PDF a11y: Added new heuristic for text runs delimitation

The previously merged text runs heuristic change to support vertical
text runs (https://crrev.com/687072) introduced a bug described in the
linked bug.

This change fixes it. It adds a new heuristic that verifies if the
difference between the angle formed by the first and last character of
a run (the |text_run_angle|) and the angle formed by the current and
previous character (the |current_angle|) is greater than 90 degrees.
If so, the text run breaks.

The file directional-text-runs.pdf has been updated to detect any
future regression.

Bug: 994958
Change-Id: I21fbe389fe5527c6dba9f05a23fd865a5b3a221e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758833Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Ian Prest <iapres@microsoft.com>
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#690011}

chrome/test/data/pdf/accessibility/directional-text-runs-expected-auralinux.txt

@@ -2,4 +2,8 @@
 ++[document frame]
 ++++[landmark] name='Page 1'
 ++++++[paragraph]
-++++++++[text] name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!'
\ No newline at end of file
+++++++++[text] name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!<newline>'
+++++++[paragraph]
+++++++++[text] name='This is some text '
+++++++[paragraph]
+++++++++[text] name='abc<newline>def'
\ No newline at end of file

chrome/test/data/pdf/accessibility/directional-text-runs-expected-blink.txt

@@ -2,8 +2,15 @@ embeddedObject
 ++document restriction=readOnly
 ++++region name='Page 1' restriction=readOnly isPageBreakingObject=true
 ++++++paragraph restriction=readOnly
-++++++++staticText name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!' restriction=readOnly
+++++++++staticText name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!<newline>' restriction=readOnly
 ++++++++++inlineTextBox name='Hello,<newline> ' restriction=readOnly
 ++++++++++inlineTextBox name='world!<newline><newline>' restriction=readOnly
 ++++++++++inlineTextBox name='Goodbye,<newline> ' restriction=readOnly
-++++++++++inlineTextBox name='world!' restriction=readOnly
\ No newline at end of file
+++++++++++inlineTextBox name='world!<newline>' restriction=readOnly
+++++++paragraph restriction=readOnly
+++++++++staticText name='This is some text ' restriction=readOnly
+++++++++++inlineTextBox name='This is some text ' restriction=readOnly
+++++++paragraph restriction=readOnly
+++++++++staticText name='abc<newline>def' restriction=readOnly
+++++++++++inlineTextBox name='abc<newline>' restriction=readOnly
+++++++++++inlineTextBox name='def' restriction=readOnly
\ No newline at end of file

chrome/test/data/pdf/accessibility/directional-text-runs-expected-mac.txt

@@ -2,4 +2,8 @@ AXGroup AXDescription='Page 1'
 ++AXGroup
 ++++AXGroup AXDescription='Page 1'
 ++++++AXGroup
-++++++++AXStaticText AXValue='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!'
\ No newline at end of file
+++++++++AXStaticText AXValue='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!<newline>'
+++++++AXGroup
+++++++++AXStaticText AXValue='This is some text '
+++++++AXGroup
+++++++++AXStaticText AXValue='abc<newline>def'
\ No newline at end of file

chrome/test/data/pdf/accessibility/directional-text-runs-expected-uia-win.txt

@@ -2,4 +2,8 @@ group
 ++document
 ++++region Name='Page 1'
 ++++++group
-++++++++description Name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!'
\ No newline at end of file
+++++++++description Name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!<newline>'
+++++++group
+++++++++description Name='This is some text '
+++++++group
+++++++++description Name='abc<newline>def'
\ No newline at end of file

chrome/test/data/pdf/accessibility/directional-text-runs-expected-win.txt

@@ -2,4 +2,8 @@ ROLE_SYSTEM_GROUPING FOCUSABLE
 ++ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
 ++++IA2_ROLE_LANDMARK name='Page 1' READONLY
 ++++++IA2_ROLE_PARAGRAPH READONLY
-++++++++ROLE_SYSTEM_STATICTEXT name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!' READONLY
\ No newline at end of file
+++++++++ROLE_SYSTEM_STATICTEXT name='Hello,<newline> world!<newline><newline>Goodbye,<newline> world!<newline>' READONLY
+++++++IA2_ROLE_PARAGRAPH READONLY
+++++++++ROLE_SYSTEM_STATICTEXT name='This is some text ' READONLY
+++++++IA2_ROLE_PARAGRAPH READONLY
+++++++++ROLE_SYSTEM_STATICTEXT name='abc<newline>def' READONLY
\ No newline at end of file

