getusermedia screenshot wrong size

The drawImage API is really poorly documented and has a confusing way of handling arguments. Because of this sometimes if you capture a screenshot from your webcam the capture / snapshot will be drawn with the wrong dimensions. To solve this you will need to redraw your canvas to the same size as your video input on capture and also specify the width/height on capture.

This tripped me up for a few minutes so I figured it was worth documenting..

<!DOCTYPE html>
<body style="margin:0;padding:0;font-family:Arial">
<video id="video" autoplay width=100%></video>
<img id="snapshot" src="">
<canvas id="canvas" style="display:none;"></canvas>

<script type="text/javascript">
var $ = function(id){return document.getElementById(id)}; // lazy dev is lazy
var localMediaStream = null;
var video = $('video'); // video stream
var canvas = $('canvas'); // invisible canvas
var snapshot = $('snapshot'); // output image
var ctx = canvas.getContext('2d');

// Start Video Stream
navigator.getUserMedia({video: true}, function(stream) {
  video.src = window.URL.createObjectURL(stream);
  localMediaStream = stream;
}, function(){
   alert("Enable and Allow your camera");
});

function snapshot() {
  if (localMediaStream) {
    canvas.width = video.offsetWidth; // update the canvas width and height
    canvas.height = video.offsetHeight;
    ctx.drawImage(video, 0, 0, video.offsetWidth, video.offsetHeight); // draw the captured content onto a canvas
    snapshot.src = canvas.toDataURL('image/png');
    video.style.display = "none";
  }
}
</script>
</body>
</html>

To test go to your development console and type “snapshot()”

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