layout: Implement `text-rendering` per SVG 1.1 § 11.7.4.

Like Gecko, we treat `geometricprecision` the same as
`optimizelegibility` for now.

5 files changed, 60 insertions, 10 deletions

diff --git a/components/gfx/font.rs b/components/gfx/font.rs
index 270e0447ed1..3c6379988bc 100644
--- a/components/gfx/font.rs
+++ b/components/gfx/font.rs
@@ -105,7 +105,9 @@ bitflags! {
         #[doc="Set if the text is entirely whitespace."]
         const IS_WHITESPACE_SHAPING_FLAG = 0x01,
         #[doc="Set if we are to ignore ligatures."]
-        const IGNORE_LIGATURES_SHAPING_FLAG = 0x02
+        const IGNORE_LIGATURES_SHAPING_FLAG = 0x02,
+        #[doc="Set if we are to disable kerning."]
+        const DISABLE_KERNING_SHAPING_FLAG = 0x04
     }
 }
 
diff --git a/components/gfx/text/shaping/harfbuzz.rs b/components/gfx/text/shaping/harfbuzz.rs
index 11c01a65948..813e4aa33f7 100644
--- a/components/gfx/text/shaping/harfbuzz.rs
+++ b/components/gfx/text/shaping/harfbuzz.rs
@@ -4,8 +4,8 @@
 
 extern crate harfbuzz;
 
-use font::{Font, FontHandleMethods, FontTableMethods, FontTableTag, IGNORE_LIGATURES_SHAPING_FLAG};
-use font::{ShapingOptions};
+use font::{DISABLE_KERNING_SHAPING_FLAG, Font, FontHandleMethods, FontTableMethods, FontTableTag};
+use font::{IGNORE_LIGATURES_SHAPING_FLAG, ShapingOptions};
 use platform::font::FontTable;
 use text::glyph::{CharIndex, GlyphStore, GlyphId, GlyphData};
 use text::shaping::ShaperMethods;
@@ -50,6 +50,8 @@ use std::ptr;
 static NO_GLYPH: i32 = -1;
 static CONTINUATION_BYTE: i32 = -2;
 
+static KERN: u32 = ((b'k' as u32) << 24) | ((b'e' as u32) << 16) | ((b'r' as u32) << 8) |
+    (b'n' as u32);
 static LIGA: u32 = ((b'l' as u32) << 24) | ((b'i' as u32) << 16) | ((b'g' as u32) << 8) |
     (b'a' as u32);
 
@@ -242,6 +244,14 @@ impl ShaperMethods for Shaper {
                     _end: hb_buffer_get_length(hb_buffer),
                 })
             }
+            if options.flags.contains(DISABLE_KERNING_SHAPING_FLAG) {
+                features.push(hb_feature_t {
+                    _tag: KERN,
+                    _value: 0,
+                    _start: 0,
+                    _end: hb_buffer_get_length(hb_buffer),
+                })
+            }
 
             hb_shape(self.hb_font, hb_buffer, features.as_mut_ptr(), features.len() as u32);
             self.save_glyph_results(text, options, glyphs, hb_buffer);
diff --git a/components/layout/text.rs b/components/layout/text.rs
index e92197b7257..ae4ad5ec0de 100644
--- a/components/layout/text.rs
+++ b/components/layout/text.rs
@@ -9,8 +9,8 @@
 use fragment::{Fragment, SpecificFragmentInfo, ScannedTextFragmentInfo};
 use inline::InlineFragments;
 
