Creating canvas dynamically in webrtc video quality tests

chrome/test/data/webrtc/video_extraction.js

 /**
  * Copyright (c) 2012 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /**
- * The ID of the video tag from which frames are captured.
- * @private
- */
-var gVideoId = 'remote-view';
-
-/**
- * The ID of the canvas tag from which the frames are captured.
- * @private
- */
-var gCanvasId = 'remote-canvas';
-
-/**
  * Counts the number of frames that have been captured. Used in timeout
  * adjustments.
  * @private
  */
 var gFrameCounter = 0;
 
 /**
  * The gStartOfTime when the capturing begins. Used for timeout adjustments.
  * @private
  */
@@ skipping 63 matching lines @@
   gFrameCaptureInterval = 1000/frame_rate;
   gCaptureDuration = 1000 * duration;
   var width = videoTag.videoWidth;
   var height = videoTag.videoHeight;
 
   if (width == 0 || height == 0) {
     throw failTest('Trying to capture from ' + videoTag.id +
                    ' but it is not playing any video.');
   }
 
-  setCanvasDimensions_(width, height);
-
   console.log('Received width is: ' + width + ', received height is: ' + height
               + ', capture interval is: ' + gFrameCaptureInterval +
               ', duration is: ' + gCaptureDuration);
 
+  var remoteCanvas = document.createElement('canvas');
+  remoteCanvas.width = width;
+  remoteCanvas.height = height;
+  document.body.appendChild(remoteCanvas);
+
   gStartOfTime = new Date().getTime();
-  setTimeout(function() { shoot(width, height); },
+  setTimeout(function() { shoot(videoTag, remoteCanvas, width, height); },
              gFrameCaptureInterval);
 }
 
 /**
- * Sets the canvas dimensions.
- * @private
- * @param {Number} The width of the canvas.
- * @param {Number} The height of the canvas.
-*/
-function setCanvasDimensions_(width, height) {

[amogh.bihani, 2014/05/13 13:23:25]

Now these changes seem futile. :(

-  var canvas = document.getElementById(gCanvasId);
-  canvas.width = width;
-  canvas.height = height;
-}
-
-/**
  * Captures an image frame from the provided video element.
  *
+ * @private
  * @param {Video} video HTML5 video element from where the image frame will
  * be captured.
+ * @param {!Object} 2d context of the canvas on which the image frame will be
+ * captured.
  * @param {Number} The width of the video/canvas area to be captured.
  * @param {Number} The height of the video/canvas area to be captured.
  *
- * @return {Canvas}
+ * @return {Object} Returns the ImageData object.
  */
-function capture(video, width, height) {
-  var canvas = document.getElementById(gCanvasId);
-  var ctx = canvas.getContext('2d');
-  ctx.drawImage(video, 0, 0, width, height);
-  return canvas;
+function captureFrame_(video, context, width, height) {
+  context.drawImage(video, 0, 0, width, height);
+  return context.getImageData(0, 0, width, height);
 }
 
 /**
  * The function which is called at the end of every gFrameCaptureInterval. Gets
  * the current frame from the video and extracts the data from it. Then it saves
  * it in the frames array and adjusts the capture interval (timers in JavaScript
  * aren't precise).
  *
+ * @param {!Object} The video whose frames are to be captured.
+ * @param {Canvas} The canvas on which the image will be captured.
  * @param {Number} The width of the video/canvas area to be captured.
  * @param {Number} The height of the video area to be captured.
  */
-function shoot(width, height) {
+function shoot(video, canvas, width, height) {
   // The first two captured frames have big difference between the ideal time
   // interval between two frames and the real one. As a consequence this affects
   // enormously the interval adjustment for subsequent frames. That's why we
   // have to reset the time after the first two frames and get rid of these two
   // frames.
   if (gFrameCounter == 1 && !gFrameIntervalAdjustment) {
     gStartOfTime = new Date().getTime();
     gFrameCounter = 0;
     gFrameIntervalAdjustment = true;
     gFrames.pop();
     gFrames.pop();
   }
-  var video  = document.getElementById(gVideoId);
-  var canvas = capture(video, width, height);
 
-  // Extract the data from the canvas.
-  var ctx = canvas.getContext('2d');
   // We capture the whole video frame.
-  var img = ctx.getImageData(0, 0, width, height);
+  var img = captureFrame_(video, canvas.getContext('2d'), width, height);
   gFrames.push(img.data.buffer);
   gFrameCounter++;
 
   // Adjust the timer.
   var current_time = new Date().getTime();
   var ideal_time = gFrameCounter*gFrameCaptureInterval;
   var real_time_elapsed = current_time - gStartOfTime;
   var diff = real_time_elapsed - ideal_time;
 
   if (real_time_elapsed < gCaptureDuration) {
     // If duration isn't over shoot again
-    setTimeout(function() { shoot(width, height); },
+    setTimeout(function() { shoot(video, canvas, width, height); },
                gFrameCaptureInterval - diff);
   } else {  // Else reset gFrameCounter and send the frames
     dDoneFrameCapturing = true;
     gFrameCounter = 0;
     clearPage_();
     prepareProgressBar_();
     sendFrames();
   }
 }
 
@@ skipping 91 matching lines @@
 /**
  * @private
  */
 function prepareProgressBar_() {
   document.body.innerHTML =
     '<html><body>' +
     '<p id="progress-bar" style="position: absolute; top: 50%; left: 40%;">' +
     'Preparing to send frames.</p>' +
     '</body></html>';
 }