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html5-canvas Compositing Change opacity with "globalAlpha" You can change the opacity of new drawings by setting the globalAlpha to a value between 0.00 (fully transparent) and 1.00 (fully opaque). The default globalAlpha is 1.00 (fully opaque). Existing drawings are not affected by globalAlpha .
Canvases are transparent by default. Try setting a page background image, and then put a canvas over it. If nothing is drawn on the canvas, you can fully see the page background.
Alpha affects everything drawn on the view. The background color's alpha affects the background color's transparency and anything else drawn on the view. i.e it brings about a frosty effect to whole view. Opacity means don't draw anything underneath, even if you are transparent, it just effects the current view.
CSS Opacity / TransparencyThe opacity property specifies the opacity/transparency of an element.
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 );
}
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