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javascripthtmlcanvastransparencypixel

putImageData(), how to keep old pixels if new pixels are transparent?

In html5, when you draw to a canvas using putImageData(), if some of the pixels you are drawing are transparent (or semi-transparent), how do you keep old pixels in the canvas unaffected?

example:

var imgData = context.createImageData(30,30);
for(var i=0; i<imgData.data.length; i+=4)
{
imgData.data[i]=255;
imgData.data[i+1]=0;
imgData.data[i+2]=0;
imgData.data[i+3]=255;
if((i/4)%30 > 15)imgData.data[i+3] = 0;
}
context.putImageData(imgData,0,0);

The right half of the 30x30 rect is transparent. If this is drawn over something on the canvas, pixels behind the right half are removed (or become thransparent). How do I keep them?

Solution

  • You can use getImageData to create a semi-transparent overlay:

    • create a temporary offscreen canvas
    • getImageData to get the pixel data from the offscreen canvas
    • modify the pixels as you desire
    • putImageData the pixels back on the offscreen canvas
    • use drawImage to draw the offscreen canvas to the onscreen canvas

    enter image description here

    Here's example code and a Demo: http://jsfiddle.net/m1erickson/CM7uY/

    <!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
    body{ background-color: ivory; }
    canvas{border:1px solid red;}
</style>
<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var context=canvas.getContext("2d");

    // draw an image on the canvas
    var img=new Image();
    img.onload=start;
    img.src="https://dl.dropboxusercontent.com/u/139992952/stack1/landscape1.jpg";
    function start(){
        canvas.width=img.width;
        canvas.height=img.height;
        context.drawImage(img,0,0);

        // overlay a red gradient 
        drawSemiTransparentOverlay(canvas.width/2,canvas.height)

    }

    function drawSemiTransparentOverlay(w,h){

        // create a temporary canvas to hold the gradient overlay
        var canvas2=document.createElement("canvas");
        canvas2.width=w;
        canvas2.height=h
        var ctx2=canvas2.getContext("2d");

        // make gradient using ImageData
        var imgData = ctx2.getImageData(0,0,w,h);
        var data=imgData.data;
        for(var y=0; y<h; y++) {
            for(var x=0; x<w; x++) {
                var n=((w*y)+x)*4;
                data[n]=255;
                data[n+1]=0;
                data[n+2]=0;
                data[n+3]=255;
                if(x>w/2){
                    data[n+3]=255*(1-((x-w/2)/(w/2)));
                }
            }
        }

        // put the modified pixels on the temporary canvas
        ctx2.putImageData(imgData,0,0);

        // draw the temporary gradient canvas on the visible canvas
        context.drawImage(canvas2,0,0);

    }


}); // end $(function(){});
</script>
</head>
<body>
    <canvas id="canvas" width=200 height=200></canvas>
</body>
</html>

    Alternatively, you might check out using a linear gradient to do your effect more directly.

    http://jsfiddle.net/m1erickson/j6wLR/