Paint dotted borders using circular dots

third_party/WebKit/Source/platform/graphics/StrokeData.cpp

 // Copyright (C) 2013 Google Inc. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //    * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //    * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
@@ skipping 52 matching lines @@
   flags->setStrokeCap(m_lineCap);
   flags->setStrokeJoin(m_lineJoin);
   flags->setStrokeMiter(SkFloatToScalar(m_miterLimit));
 
   setupPaintDashPathEffect(flags, length);
 }
 
 void StrokeData::setupPaintDashPathEffect(PaintFlags* flags, int length) const {
   if (m_dash) {
     flags->setPathEffect(m_dash);
-  } else if (m_style == DashedStroke || m_style == DottedStroke) {
+  } else if (strokeIsDashed(m_thickness, m_style)) {
     float width =
         m_style == DashedStroke ? dashRatio * m_thickness : m_thickness;
 
     // Truncate the width, since we don't want fuzzy dots or dashes.
     int dashLength = static_cast<int>(width);
     // Subtract off the endcaps, since they're rendered separately.
     int distance = length - 2 * static_cast<int>(m_thickness);
     int phase = 1;
     if (dashLength > 1) {
       // Determine how many dashes or dots we should have.
       int numDashes = distance / dashLength;
       int remainder = distance % dashLength;
       // Adjust the phase to center the dashes within the line.
       if (numDashes % 2) {
         // Odd: shift right a full dash, minus half the remainder.
         phase = dashLength - remainder / 2;
       } else {
         // Even: shift right half a dash, minus half the remainder.
         phase = (dashLength - remainder) / 2;
       }
     }
     SkScalar dashLengthSk = SkIntToScalar(dashLength);
     SkScalar intervals[2] = {dashLengthSk, dashLengthSk};
     flags->setPathEffect(
         SkDashPathEffect::Make(intervals, 2, SkIntToScalar(phase)));
+  } else if (m_style == DottedStroke) {
+    flags->setStrokeCap((PaintFlags::Cap)RoundCap);
+    // Adjust the width to get equal dot spacing as much as possible.
+    float perDotLength = m_thickness * 2;
+    static float epsilon = 1.0e-2f;
+    if (length < perDotLength + m_thickness) {
+      // Exactly 2 dots with whatever space we can get
+      SkScalar intervals[2] = {0, length - m_thickness - epsilon};
+      flags->setPathEffect(SkDashPathEffect::Make(intervals, 2, 0));
+      return;
+    }
+
+    // Determine what number of dots gives the minimum deviation from
+    // idealGap between dots. Set the gap to that width.
+    float minNumDots = floorf((length + m_thickness) / perDotLength);
+    float maxNumDots = minNumDots + 1;
+    float minGap = (length - minNumDots * m_thickness) / (minNumDots - 1);
+    float maxGap = (length - maxNumDots * m_thickness) / (maxNumDots - 1);
+    if (fabs(minGap - m_thickness) < fabs(maxGap - m_thickness)) {
+      SkScalar intervals[2] = {0, minGap + m_thickness - epsilon};
+      flags->setPathEffect(SkDashPathEffect::Make(intervals, 2, 0));
+    } else {
+      SkScalar intervals[2] = {0, maxGap + m_thickness - epsilon};
+      flags->setPathEffect(SkDashPathEffect::Make(intervals, 2, 0));
+    }
   } else {
-    // TODO(schenney): WavyStroke:  https://crbug.com/229574
+    // TODO(schenney): WavyStroke https://crbug.com/229574
     flags->setPathEffect(0);
   }
 }
 
+bool StrokeData::strokeIsDashed(float width, StrokeStyle style) {
+  return style == DashedStroke || (style == DottedStroke && width <= 3);
+}
+
 }  // namespace blink