Changing font size with pinch gesture in Reader Mode

components/dom_distiller/core/javascript/dom_distiller_viewer.js

Index: components/dom_distiller/core/javascript/dom_distiller_viewer.js
diff --git a/components/dom_distiller/core/javascript/dom_distiller_viewer.js b/components/dom_distiller/core/javascript/dom_distiller_viewer.js
index 55f53bbf3854feaebdf305bed92df2acce7a9ef2..551a8cbeee1748c2e62df347e62062e9069514af 100644
--- a/components/dom_distiller/core/javascript/dom_distiller_viewer.js
+++ b/components/dom_distiller/core/javascript/dom_distiller_viewer.js
@@ -74,3 +74,123 @@ document.getElementById('showOriginal').addEventListener('click', function(e) {
   document.body.appendChild(img);
 }, true);
 
+window.addEventListener('load', function() {
+  // When users pinch in Reader Mode, the page would zoom in or out as if it
+  // is a normal web page allowing user-zoom. At the end of pinch gesture, the
+  // page would do text reflow. These pinch-to-zoom and text reflow effects
+  // are not native, but are emulated using CSS and JavaScript.
+  //
+  // In order to achieve near-native zooming and panning frame rate, fake 3D
+  // transform is used so that the layer doesn't repaint for each frame.
+  //
+  // After the text reflow, the web content shown in viewport should roughly be
+  // the same paragraph before zooming.
+  //
+  // The control point of font size is the html element, so that both "em" and
+  // "rem" are adjusted.
+  //
+  // TODO(wychen): Improve scroll position when elementFromPoint is body.
+
+  var pinching = false;
+  var pinchDistAnchor = 0;
+  var fontSizeAnchor = 1.0;
+  var focusElement = null;
+  var focusPos = 0;
+  var pinchOrigin = {x: 0, y: 0};
+  var scaleFactor = 1;
+  var lastClientMidY = 0;
+
+  // The zooming speed relative to pinching speed.
+  // @const
+  var FONT_SCALE_MULTIPLIER = 0.3;
+
+  // The font size is guaranteed to be in px.
+  var baseSize =
+      parseFloat(getComputedStyle(document.documentElement).fontSize);
+
+  var handleTouchStart = function(e) {
+    if (e.touches.length < 2) return;
+    e.preventDefault();
+    if (e.touches.length != 2) return;

[jdduke (slow), 2015/03/16 20:27:52]

How much more complicated is supporting >2 touches?

For scale gesture detection on Android, we abstract the notion of "touchDist" to
be a span indicating the average distance between touch points through the focal
point, i.e., compute the "focal" position by averaging all the point positions,
compute the average distance of each point to that focal point, then double that
(giving you the diameter of the approximate circle formed by the touch points). 

We can probably modify the touchDist/Mid functions to do this and it'll just
work.

[wychen, 2015/03/17 17:36:11]

Good suggestion! I've never used more than two fingers for zooming on a phone,
so this doesn't occur to me.

Right now, using more than two fingers wouldn't break this feature too much
because handleTouchMove() doesn't check number of points, but definitely not as
good as it can be.

I'll give it a shot.

+
+    pinchDistAnchor = touchDist(e);
+    pinching = pinchDistAnchor > 0;
+    fontSizeAnchor =
+        parseFloat(getComputedStyle(document.documentElement).fontSize)
+        / baseSize;
+
+    pinchOrigin = touchPageMid(e);
+    var clientMid = touchClientMid(e);
+    var pinchMidY = clientMid.y;
+    // Try to preserve the pinching center after text reflow.
+    // This is accurate to the HTML element level.
+    focusElement = document.elementFromPoint(clientMid.x, clientMid.y);
+    var rect = focusElement.getBoundingClientRect();
+    focusPos = (pinchMidY - rect.top) / (rect.bottom - rect.top);
+
+    lastClientMidY = clientMid.y;
+  };
+
+  var handleTouchMove = function(e) {
+    if (!pinching) return;
+    e.preventDefault();
+
+    var pinchScale = touchDist(e) / pinchDistAnchor;
+    var fontScale = 1 + ((pinchScale - 1) * FONT_SCALE_MULTIPLIER);

[jdduke (slow), 2015/03/16 20:27:52]

We might want to be more careful about the min font scale here, I was pretty
casual in the limits I enforced.