chrome/test/data/pdf/accessibility/directional-text-runs.in

@@ -50,6 +50,16 @@ BT
 0 1 -1 0 72 95 Tm
 ( world!) Tj
 ET
+BT
+/F1 10 Tf
+10 30 Td
+(This is some text) Tj
+/F1 5 Tf
+69 -1 Td
+(abc) Tj
+0 5 Td
+(def) Tj
+ET
 endstream
 endobj
 {{xref}}

pdf/pdfium/pdfium_page.cc

@@ -33,6 +33,12 @@ namespace chrome_pdf {
 
 namespace {
 
+constexpr double k45DegreesInRadians = base::kPiDouble / 4;
+constexpr double k90DegreesInRadians = base::kPiDouble / 2;
+constexpr double k180DegreesInRadians = base::kPiDouble;
+constexpr double k270DegreesInRadians = 3 * base::kPiDouble / 2;
+constexpr double k360DegreesInRadians = 2 * base::kPiDouble;
+
 PDFiumPage::IsValidLinkFunction g_is_valid_link_func_for_testing = nullptr;
 
 // If the link cannot be converted to a pp::Var, then it is not possible to
@@ -101,13 +107,31 @@ int GetFirstNonUnicodeWhiteSpaceCharIndex(FPDF_TEXTPAGE text_page,
 }
 
 PP_PrivateDirection GetDirectionFromAngle(double angle) {
-  angle = fmod(angle + base::kPiDouble / 4.0, 2 * base::kPiDouble);
-  if (angle >= 0 && angle <= base::kPiDouble / 2.0)
+  // Rotating the angle by 45 degrees to simplify the conditions statements.
+  // It's like if we rotated the whole cartesian coordinate system like below.
+  //   X                   X
+  //     X      IV       X
+  //       X           X
+  //         X       X
+  //           X   X
+  //   III       X       I
+  //           X   X
+  //         X       X
+  //       X           X
+  //     X      II       X
+  //   X                   X
+
+  angle = fmod(angle + k45DegreesInRadians, k360DegreesInRadians);
+  // Quadrant I.
+  if (angle >= 0 && angle <= k90DegreesInRadians)
     return PP_PRIVATEDIRECTION_LTR;
-  if (angle > base::kPiDouble / 2.0 && angle <= base::kPiDouble / 2.0)
+  // Quadrant II.
+  if (angle > k90DegreesInRadians && angle <= k180DegreesInRadians)
     return PP_PRIVATEDIRECTION_TTB;
-  if (angle > base::kPiDouble && angle <= 3 * base::kPiDouble / 2.0)
+  // Quadrant III.
+  if (angle > k180DegreesInRadians && angle <= k270DegreesInRadians)
     return PP_PRIVATEDIRECTION_RTL;
+  // Quadrant IV.
   return PP_PRIVATEDIRECTION_BTT;
 }
 
@@ -135,6 +159,30 @@ double GetRotatedCharWidth(double angle, const pp::FloatSize& size) {
   return abs(cos(angle) * size.width()) + abs(sin(angle) * size.height());
 }
 
+double GetAngleOfVector(const pp::FloatPoint& v) {
+  double angle = atan2(v.y(), v.x());
+  if (angle < 0)
+    angle += k360DegreesInRadians;
+  return angle;
+}
+
+double GetAngleDifference(double a, double b) {
+  // This is either the difference or (360 - difference).
+  double x = fmod(fabs(b - a), k360DegreesInRadians);
+  return x > k180DegreesInRadians ? k360DegreesInRadians - x : x;
+}
+
+bool DoubleEquals(double d1, double d2) {
+  // The idea behind this is to use this fraction of the larger of the
+  // two numbers as the limit of the difference.  This breaks down near
+  // zero, so we reuse this as the minimum absolute size we will use
+  // for the base of the scale too.
+  static const float epsilon_scale = 0.00001f;
+  return fabs(d1 - d2) <
+         epsilon_scale *
+             std::fmax(std::fmax(std::fabs(d1), std::fabs(d2)), epsilon_scale);
+}
+
 }  // namespace
 