-use gfx::font::{FontMetrics, IGNORE_LIGATURES_SHAPING_FLAG, RunMetrics, ShapingFlags};
-use gfx::font::{ShapingOptions};
+use gfx::font::{DISABLE_KERNING_SHAPING_FLAG, FontMetrics, IGNORE_LIGATURES_SHAPING_FLAG};
+use gfx::font::{RunMetrics, ShapingFlags, ShapingOptions};
 use gfx::font_context::FontContext;
 use gfx::text::glyph::CharIndex;
 use gfx::text::text_run::TextRun;
@@ -23,7 +23,8 @@ use servo_util::smallvec::{SmallVec, SmallVec1};
 use std::collections::DList;
 use std::mem;
 use style::ComputedValues;
-use style::computed_values::{line_height, text_orientation, text_transform, white_space};
+use style::computed_values::{line_height, text_orientation, text_rendering, text_transform};
+use style::computed_values::{white_space};
 use style::style_structs::Font as FontStyle;
 use std::sync::Arc;
 
@@ -108,6 +109,7 @@ impl TextRunScanner {
             let text_transform;
             let letter_spacing;
             let word_spacing;
+            let text_rendering;
             {
                 let in_fragment = self.clump.front().unwrap();
                 let font_style = in_fragment.style().get_font_arc();
@@ -120,6 +122,7 @@ impl TextRunScanner {
                 text_transform = inherited_text_style.text_transform;
                 letter_spacing = inherited_text_style.letter_spacing;
                 word_spacing = inherited_text_style.word_spacing.unwrap_or(Au(0));
+                text_rendering = inherited_text_style.text_rendering;
             }
 
             // First, transform/compress text of all the nodes.
@@ -161,13 +164,19 @@ impl TextRunScanner {
             // as the default space, user agents should not use ligatures." This ensures that, for
             // example, `finally` with a wide `letter-spacing` renders as `f i n a l l y` and not
             // `fi n a l l y`.
+            let mut flags = ShapingFlags::empty();
+            match letter_spacing {
+                Some(Au(0)) | None => {}
+                Some(_) => flags.insert(IGNORE_LIGATURES_SHAPING_FLAG),
+            }
+            if text_rendering == text_rendering::T::optimizespeed {
+                flags.insert(IGNORE_LIGATURES_SHAPING_FLAG);
+                flags.insert(DISABLE_KERNING_SHAPING_FLAG)
+            }
             let options = ShapingOptions {
                 letter_spacing: letter_spacing,
                 word_spacing: word_spacing,
-                flags: match letter_spacing {
-                    Some(Au(0)) | None => ShapingFlags::empty(),
-                    Some(_) => IGNORE_LIGATURES_SHAPING_FLAG,
-                },
+                flags: flags,
             };
 
             Arc::new(box TextRun::new(&mut *fontgroup.fonts.get(0).borrow_mut(),
diff --git a/components/style/properties/mod.rs.mako b/components/style/properties/mod.rs.mako
index 3726152f026..fc4a49d1848 100644
--- a/components/style/properties/mod.rs.mako
+++ b/components/style/properties/mod.rs.mako
@@ -1483,6 +1483,8 @@ pub mod longhands {
     // TODO(pcwalton): `full-width`
     ${single_keyword("text-transform", "none capitalize uppercase lowercase")}
 
+    ${single_keyword("text-rendering", "auto optimizespeed optimizelegibility geometricprecision")}
+
     // CSS 2.1, Section 17 - Tables
     ${new_style_struct("Table", is_inherited=False)}
 
diff --git a/tests/html/text_rendering.html b/tests/html/text_rendering.html
new file mode 100644
index 00000000000..13e69236f4f
--- /dev/null
+++ b/tests/html/text_rendering.html
@@ -0,0 +1,27 @@
+<!DOCTYPE html>
+<html>
+<head>
+<style>
+body {
+    font-size: 24pt;
+    font-family: "PT Sans";
+    font-weight: bold;
+}
+#a {
+    text-rendering: optimizelegibility;
+}
+#b {
+    text-rendering: geometricprecision;
+}
+#c {
+    text-rendering: optimizespeed;
+}
+</style>
+</head>
+<body>
+<div id=a>Harry finally caught the Snitch.</div>
+<div id=b>Harry finally caught the Snitch.</div>
+<div id=c>Harry finally caught the Snitch.</div>
+</body>
+</html>
+