[wychen, 2015/03/17 17:36:11]

0.4 might be in the unreadable range, and 0.4~2.5 seems to be broader than
necessary, but later on, the font size would be the product of "Android
accessibility scaling", "Chrome accessibility scaling", and the font scaling
factor defined here. Therefore, broader range leaves users more flexible
choices.

+    scaleFactor = Math.max(0.4, Math.min(2.5, fontSizeAnchor * fontScale));
+    var mid = touchPageMid(e);
+
+    document.body.style.transformOrigin =
+        pinchOrigin.x + 'px ' + pinchOrigin.y  + 'px';
+    // Use "fake" 3D transform so that the layer is not repainted.
+    // With 2D transform, the frame rate would be much lower.

[jdduke (slow), 2015/03/16 20:27:53]

I can't speak to this logic.

[wychen, 2015/03/17 17:36:12]

The frame rate was enhanced from <30fps to 60fps through. :p
Using any kind of null 3D transform is just to trigger the layer composition
mechanism without actually changing how it looks.

+    document.body.style.transform =
+        'translate3d(' + (mid.x - pinchOrigin.x) + 'px,' +
+                         (mid.y - pinchOrigin.y) + 'px, 0px)' +
+        'scale(' + scaleFactor/fontSizeAnchor + ')';
+
+    lastClientMidY = touchClientMid(e).y;
+  };
+
+  var handleTouchEnd = function(e) {
+    if (!pinching) return;

[jdduke (slow), 2015/03/16 20:27:53]

We'll need to be careful here as well, as we probably want to continue pinching
if the user lifts their 4th or 3rd finger (for which we'll see a touchend).

[wychen, 2015/03/17 17:36:11]

Indeed.

+    e.preventDefault();
+    pinching = false;
+
+    document.body.style.transformOrigin = '';
+    document.body.style.transform = '';
+    document.documentElement.style.fontSize = scaleFactor * baseSize + "px";

[jdduke (slow), 2015/03/16 20:27:52]

I'm an HTML newb so we'll want somebody else to review this.

[wychen, 2015/03/17 17:36:11]

Why would this line in particular concern you?

The font size needs to be adjusted at the html element rather than the body
element, and measuring the original size in px helps decouple CSS and JS.

[jdduke (slow), 2015/03/17 17:49:51]

Oh it's not that particular line that concerns me, I'm just saying my  HTML
knowledge is sadly limited so it would be good to have a Blink engineer review
this just to make sure there's nothing we're missing and/or a different approach
we could take.

+
+    var rect = focusElement.getBoundingClientRect();
+    var targetTop = focusPos * (rect.bottom - rect.top) + rect.top +
+                    document.body.scrollTop - lastClientMidY;
+    document.body.scrollTop = targetTop;
+  };
+
+  var handleTouchCancel = function(e) {
+    pinching = false;
+  };
+
+  function touchDist(e) {
+    var dx = (e.touches[0].screenX-e.touches[1].screenX);
+    var dy = (e.touches[0].screenY-e.touches[1].screenY);
+    return Math.sqrt(dx * dx + dy * dy);
+  }
+
+  function touchClientMid(e) {
+    var mx = (e.touches[0].clientX+e.touches[1].clientX)/2;
+    var my = (e.touches[0].clientY+e.touches[1].clientY)/2;
+    return {x: mx, y: my};
+  }
+
+  function touchPageMid(e) {
+    var mx = (e.touches[0].pageX+e.touches[1].pageX)/2;
+    var my = (e.touches[0].pageY+e.touches[1].pageY)/2;
+    return {x: mx, y: my};
+  }
+
+  window.addEventListener('touchstart', handleTouchStart, false);
+  window.addEventListener('touchmove', handleTouchMove, false);
+  window.addEventListener('touchend', handleTouchEnd, false);
+  window.addEventListener('touchcancel', handleTouchCancel, false);
+});