https://www.w3schools.com/tags/tryit.asp?filename=tryhtml5_canvas_drawimage
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/filter
js canvas filter canvas filter
<!DOCTYPE html>
<html>
<body>
<p>Image to use:</p>
<img id="scream" width="220" height="277" src="img_the_scream.jpg" alt="The Scream">
<p>Canvas:</p>
<canvas id="myCanvas" width="340" height="397" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
window.onload = function() {
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
// 合成模式
ctx.filter = 'blur(4px) grayscale(50%)';
// 分别设置,后面会把前面内容覆盖
// ctx.filter = 'blur(4px)';
// ctx.filter = 'grayscale(50%)'
var img = document.getElementById("scream");
ctx.drawImage(img, 10, 10);
// 这里设置 filter 无效
// ctx.filter = 'grayscale(1)';
}
</script>
<p><strong>Note:</strong> The canvas tag is not supported in Internet
Explorer 8 and earlier versions.</p>
</body>
</html>
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