Changeset 68148 in webkit

Timestamp:

Sep 23, 2010, 9:18:01 AM (14 years ago)

Author:

mitz@apple.com

Message:

Address a remaining discrepancy in piecewise text measurement
​https://bugs.webkit.org/show_bug.cgi?id=45796

Patch by Brad Moore <​bradm@apple.com> on 2010-09-23
Reviewed by Dan Bernstein.

Don't include always-integral space widths in the floating point accumulator designed
to minimize precision loss. This brings whole-string measurement in line with piecewise
text measurement when dealing with fonts with fractional advances.

• platform/graphics/WidthIterator.cpp:

(WebCore::WidthIterator::advance): Change the associativity of width addition to minimize precision loss.

Location:

trunk/WebCore

Files:

• ChangeLog (modified) (1 diff)
• platform/graphics/WidthIterator.cpp (modified) (1 diff)

trunk/WebCore/ChangeLog

@@ +1 @@
+2010-09-23  Brad Moore  <bradm@apple.com>
+
+        Reviewed by Dan Bernstein.
+
+        Address a remaining discrepancy in piecewise text measurement
+        https://bugs.webkit.org/show_bug.cgi?id=45796
+
+        Don't include always-integral space widths in the floating point accumulator designed
+        to minimize precision loss.  This brings whole-string measurement in line with piecewise
+        text measurement when dealing with fonts with fractional advances.
+
+        * platform/graphics/WidthIterator.cpp:
+        (WebCore::WidthIterator::advance): Change the associativity of width addition to minimize precision loss.
+
 2010-09-23  Patrick Gansterer  <paroga@webkit.org>
 

trunk/WebCore/platform/graphics/WidthIterator.cpp

@@ -213 +213 @@
         // Force characters that are used to determine word boundaries for the rounding hack
         // to be integer width, so following words will start on an integer boundary.
-        if (m_run.applyWordRounding() && Font::isRoundingHackCharacter(c))
+        if (m_run.applyWordRounding() && Font::isRoundingHackCharacter(c)) {
             width = ceilf(width);
 
-        // Check to see if the next character is a "rounding hack character", if so, adjust
-        // width so that the total run width will be on an integer boundary.
-        if ((m_run.applyWordRounding() && currentCharacter < m_run.length() && Font::isRoundingHackCharacter(*cp))
-                || (m_run.applyRunRounding() && currentCharacter >= m_end)) {
-            float totalWidth = widthSinceLastRounding + width;
-            widthSinceLastRounding = ceilf(totalWidth);
-            width += widthSinceLastRounding - totalWidth;
-            m_runWidthSoFar += widthSinceLastRounding;
-            widthSinceLastRounding = 0;
-        } else
-            widthSinceLastRounding += width;
+            // Since widthSinceLastRounding can lose precision if we include measurements for
+            // preceding whitespace, we bypass it here.
+            m_runWidthSoFar += width;
+
+            // Since this is a rounding hack character, we should have reset this sum on the previous
+            // iteration.
+            ASSERT(!widthSinceLastRounding);
+        } else {
+            // Check to see if the next character is a "rounding hack character", if so, adjust
+            // width so that the total run width will be on an integer boundary.
+            if ((m_run.applyWordRounding() && currentCharacter < m_run.length() && Font::isRoundingHackCharacter(*cp))
+                    || (m_run.applyRunRounding() && currentCharacter >= m_end)) {
+                float totalWidth = widthSinceLastRounding + width;
+                widthSinceLastRounding = ceilf(totalWidth);
+                width += widthSinceLastRounding - totalWidth;
+                m_runWidthSoFar += widthSinceLastRounding;
+                widthSinceLastRounding = 0;
+            } else
+                widthSinceLastRounding += width;
+        }
 
         if (glyphBuffer)