高度坐标。但到那时,视频的高度是160像素。但当加载视频时,高度将更改为240像素。同样,当视频状态准备好在gotStream()中播放时,我正在更改画布的宽度和高度,如下所示
一旦视频准备好播放,画布大小与本地视频的大小完全相同。但是当我尝试在画布上使用鼠标绘制时,它被绘制在错误的位置,特别是当我尝试在画布的下半部分或右边缘绘制时。谁能帮我解决这个问题
更新:我在onload事件中将画布的高度硬编码为“240px”,这也不起作用。画布正在拉伸,因为宽度和高度被设置为样式而不是原始属性。通过以下代码修复了该问题
document.getElementById('localCanvas').setAttribute("width", document.querySelector('#localVideo').offsetWidth;
document.getElementById('localCanvas').setAttribute("height", document.querySelector('#localVideo').offsetHeight;
document.getElementById('localCanvas').setAttribute("width", document.querySelector('#localVideo').offsetWidth;
document.getElementById('localCanvas').setAttribute("height", document.querySelector('#localVideo').offsetHeight;