How to change the opacity (alpha, transparency) of an element in a canvas element?_问答_开发者

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.

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(开发者_运维技巧);
    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?

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.

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 your onload handler is called on all platforms, even if the image is already in the cache.
Wrap changes to stuff like globalAlpha between a save and restore (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.

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.

This 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 usage:

var myImageData = context.getImageData(left, top, width, height);

The putImageData syntax:

context.putImageData(myImageData, x, y);

Where context is your canvas 2d context, and x and y are the position on the canvas.

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 the horizontal offset, y is the vertical offset.

The 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

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';

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) {
    let 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 );
}

Set global Alpha draw the object that has opacity then set back to normal.

  //////////////////////// circle ///////////////////////
  ctx.globalAlpha = 0.75;
  ctx.beginPath();
  ctx.arc(x1, y1, r1, 0, Math.PI*2);
  ctx.fillStyle = colour;
  ctx.fill();
  ctx.closePath();
  ctx.globalAlpha = 1;

How i made it..on canvas i first draw rect in a selfrun function 0,0,canvas.width,canvas.height as a background of canvas and i set globalAlpha to 1 .then i draw other shapes in ather own functions and set their globalAlpha to 0.whatever number they dont affect each other even images.

Like Ian said, use c.globalAlpha = 0.5 to set the opacity, type up the rest of the settings for the square, then follow up with c.save();. This will save the settings for the square then you can c.rect and c.fillStyle the square how you want it. I chose not to wrap it with c.restore afterwards and it worked well

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.

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.