<!DOCTYPE html><html><head lang="en"> <meta charset="UTF-8"> <title>canvas-图像截取</title> <style> canvas{ border: 1px dashed red; float: left; position: relative; } </style> </head><body> <div id="cantox" style="position: relative"> <canvas id="oldcanvas" width="500px" height="300px"> </canvas> <div id="cliptox" style="position: absolute;display:none"></div> </div> <button id ="btnclip" style="float: left">截取该区域</button> <canvas id="nowcanvas" width="500px" height="300px" > </canvas> <script> //获取div元素 var cantox = document.getElementById("cantox"); var cliptox = document.getElementById("cliptox"); var btnclip = document.getElementById("btnclip"); //获取到canvas元素 var oldcanvas = document.getElementById("oldcanvas"); var nowcanvas = document.getElementById("nowcanvas"); //获取canvas中的画图环境 var oldcontext = oldcanvas.getContext('2d'); var nowcontext = nowcanvas.getContext('2d'); var img = new Image(); img.src = "./image/liuyifei.jpg"; //加载图像到canvas画布中 window.onload = function (){ oldcontext.drawImage(img,0,0,oldcanvas.width,oldcanvas.height); } var startPoint;//截取图像开始坐标 var endPoint;//截图图像的结束坐标 var w; //截取图像的宽度 var h; //截取图像的高度 var flag = false; //用于判断移动事件事物控制 //鼠标按下事件 cantox.onmousedown = function (e){ flag = true; cliptox.style.display = 'block'; startPoint = windowToCanvas(oldcanvas, e.clientX, e.clientY); cliptox.style.left = startPoint.x+'px'; cliptox.style.top = startPoint.y+'px'; } //鼠标移动事件 cantox.onmousemove = function (e){ if(flag){ cliptox.style.background = 'rgba(0,0,0,0.5)'; endPoint = windowToCanvas(oldcanvas, e.clientX, e.clientY); w = endPoint.x - startPoint.x; h = endPoint.y - startPoint.y; cliptox.style.width = w +'px'; cliptox.style.height = h+'px'; } } //鼠标释放事件 cantox.onmouseup = function (e){ flag = false; } //按钮截取事件 btnclip.onclick = function (){ imgCut(nowcontext,img,oldcanvas.width,oldcanvas.height,startPoint.x,startPoint.y,w,h); } /* * 图像截取函数 * context:绘制环境对象 * image：图像对象 * imgElementW：图像显示的宽度 * imgElementH：图像显示的高度 * sx:截取图像的开始X坐标 * sy:截取图像的开始Y坐标 * w:截取图像的宽度 * h:截取图像的高度 * */ function imgCut(context,image,imgElementW,imgElementH,sx,sy,w,h){ //清理画布，便于重新绘制 context.clearRect(0,0,imgElementW,imgElementH); //计算 ：比例 = 原图像/显示图像 var ratioW = image.width/imgElementW; var ratioH = image.height/imgElementH; //根据截取图像的所占位置及大小计算出在原图所占的位置及大小 //.drawImage(图像对象,原图像截取的起始X坐标,原图像截取的起始Y坐标,原图像截取的宽度,原图像截取的高度， // 绘制图像的起始X坐标,绘制图像的起始Y坐标,绘制图像所需要的宽度,绘制图像所需要的高度); context.drawImage(image,ratioW*sx,ratioH*sy,ratioW*w,ratioH*h,0,0,w,h); } /* * 坐标转换：将window中的坐标转换到元素盒子中的坐标，并返回(x,y)坐标 * element：canvas元素对象 * x:鼠标在当前窗口X坐标值 * y:鼠标在当前窗口Y坐标值 * */ function windowToCanvas(element,x,y){ //获取当前鼠标在window中的坐标值 // alert(event.clientX+"-------"+event.clientY); //获取元素的坐标属性 var box = element.getBoundingClientRect(); var bx = x - box.left; var by = y - box.top; return {x:bx,y:by}; } </script></body></html>
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