Fixing superscript and subscript baseline for tiny fonts in SVG

third_party/WebKit/Source/core/layout/svg/LayoutSVGInlineText.cpp

 /*
  * Copyright (C) 2006 Oliver Hunt <ojh16@student.canterbury.ac.nz>
  * Copyright (C) 2006 Apple Computer Inc.
  * Copyright (C) 2007 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2008 Rob Buis <buis@kde.org>
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 380 matching lines @@
   FontDescription fontDescription(style->getFontDescription());
 
   Document& document = layoutObject->document();
   // FIXME: We need to better handle the case when we compute very small fonts
   // below (below 1pt).
   fontDescription.setComputedSize(FontSize::getComputedSizeFromSpecifiedSize(
       &document, scalingFactor, fontDescription.isAbsoluteSize(),
       fontDescription.specifiedSize(), DoNotUseSmartMinimumForFontSize));
 
   scaledFont = Font(fontDescription);
+  // If the font size is smaller than 3, rounding ascent and descent values
+  // ignores the baseline shifts for superscript and subscript. In this case,
+  // we use subpixel ascent and descent.
+  if (scaledFont.getFontDescription().getSize().value < 3)
+    scaledFont.getFontDescription().setSubpixelAscentDescent(true);

[eae, 2016/10/17 20:14:23]

It's not clear that updateScaledFont is guaranteed to run before m_scaledFont is
used, perhaps you could set a breakpoint there or add an assert to see if that
is the case.

There is also SVGTextLayoutEngine::layoutTextOnLineOrPath that access the font
directly which might need updating.

[zakerinasab, 2016/10/18 15:51:16]

Correct. The problem is that the rounding at the beginning of platformInit()
affects the calculation of other metrics as well. The current call to
setSubpixelAscentDescent() does not update the previous calculations, so you
have to do that before the font gets initiated, which brings you to the change
needed in LayoutText.cpp which is outside SVG code and affects others too (like
Canvas). We can either go with something universal, or to remain more cautious,
we can change setSubpixelAscentDescent() to update the previous calculations if
needed. WDYT?
No benefit there as the font is already created there, unless we update the
previous metrics in setSubpixelAscentDescent(). In this case IMO this will
complete the CL.

   scaledFont.update(document.styleEngine().fontSelector());
 }
 
 LayoutRect LayoutSVGInlineText::absoluteClippedOverflowRect() const {
   return parent()->absoluteClippedOverflowRect();
 }
 
 FloatRect LayoutSVGInlineText::paintInvalidationRectInLocalSVGCoordinates()
     const {
   return parent()->paintInvalidationRectInLocalSVGCoordinates();
 }
 
 PassRefPtr<StringImpl> LayoutSVGInlineText::originalText() const {
   RefPtr<StringImpl> result = LayoutText::originalText();
   if (!result)
     return nullptr;
   return normalizeWhitespace(result);
 }
 
 }  // namespace blink