javascript 在画布上绘制不透明的图像
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draw image with opacity on to a canvas
提问by Becky
I've got an image. I use drawImage()to draw it on to a canvas.
我有一个图像。我drawImage()习惯把它画在画布上。
My question is: If the image has an opacity of 0.4, how do I draw it in the same opacity on to the canvas.
我的问题是：如果图像的不透明度为 0.4，我如何以相同的不透明度将其绘制到画布上。
I've created a sample Fiddle here. How can I draw #screamon to mycanvasmaintaining 0.4 opacity on the image.
我在这里创建了一个示例Fiddle。我怎样才能保持图像的不透明度#scream为mycanvas0.4。
html:
html：
<p>Image with 0.4 opacity to be used:</p>
<img id="scream" width="200" height="200" src="http://img42.com/NMMU8+">
<p>Canvas:</p>
<canvas id="myCanvas" width="220" height="220" style="border:1px solid #d3d3d3;">
</canvas>
css:
css：
#scream{
opacity: 0.4;
}
#myCanvas{
background-color: #f60;
}
js:
js：
window.onload = function() {
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var img = document.getElementById("scream");
ctx.drawImage(img, 10, 10);
}
回答by Puni
Use globalAlphabut make sure that you set it before drawing the image:
使用globalAlpha但请确保在绘制图像之前设置它：
ctx.globalAlpha = 0.4;
