如何更改画布元素绘制完成后的不透明度(alpha，透明度)?

Using the HTML5 <canvas> element, I would like to load an image file (PNG, JPEG, etc.), draw it to the canvas completely transparently, and then fade it in. I have figured out how to load the image and draw it to the canvas, but I don't know how to change its opacity once it as been drawn. element, I would like