How do i tint an image with HTML5 Canvas?
My question is, what is the best way to tint an image that is drawn using the drawImage method. The target useage for this is advanced 2d particle-effects (game development) where particles change colors over time etc. I am not asking how to tint the whole canvas, only the current image i am about to draw.
I have concluded that the globalAlpha parameter affects the current image that is drawn.
//works with drawImage()
canvas2d.globalAlpha = 0.5;
But how do i tint each image with an arbitrary color value ? It would be awesome if there was some kind of globalFillStyle or globalColor or that kind of thing...
EDIT:
Here is a screenshot of the application i am working with: http://twitpic.com/1j2aeg/full alt text http://web20.twitpic.com/img/92485672-1d59e2f85d099210d4dafb5211bf770f.4bd804ef-scaled.png
Answer
You have compositing operations, and one of them is destination-atop. If you composite an image onto a solid color with the 'context.globalCompositeOperation = "destination-atop"', it will have the alpha of the foreground image, and the color of the background image. I used this to make a fully tinted copy of an image, and then drew that fully tinted copy on top of the original at an opacity equal to the amount that I want to tint.
Here is the full code:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>HTML5 Canvas Test</title>
<script type="text/javascript">
var x; //drawing context
var width;
var height;
var fg;
var buffer
window.onload = function() {
var drawingCanvas = document.getElementById('myDrawing');
// Check the element is in the DOM and the browser supports canvas
if(drawingCanvas && drawingCanvas.getContext) {
// Initaliase a 2-dimensional drawing context
x = drawingCanvas.getContext('2d');
width = x.canvas.width;
height = x.canvas.height;
// grey box grid for transparency testing
x.fillStyle = '#666666';
x.fillRect(0,0,width,height);
x.fillStyle = '#AAAAAA';
var i,j;
for (i=0; i<100; i++){
for (j=0; j<100; j++){
if ((i+j)%2==0){
x.fillRect(20*i,20*j,20,20);
}
}
}
fg = new Image();
fg.src = 'http://uncc.ath.cx/LayerCake/images/16/3.png';
// create offscreen buffer,
buffer = document.createElement('canvas');
buffer.width = fg.width;
buffer.height = fg.height;
bx = buffer.getContext('2d');
// fill offscreen buffer with the tint color
bx.fillStyle = '#FF0000'
bx.fillRect(0,0,buffer.width,buffer.height);
// destination atop makes a result with an alpha channel identical to fg, but with all pixels retaining their original color *as far as I can tell*
bx.globalCompositeOperation = "destination-atop";
bx.drawImage(fg,0,0);
// to tint the image, draw it first
x.drawImage(fg,0,0);
//then set the global alpha to the amound that you want to tint it, and draw the buffer directly on top of it.
x.globalAlpha = 0.5;
x.drawImage(buffer,0,0);
}
}
</script>
</head>
</body>
<canvas id="myDrawing" width="770" height="400">
<p>Your browser doesn't support canvas.</p>
</canvas>
</body>
</html>