Check image transparency

Question

I have a website where people can upload an PNG image and save it. But before they can save it, i need a check if the image contains transparency. Is there an way to check (i prefer JavaScript) if an image is not 24 bits?

<img id="imageId" src=#" onload="checkRestriction(this,'1')" alt=""/>

var isPng24Bit = false;

function checkRestriction(image, id) {
    if(image.colorDepth != 24) {
      PNGis24Bit = false;
    } else {
      PNGis24Bit = true; 
    }
}

Answer 1

you can use this canvas technique for this purpose and customize this code as your demand
Make sure you resize your canvas to the same size as the image or else some pixels will be transparent where the image doesn't cover the canvas.

c.width=element.width;
c.height=element.height;

Example code and Demo:

var canvas1=document.getElementById("canvas1");
var ctx1=canvas1.getContext("2d");
var canvas2=document.getElementById("canvas2");
var ctx2=canvas2.getContext("2d");

$p1=$('#results1');
$p2=$('#results2');

var img1=new Image();
img1.crossOrigin='anonymous'
img1.onload=start1;
img1.src="https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png";
function start1(){

  canvas1.width=img1.width;
  canvas1.height=img1.height;

  ctx1.drawImage(img1,0,0);

  var imgData=ctx1.getImageData(0,0,canvas1.width,canvas1.height);
  var data=imgData.data;
  var found1='Left canvas does not have transparency';
  for(var i=0;i<data.length;i+=4){
    if(data[i+3]<255){found1='Left canvas does have transparency'; 
        break;
        }
  }

  $p1.text(found1);

}


var img2=new Image();
img2.crossOrigin='anonymous'
img2.onload=start2;
img2.src="https://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png";
function start2(){

  canvas2.width=img2.width;
  canvas2.height=img2.height;

  ctx2.drawImage(img2,0,0);

  var imgData=ctx2.getImageData(0,0,canvas2.width,canvas2.height);
  var data=imgData.data;
  var found2='Right canvas does not have transparency';
  for(var i=0;i<data.length;i+=4){
    if(data[i+3]<255){found2='Right canvas does have transparency'; 
                      break;
                     }
  }

  $p2.text(found2);

}

body{ background-color: ivory; }
canvas{border:1px solid red;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<p id=results1>Results:</p>
<p id=results2>Results:</p>
<canvas id="canvas1" width=300 height=300></canvas>
<canvas id="canvas2" width=300 height=300></canvas>