问题
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.
Here\'s the code I have so far:
var canvas = document.getElementById(\'myCanvas\');
if (canvas.getContext)
{
var c = canvas.getContext(\'2d\');
c.globalAlpha = 0;
var img = new Image();
img.onload = function() {
c.drawImage(img, 0, 0);
}
img.src = \'image.jpg\';
}
Will somebody please point me in the right direction like a property to set or a function to call that will change the opacity?
回答1:
I am also looking for an answer to this question, (to clarify, I want to be able to draw an image with user defined opacity such as how you can draw shapes with opacity) if you draw with primitive shapes you can set fill and stroke color with alpha to define the transparency. As far as I have concluded right now, this does not seem to affect image drawing.
//works with shapes but not with images
ctx.fillStyle = "rgba(255, 255, 255, 0.5)";
I have concluded that setting the globalCompositeOperation works with images.
//works with images
ctx.globalCompositeOperation = "lighter";
I wonder if there is some kind third way of setting color so that we can tint images and make them transparent easily.
EDIT:
After further digging I have concluded that you can set the transparency of an image by setting the globalAlpha parameter BEFORE you draw the image:
//works with images
ctx.globalAlpha = 0.5
If you want to achieve a fading effect over time you need some kind of loop that changes the alpha value, this is fairly easy, one way to achieve it is the setTimeout function, look that up to create a loop from which you alter the alpha over time.
回答2:
Some simpler example code for using globalAlpha:
ctx.save();
ctx.globalAlpha = 0.4;
ctx.drawImage(img, x, y);
ctx.restore();
If you need img to be loaded:
var img = new Image();
img.onload = function() {
ctx.save();
ctx.globalAlpha = 0.4;
ctx.drawImage(img, x, y);
ctx.restore()
};
img.src = "http://...";
Notes:
Set the
'src'last, to guarantee that youronloadhandler is called on all platforms, even if the image is already in the cache.Wrap changes to stuff like
globalAlphabetween asaveandrestore(in fact use them lots), to make sure you don't clobber settings from elsewhere, particularly when bits of drawing code are going to be called from events.
回答3:
Edit: The answer marked as "correct" is not correct.
It's easy to do. Try this code, swapping out "ie.jpg" with whatever picture you have handy:
<!DOCTYPE HTML>
<html>
<head>
<script>
var canvas;
var context;
var ga = 0.0;
var timerId = 0;
function init()
{
canvas = document.getElementById("myCanvas");
context = canvas.getContext("2d");
timerId = setInterval("fadeIn()", 100);
}
function fadeIn()
{
context.clearRect(0,0, canvas.width,canvas.height);
context.globalAlpha = ga;
var ie = new Image();
ie.onload = function()
{
context.drawImage(ie, 0, 0, 100, 100);
};
ie.src = "ie.jpg";
ga = ga + 0.1;
if (ga > 1.0)
{
goingUp = false;
clearInterval(timerId);
}
}
</script>
</head>
<body onload="init()">
<canvas height="200" width="300" id="myCanvas"></canvas>
</body>
</html>
The key is the globalAlpha property.
Tested with IE 9, FF 5, Safari 5, and Chrome 12 on Win7.
回答4:
The post is old so far I'll go with my suggestion. Suggestion is based on pixel manipulation in canvas 2d context. From MDN:
You can directly manipulate pixel data in canvases at the byte level
To manipulate pixels we'll use two functions here - getImageData and putImageData
getImageData function usage:
var myImageData = context.getImageData(left, top, width, height);
and putImageData syntax:
context.putImageData(myImageData, dx, dy); //dx, dy - x and y offset on your canvas
Where context is your canvas 2d context
So to get red green blue and alpha values, we'll do the following:
var r = imageData.data[((x*(imageData.width*4)) + (y*4))];
var g = imageData.data[((x*(imageData.width*4)) + (y*4)) + 1];
var b = imageData.data[((x*(imageData.width*4)) + (y*4)) + 2];
var a = imageData.data[((x*(imageData.width*4)) + (y*4)) + 3];
Where x is x offset, y is y offset on canvas
So we having code making image half-transparent
var canvas = document.getElementById('myCanvas');
var c = canvas.getContext('2d');
var img = new Image();
img.onload = function() {
c.drawImage(img, 0, 0);
var ImageData = c.getImageData(0,0,img.width,img.height);
for(var i=0;i<img.height;i++)
for(var j=0;j<img.width;j++)
ImageData.data[((i*(img.width*4)) + (j*4) + 3)] = 127;//opacity = 0.5 [0-255]
c.putImageData(ImageData,0,0);//put image data back
}
img.src = 'image.jpg';
You can make you own "shaders" - see full MDN article here
回答5:
You can. Transparent canvas can be quickly faded by using destination-out global composite operation. It's not 100% perfect, sometimes it leaves some traces but it could be tweaked, depending what's needed (i.e. use 'source-over' and fill it with white color with alpha at 0.13, then fade to prepare the canvas).
// Fill canvas using 'destination-out' and alpha at 0.05
ctx.globalCompositeOperation = 'destination-out';
ctx.fillStyle = "rgba(255, 255, 255, 0.05)";
ctx.beginPath();
ctx.fillRect(0, 0, width, height);
ctx.fill();
// Set the default mode.
ctx.globalCompositeOperation = 'source-over';
回答6:
I think this answers the question best, it actually changes the alpha value of something that has been drawn already. Maybe this wasn't part of the api when this question was asked.
given 2d context c.
function reduceAlpha(x, y, w, h, dA) {
var screenData = c.getImageData(x, y, w, h);
for(let i = 3; i < screenData.data.length; i+=4){
screenData.data[i] -= dA; //delta-Alpha
}
c.putImageData(screenData, x, y );
}
回答7:
If you use jCanvas library you can use opacity property when drawing. If you need fade effect on top of that, simply redraw with different values.
回答8:
You can't. It's immediate mode graphics. But you can sort of simulate it by drawing a rectangle over it in the background color with an opacity.
If the image is over something other than a constant color, then it gets quite a bit trickier. You should be able to use the pixel manipulation methods in this case. Just save the area before drawing the image, and then blend that back on top with an opacity afterwards.
来源：https://stackoverflow.com/questions/2359537/how-to-change-the-opacity-alpha-transparency-of-an-element-in-a-canvas-elemen