在画布上绘制不透明的图像[复制]

This question already has an answer here:

这个问题已经有了答案:

• How to change the opacity (alpha, transparency) of an element in a canvas element after it has been drawn? 7 answers
• 如何更改画布元素中元素的不透明度(alpha，透明度)?7的答案

I've got an image. I use drawImage() to draw it on to a canvas.

我有一个图像。我使用drawImage()将其绘制到画布上。

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是0.4，我如何在画布上使用同样的不透明度来绘制它呢?

I've created a sample Fiddle here. How can I draw #scream on to mycanvas maintaining 0.4 opacity on the image.

我在这里创建了一个样本。我怎样才能把# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #

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);
}

1 个解决方案

#1

Use globalAlpha but 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:

ctx.globalAlpha = 0.4;