From a300b195257f085161c0e9673982151dd197c9e0 Mon Sep 17 00:00:00 2001
From: Calixte Denizet <calixte.denizet@gmail.com>
Date: Thu, 28 Nov 2024 18:18:10 +0100
Subject: [PATCH] Simplify the code to zoom in using a pinch gesture

---
 web/app.js | 90 +++++++++++++++++++++++-------------------------------
 1 file changed, 38 insertions(+), 52 deletions(-)

diff --git a/web/app.js b/web/app.js
index 82fa056801e7c..245002fb6ec66 100644
--- a/web/app.js
+++ b/web/app.js
@@ -92,6 +92,14 @@ import { ViewHistory } from "./view_history.js";
 
 const FORCE_PAGES_LOADED_TIMEOUT = 10000; // ms
 
+// The 35 is coming from:
+//  https://searchfox.org/mozilla-central/source/gfx/layers/apz/src/GestureEventListener.cpp#36
+//
+// The properties TouchEvent::screenX/Y are in screen CSS pixels:
+//  https://developer.mozilla.org/en-US/docs/Web/API/Touch/screenX#examples
+// MIN_TOUCH_DISTANCE_TO_PINCH is in CSS pixels.
+const MIN_TOUCH_DISTANCE_TO_PINCH = 35 / (window.devicePixelRatio || 1);
+
 const ViewOnLoad = {
   UNKNOWN: -1,
   PREVIOUS: 0, // Default value.
@@ -179,6 +187,7 @@ const PDFViewerApplication = {
   _title: document.title,
   _printAnnotationStoragePromise: null,
   _touchInfo: null,
+  _isPinching: false,
   _isCtrlKeyDown: false,
   _caretBrowsing: null,
   _isScrolling: false,
@@ -2639,10 +2648,10 @@ function onTouchStart(evt) {
     [touch0, touch1] = [touch1, touch0];
   }
   this._touchInfo = {
-    touch0X: touch0.pageX,
-    touch0Y: touch0.pageY,
-    touch1X: touch1.pageX,
-    touch1Y: touch1.pageY,
+    touch0X: touch0.screenX,
+    touch0Y: touch0.screenY,
+    touch1X: touch1.screenX,
+    touch1Y: touch1.screenY,
   };
 }
 
@@ -2656,8 +2665,8 @@ function onTouchMove(evt) {
   if (touch0.identifier > touch1.identifier) {
     [touch0, touch1] = [touch1, touch0];
   }
-  const { pageX: page0X, pageY: page0Y } = touch0;
-  const { pageX: page1X, pageY: page1Y } = touch1;
+  const { screenX: screen0X, screenY: screen0Y } = touch0;
+  const { screenX: screen1X, screenY: screen1Y } = touch1;
   const {
     touch0X: pTouch0X,
     touch0Y: pTouch0Y,
@@ -2665,60 +2674,36 @@ function onTouchMove(evt) {
     touch1Y: pTouch1Y,
   } = _touchInfo;
 
+  const prevGapX = pTouch1X - pTouch0X;
+  const prevGapY = pTouch1Y - pTouch0Y;
+  const currGapX = screen1X - screen0X;
+  const currGapY = screen1Y - screen0Y;
+
+  const distance = Math.hypot(currGapX, currGapY) || 1;
+  const pDistance = Math.hypot(prevGapX, prevGapY) || 1;
   if (
-    Math.abs(pTouch0X - page0X) <= 1 &&
-    Math.abs(pTouch0Y - page0Y) <= 1 &&
-    Math.abs(pTouch1X - page1X) <= 1 &&
-    Math.abs(pTouch1Y - page1Y) <= 1
+    !this._isPinching &&
+    Math.abs(pDistance - distance) <= MIN_TOUCH_DISTANCE_TO_PINCH
   ) {
-    // Touches are really too close and it's hard do some basic
-    // geometry in order to guess something.
     return;
   }
 
-  _touchInfo.touch0X = page0X;
-  _touchInfo.touch0Y = page0Y;
-  _touchInfo.touch1X = page1X;
-  _touchInfo.touch1Y = page1Y;
-
-  if (pTouch0X === page0X && pTouch0Y === page0Y) {
-    // First touch is fixed, if the vectors are collinear then we've a pinch.
-    const v1X = pTouch1X - page0X;
-    const v1Y = pTouch1Y - page0Y;
-    const v2X = page1X - page0X;
-    const v2Y = page1Y - page0Y;
-    const det = v1X * v2Y - v1Y * v2X;
-    // 0.02 is approximatively sin(0.15deg).
-    if (Math.abs(det) > 0.02 * Math.hypot(v1X, v1Y) * Math.hypot(v2X, v2Y)) {
-      return;
-    }
-  } else if (pTouch1X === page1X && pTouch1Y === page1Y) {
-    // Second touch is fixed, if the vectors are collinear then we've a pinch.
-    const v1X = pTouch0X - page1X;
-    const v1Y = pTouch0Y - page1Y;
-    const v2X = page0X - page1X;
-    const v2Y = page0Y - page1Y;
-    const det = v1X * v2Y - v1Y * v2X;
-    if (Math.abs(det) > 0.02 * Math.hypot(v1X, v1Y) * Math.hypot(v2X, v2Y)) {
-      return;
-    }
-  } else {
-    const diff0X = page0X - pTouch0X;
-    const diff1X = page1X - pTouch1X;
-    const diff0Y = page0Y - pTouch0Y;
-    const diff1Y = page1Y - pTouch1Y;
-    const dotProduct = diff0X * diff1X + diff0Y * diff1Y;
-    if (dotProduct >= 0) {
-      // The two touches go in almost the same direction.
-      return;
-    }
-  }
+  _touchInfo.touch0X = screen0X;
+  _touchInfo.touch0Y = screen0Y;
+  _touchInfo.touch1X = screen1X;
+  _touchInfo.touch1Y = screen1Y;
 
   evt.preventDefault();
 
-  const origin = [(page0X + page1X) / 2, (page0Y + page1Y) / 2];
-  const distance = Math.hypot(page0X - page1X, page0Y - page1Y) || 1;
-  const pDistance = Math.hypot(pTouch0X - pTouch1X, pTouch0Y - pTouch1Y) || 1;
+  if (!this._isPinching) {
+    // Start pinching.
+    this._isPinching = true;
+
+    // We return here else the first pinch is a bit too much
+    return;
+  }
+
+  const origin = [(screen0X + screen1X) / 2, (screen0Y + screen1Y) / 2];
   if (supportsPinchToZoom) {
     const newScaleFactor = this._accumulateFactor(
       pdfViewer.currentScale,
@@ -2745,6 +2730,7 @@ function onTouchEnd(evt) {
   this._touchInfo = null;
   this._touchUnusedTicks = 0;
   this._touchUnusedFactor = 1;
+  this._isPinching = false;
 }
 
 function onClick(evt) {