adding baseline options for super/sub scripting

ui/gfx/text_constants.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_TEXT_CONSTANTS_H_
 #define UI_GFX_TEXT_CONSTANTS_H_
 
 namespace gfx {
 
 // TODO(msw): Distinguish between logical character stops and glyph stops?
@@ skipping 14 matching lines @@
 
 // The directionality modes used to determine the base text direction.
 enum DirectionalityMode {
   DIRECTIONALITY_FROM_TEXT = 0, // Use the first strong character's direction.
   DIRECTIONALITY_FROM_UI,       // Use the UI locale's text reading direction.
   DIRECTIONALITY_FORCE_LTR,     // Use LTR regardless of content or UI locale.
   DIRECTIONALITY_FORCE_RTL,     // Use RTL regardless of content or UI locale.
 };
 
 // Text styles and adornments.
+// Figure of font metrics:

[msw, 2015/02/17 23:32:06]

nit: vertically center "height", "ascent", and "cap height".

[dschuyler, 2015/02/18 01:37:27]

Done.

+//   +--------+--------+------------------------+-------------+
+//   |        |        | internal leading       | SUPERSCRIPT |
+//   |        | ascent +------------+-----------|             |
+//   |        |        |            | SUPERIOR  |-------------+
+//   |        |        |            |-----------|
+//   | height |        | cap height | INFERIOR  |-------------+
+//   |        |--------+------------+-----------| SUBSCRIPT   |
+//   |        | descent                         |             |
+//   +--------+---------------------------------+-------------+
 // TODO(msw): Merge with gfx::Font::FontStyle.
 enum TextStyle {
   BOLD = 0,
   ITALIC,
   STRIKE,
   DIAGONAL_STRIKE,
   UNDERLINE,
+  SUPERSCRIPT,     // e.g. a mathematical exponent would be superscript

[msw, 2015/02/17 23:32:06]

I don't think you'll want to merge these directly with TextStyle. Instead add
something like a BreakList<ScriptBaseline> RenderText::baseline_types_; or
similar for a new enum of these script baseline types, including a NORMAL type.

Otherwise, using a bool value for each style would allow users to set
overlapping ranges with different script baseline styles. I imagine we don't
(yet) want to handle combined/nested script baseline types (eg. a range marked
true for both subscript and superscript, etc.).

[dschuyler, 2015/02/18 01:37:27]

I'm a little less confident for the number of places I ended up adding baseline
changes.  Something I still haven't worked out the implications of is this, "Use
an empty color BreakList to avoid breaking runs at color boundaries."  I guess
I'm wondering if I should do something similar with the baseline styles or not.

Done.

[msw, 2015/02/18 17:07:23]

What you have looks pretty good. That comment is saying that we shouldn't break
runs for text color ranges, so it doesn't use the color breaklist. Since we
*should* break runs for baseline changes (because it uses a different font
size), we'll use the actual baseline style breaklist there (as you currently
do).

+  SUPERIOR,        // e.g. 8th, the "th" would be superior script
+  INFERIOR,        // e.g. 1/2, the "2" would be inferior ("1" is superior)
+  SUBSCRIPT,       // e.g. H2O, the "2" would be subscript
   NUM_TEXT_STYLES,
 };
 
 // Elision behaviors of text that exceeds constrained dimensions.
 enum ElideBehavior {
   NO_ELIDE = 0, // Do not modify the text, it may overflow its available bounds.
   TRUNCATE,     // Do not elide or fade, just truncate at the end of the string.
   ELIDE_HEAD,   // Add an ellipsis at the start of the string.
   ELIDE_MIDDLE, // Add an ellipsis in the middle of the string.
   ELIDE_TAIL,   // Add an ellipsis at the end of the string.
   ELIDE_EMAIL,  // Add ellipses to username and domain substrings.
   FADE_TAIL,    // Fade the string's end opposite of its horizontal alignment.
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_TEXT_CONSTANTS_H_