原生js 基于canvas写一个简单的前端 截图
2019-04-16 15:07 muamaker 阅读(1666) 评论(0) 编辑 收藏 举报
先看效果
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title></title>
<style type="text/css">
* {
padding: 0;
margin: 0;
font-size: 14px;
}
#box {
width: 400px;
height: 400px;
border: 1px solid red;
margin: 20px auto;
position: relative;
}
#operate {
text-align: center;
;
}
#operate p {
margin-bottom: 6px;
}
input[type=file] {
display: none;
}
label {
display: inline-block;
cursor: pointer;
background: #38f;
color: #fff;
width: 102px;
height: 38px;
line-height: 38px;
border-radius: 4px;
}
#clipcanvas {
position: absolute;
top: 0;
left: 0;
z-index: 10;
width: 100%;
height: 100%;
}
</style>
</head>
<body>
<div id="box">
<canvas id="canvas" width="400" height="400"></canvas>
<canvas id="clipcanvas"></canvas>
</div>
<div id="operate">
<p><label><input type="file" name="" id="bg" value="" />选择头像</label></p>
<p><label><input type="file" name="" id="flag" value="" />选择上层</label></p>
<p><input type="text" name="" id="posX" value="0" />请输入上层x位置</p>
<p><input type="text" name="" id="posY" value="0" />请输入上层y位置</p>
<p><label id="create">直接生成</label></p>
<p><label id="reset">重新截图</label></p>
</div>
<a href="" download="logo.png" title="点击下载" id="down">
<img src="" id="result">
</a>
</body>
<script type="text/javascript">
const canvas = document.getElementById("canvas");
const ctx = canvas.getContext("2d");
const baseW = 400;
const flagW = 100;
let bgConfig;
let flagConfig;
document.getElementById("bg").onchange = async function() {
const file = this.files[0];
try {
const img = await getImageObj(file);
const rate = compress(img, baseW);
bgConfig = [img, 0, 0, img.width, img.height, 0, 0, rate.w, rate.h];
drawn();
} catch (e) {
console.error(e);
}
};
document.getElementById("flag").onchange = async function() {
const file = this.files[0];
try {
const img = await getImageObj(file);
const rate = compress(img, flagW);
flagConfig = [img, 0, 0, img.width, img.height, 0, 0, rate.w, rate.h];
drawn();
} catch (e) {
console.error(e);
}
};
document.getElementById("posX").onchange = function() {
let val = Number(this.value) || 0;
flagConfig[5] = val;
drawn();
}
document.getElementById("posY").onchange = function() {
let val = Number(this.value) || 0;
flagConfig[6] = val;
drawn();
}
function drawn() {
ctx.clearRect(0, 0, canvas.clientWidth, canvas.clientHeight);
if (bgConfig) {
ctx.drawImage(...bgConfig);
}
if (flagConfig) {
ctx.drawImage(...flagConfig);
}
}
//图片压缩,获取等比缩放后的结果
function compress(img, base) {
let w = img.width;
let h = img.height;
if (img.width > img.height) {
if (img.width > base) {
//要将宽度缩放
w = base;
h = (w / img.width) * img.height; // 新的 宽比 高 = 旧的宽比高 h / w = img.heigth/img.width ;
}
} else {
if (img.height > base) {
h = base;
w = (h / img.height) * img.width;
}
}
return {
w,
h
};
}
function getImageObj(file) {
const url = getObjectURL(file);
const img = new Image();
img.src = url;
return new Promise((resolve, reject) => {
img.onload = function() {
resolve(img);
}
img.onerror = function(e) {
reject(e);
}
});
}
//取得该文件的url
function getObjectURL(file) {
var url = null;
if (window.createObjectURL != undefined) {
url = window.createObjectURL(file);
} else if (window.URL != undefined) {
url = window.URL.createObjectURL(file);
} else if (window.webkitURL != undefined) {
url = window.webkitURL.createObjectURL(file);
}
return url;
}
//截图
var clipcanvas = document.getElementById("clipcanvas");
var clipctx = clipcanvas.getContext("2d");
clipcanvas.width = canvas.clientWidth;
clipcanvas.height = canvas.clientHeight;
var start = null;
var clipArea = {}; //裁剪范围
clipcanvas.onmousedown = function(e) {
start = {
x: e.offsetX,
y: e.offsetY
};
}
clipcanvas.onmousemove = function(e) {
if (start) {
fill(start.x, start.y, e.offsetX - start.x, e.offsetY - start.y)
}
}
document.addEventListener("mouseup", function() {
if (start) {
start = null;
exportImg(clipArea);
}
});
//重新截图
document.getElementById("reset").onclick = function(){
clipctx.clearRect(0, 0, clipcanvas.width, clipcanvas.height);
}
//直接生成
document.getElementById("create").onclick = function(){
exportImg({
x:0,
y:0,
w:canvas.clientWidth,
h:canvas.clientHeight,
})
}
function fill(x, y, w, h) {
clipctx.clearRect(0, 0, clipcanvas.width, clipcanvas.height);
clipctx.beginPath();
clipctx.fillStyle = 'rgba(0,0,0,0.6)';
clipctx.strokeStyle = "green";
//遮罩层
clipctx.globalCompositeOperation = "source-over";
clipctx.fillRect(0, 0, clipcanvas.width, clipcanvas.height);
//画框
clipctx.globalCompositeOperation = 'destination-out';
clipctx.fillRect(x, y, w, h);
//描边
clipctx.globalCompositeOperation = "source-over";
clipctx.moveTo(x, y);
clipctx.lineTo(x + w, y);
clipctx.lineTo(x + w, y + h);
clipctx.lineTo(x, y + h);
clipctx.lineTo(x, y);
clipctx.stroke();
clipctx.closePath();
clipArea = {
x,
y,
w,
h
};
}
function startClip(area) {
var canvas = document.createElement("canvas");
canvas.width = area.w;
canvas.height = area.h;
var data = ctx.getImageData(area.x, area.y, area.w, area.h);
var context = canvas.getContext("2d");
context.putImageData(data, 0, 0);
return canvas.toDataURL("image/png");
}
function exportImg(clipArea){
var url = startClip(clipArea);
document.getElementById("result").src = url;
document.getElementById("down").href = url;
}
</script>
</html>
