canvas实现图片添加水印效果HTML5
时间：6年前 阅读：4942
利用canvas可以实现向图片添加水印的功能，先通过file控件的reader选择图片，然后使用canvas添加图片及水印，并且使用toDataURL()和a标签实现添加水印后的图片的下载功能
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<canvas id="drawing" style="width:300px;border:1px solid black">
<p>The canvas element is not supported!</p>
</canvas>
<div>
<span>
<input type="file" id="addImgHelper" style="display:none">
<button id="addImg">选择图片</button>
</span>
<span>
<button id="addWaterMark" disabled>添加水印</button>
<span>水印文字为</span>
<input id="waterMarkWords" type="text" value="期权记">
</span>
<span>
<button id="downloadImg" disabled>下载图片</button>
<a id="downloadImgHelper" href="#" download="带水印图片" style="display:none"></a>
</span>
</div>
</body>
<script type="text/javascript">
if(drawing.getContext){
var cxt = drawing.getContext('2d');
var W,H;
addImg.onclick = function(){
addImgHelper.click();
}
addImgHelper.onchange = function(){
addWaterMark.disabled = true;
downloadImg.disabled = true;
var file = addImgHelper.files[0];
if(file && /image/.test(file.type)){
var reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = function(){
var img = new Image();
img.src= reader.result;
img.onload = function(){
addWaterMark.disabled = false;
drawing.width = W = img.width;
drawing.height = H = img.height;
cxt.drawImage(img,0,0);
addWaterMark.onclick = function(){
downloadImg.disabled = false;
cxt.clearRect(0,0,W,H);
cxt.drawImage(img,0,0);
var str = waterMarkWords.value;
cxt.font = "bold 50px Arial";
cxt.lineWidth = '1';
cxt.fillStyle = 'rgba(255,255,255,0.5)';
cxt.textBaseline = "bottom";
cxt.textAlign = 'end';
cxt.fillText(str,W-10,H-10,W/2);
downloadImg.onclick = function(){
downloadImgHelper.href = drawing.toDataURL('image/png');
downloadImgHelper.click();
}
}
}
}
}
}
}
</script>
</html>
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