Improve dashed line drawing

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

Index: third_party/WebKit/Source/platform/graphics/StrokeData.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/StrokeData.cpp b/third_party/WebKit/Source/platform/graphics/StrokeData.cpp
index cbdb47b677f27bed5c95c4cb058e56f8c8842681..ac64a3d70ae985551a8921bfb5bb0be3f0b76dc3 100644
--- a/third_party/WebKit/Source/platform/graphics/StrokeData.cpp
+++ b/third_party/WebKit/Source/platform/graphics/StrokeData.cpp
@@ -34,8 +34,6 @@
 
 namespace blink {
 
-static const int dashRatio = 3;  // Ratio of the length of a dash to its width.
-
 void StrokeData::setLineDash(const DashArray& dashes, float dashOffset) {
   // FIXME: This is lifted directly off SkiaSupport, lines 49-74
   // so it is not guaranteed to work correctly.
@@ -71,56 +69,46 @@ void StrokeData::setupPaintDashPathEffect(PaintFlags* flags, int length) const {
   if (m_dash) {
     flags->setPathEffect(m_dash);
   } 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;
-      }
+    float dashLength = m_thickness;
+    float gapLength = dashLength;
+    if (m_style == DashedStroke) {
+      dashLength *= StrokeData::dashLengthRatio(m_thickness);
+      gapLength *= StrokeData::dashGapRatio(m_thickness);
+    }
+    // Account for modification to effective length in
+    // GraphicsContext::adjustLineToPixelBoundaries
+    length -= 2 * m_thickness;
+    if (length <= dashLength) {
+      // No space for dashes
+      flags->setPathEffect(0);
+    } else if (length <= 2 * dashLength + gapLength) {
+      // Exactly 2 dashes proportionally sized
+      float multiplier = length / (2 * dashLength + gapLength);
+      SkScalar intervals[2] = {dashLength * multiplier, gapLength * multiplier};
+      flags->setPathEffect(SkDashPathEffect::Make(intervals, 2, 0));
+    } else {
+      float gap = gapLength;
+      if (m_style == DashedStroke)
+        gap = selectBestDashGap(length, dashLength, gapLength);
+      SkScalar intervals[2] = {dashLength, gap};
+      flags->setPathEffect(SkDashPathEffect::Make(intervals, 2, 0));
     }
-    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};
+    if (length < perDotLength) {
+      // Not enoguh space for 2 dots. Just draw 1 by giving a gap that is
+      // bigger than the length.
+      SkScalar intervals[2] = {0, perDotLength};
       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));
-    }
+    static const float epsilon = 1.0e-2f;
+    float gap = selectBestDashGap(length, m_thickness, m_thickness);
+    SkScalar intervals[2] = {0, gap + m_thickness - epsilon};
+    flags->setPathEffect(SkDashPathEffect::Make(intervals, 2, 0));
   } else {
     // TODO(schenney): WavyStroke https://crbug.com/229574
     flags->setPathEffect(0);
@@ -131,4 +119,19 @@ bool StrokeData::strokeIsDashed(float width, StrokeStyle style) {
   return style == DashedStroke || (style == DottedStroke && width <= 3);
 }
 
+float StrokeData::selectBestDashGap(float strokeLength,
+                                    float dashLength,
+                                    float gapLength) {
+  // Determine what number of dashes gives the minimum deviation from
+  // gapLength between dashes. Set the gap to that width.
+  float minNumDashes =
+      floorf((strokeLength + gapLength) / (dashLength + gapLength));
+  float maxNumDashes = minNumDashes + 1;
+  float minGap =
+      (strokeLength - minNumDashes * dashLength) / (minNumDashes - 1);
+  float maxGap =
+      (strokeLength - maxNumDashes * dashLength) / (maxNumDashes - 1);
+  return fabs(minGap - gapLength) < fabs(maxGap - gapLength) ? minGap : maxGap;
+}
+
 }  // namespace blink