 PDFiumPage::LinkTarget::LinkTarget() : page(-1) {}
@@ -216,14 +264,14 @@ void PDFiumPage::GetTextRunInfo(int start_char_index,
 
   pp::FloatRect start_char_rect =
       GetFloatCharRectInPixels(page, text_page, char_index);
-  int text_run_font_size = FPDFText_GetFontSize(text_page, char_index);
+  double text_run_font_size = FPDFText_GetFontSize(text_page, char_index);
 
   // Heuristic: Initialize the average character size to one-third of the font
   // size to avoid having the first few characters misrepresent the average.
   // Without it, if a text run starts with a '.', its small bounding box could
   // lead to a break in the text run after only one space. Ex: ". Hello World"
   // would be split in two runs: "." and "Hello World".
-  int font_size_minimum = FPDFText_GetFontSize(text_page, char_index) / 3.0;
+  double font_size_minimum = FPDFText_GetFontSize(text_page, char_index) / 3.0;
   pp::FloatSize avg_char_size =
       pp::FloatSize(font_size_minimum, font_size_minimum);
   int non_whitespace_chars_count = 1;
@@ -241,6 +289,10 @@ void PDFiumPage::GetTextRunInfo(int start_char_index,
   float estimated_font_size =
       std::max(start_char_rect.width(), start_char_rect.height());
 
+  // The angle of the vector starting at the first character center-point and
+  // ending at the current last character center-point.
+  double text_run_angle = 0;
+
   // Continue adding characters until heuristics indicate we should end the text
   // run.
   while (char_index < chars_count) {
@@ -250,8 +302,8 @@ void PDFiumPage::GetTextRunInfo(int start_char_index,
 
     if (!base::IsUnicodeWhitespace(character)) {
       // Heuristic: End the text run if different font size is encountered.
-      int font_size = FPDFText_GetFontSize(text_page, char_index);
-      if (font_size != text_run_font_size)
+      double font_size = FPDFText_GetFontSize(text_page, char_index);
+      if (!DoubleEquals(font_size, text_run_font_size))
         break;
 
       // Heuristic: End text run if character isn't going in the same direction.
@@ -259,21 +311,33 @@ void PDFiumPage::GetTextRunInfo(int start_char_index,
           GetDirectionFromAngle(FPDFText_GetCharAngle(text_page, char_index)))
         break;
 
+      // Heuristic: End the text run if the difference between the text run
+      // angle and the angle between the center-points of the previous and
+      // current characters is greater than 90 degrees.
+      double current_angle = GetAngleOfVector(char_rect.CenterPoint() -
+                                              prev_char_rect.CenterPoint());
+      if (start_char_rect != prev_char_rect) {
+        text_run_angle = GetAngleOfVector(prev_char_rect.CenterPoint() -
+                                          start_char_rect.CenterPoint());
+
+        if (GetAngleDifference(text_run_angle, current_angle) >
+            k90DegreesInRadians) {
+          break;
+        }
+      }
+
       // Heuristic: End the text run if the center-point distance to the
       // previous character is less than 2.5x the average character size.
       AddCharSizeToAverageCharSize(char_rect.Floatsize(), &avg_char_size,
                                    &non_whitespace_chars_count);
 
-      pp::FloatPoint current_prev_diff =
-          char_rect.CenterPoint() - prev_char_rect.CenterPoint();
-      double angle = atan2(current_prev_diff.y(), current_prev_diff.x());
-      double avg_char_width = GetRotatedCharWidth(angle, avg_char_size);
+      double avg_char_width = GetRotatedCharWidth(current_angle, avg_char_size);
 
       double distance =
           GetDistanceBetweenPoints(char_rect.CenterPoint(),
                                    prev_char_rect.CenterPoint()) -
-          GetRotatedCharWidth(angle, char_rect.Floatsize()) / 2 -
-          GetRotatedCharWidth(angle, prev_char_rect.Floatsize()) / 2;
+          GetRotatedCharWidth(current_angle, char_rect.Floatsize()) / 2 -
+          GetRotatedCharWidth(current_angle, prev_char_rect.Floatsize()) / 2;
 
       if (distance > 2.5 * avg_char_width)
         break;