Add glyph offset option to user configuration

Add the ability to move glyphs within their cells on a global basis via
an option in the configuration file.

alacritty.yml

@@ -54,12 +54,20 @@ font:
 
   # Point size of the font
   size: 11.0
+
   # Offset is the extra space around each character. offset.y can be thought of
   # as modifying the linespacing, and offset.x as modifying the letter spacing.
   offset:
     x: 0.0
     y: 0.0
 
+  # Glyph offset determines the locations of the glyphs within their cells with
+  # the default being at the bottom. Increase the x offset to move the glyph to
+  # the right, increase the y offset to move the glyph upward.
+  glyph_offset:
+    x: 0.0
+    y: 0.0
+
   # OS X only: use thin stroke font rendering. Thin strokes are suitable
   # for retina displays, but for non-retina you probably want this set to
   # false.

alacritty_macos.yml

@@ -53,12 +53,20 @@ font:
 
   # Point size of the font
   size: 12.0
+
   # Offset is the extra space around each character. offset.y can be thought of
   # as modifying the linespacing, and offset.x as modifying the letter spacing.
   offset:
     x: 0.0
     y: 0.0
 
+  # Glyph offset determines the locations of the glyphs within their cells with
+  # the default being at the bottom. Increase the x offset to move the glyph to
+  # the right, increase the y offset to move the glyph upward.
+  glyph_offset:
+    x: 0.0
+    y: 0.0
+
   # OS X only: use thin stroke font rendering. Thin strokes are suitable
   # for retina displays, but for non-retina you probably want this set to
   # false.

src/config.rs

@@ -1195,6 +1195,25 @@ impl Default for FontOffset {
     }
 }
 
+/// Modifications to glyph positions within their cells
+///
+/// By default the glyphs are located at the bottom of the cell which can be
+/// undesirable. This gives the user a way to shift where the glyphs are
+/// displayed in their cells.
+#[derive(Clone, Copy, Debug, Deserialize)]
+pub struct GlyphOffset {
+    /// Horizontal position
+    pub x: f32,
+    /// Vertical position
+    pub y: f32,
+}
+
+impl Default for GlyphOffset {
+    fn default() -> GlyphOffset {
+        GlyphOffset { x: 0.0, y: 0.0 }
+    }
+}
+
 trait DeserializeFromF32 : Sized {
     fn deserialize_from_f32<D>(D) -> ::std::result::Result<Self, D::Error>
         where D: serde::de::Deserializer;
@@ -1256,6 +1275,10 @@ pub struct Font {
     /// Extra spacing per character
     offset: FontOffset,
 
+    /// Glyph offset within character cell
+    #[serde(default)]
+    glyph_offset: GlyphOffset,
+
     #[serde(default="true_bool")]
     use_thin_strokes: bool
 }
@@ -1296,6 +1319,12 @@ impl Font {
     pub fn offset(&self) -> &FontOffset {
         &self.offset
     }
+
+    /// Get cell offsets for glyphs
+    #[inline]
+    pub fn glyph_offset(&self) -> &GlyphOffset {
+        &self.glyph_offset
+    }
 }
 
 #[cfg(target_os = "macos")]
@@ -1307,7 +1336,8 @@ impl Default for Font {
             italic: FontDescription::new_with_family("Menlo"),
             size: Size::new(11.0),
             use_thin_strokes: true,
-            offset: Default::default()
+            offset: Default::default(),
+            glyph_offset: Default::default()
         }
     }
 }
@@ -1321,7 +1351,8 @@ impl Default for Font {
             italic: FontDescription::new_with_family("monospace"),
             size: Size::new(11.0),
             use_thin_strokes: false,
-            offset: Default::default()
+            offset: Default::default(),
+            glyph_offset: Default::default()
         }
     }
 }

src/renderer/mod.rs

@@ -28,7 +28,7 @@ use gl;
 use index::{Line, Column, RangeInclusive};
 use notify::{Watcher as WatcherApi, RecommendedWatcher as Watcher, op};
 
-use config::Config;
+use config::{Config, GlyphOffset};
 use term::{self, cell, RenderableCell};
 use window::{Size, Pixels};
 
@@ -154,6 +154,9 @@ pub struct GlyphCache {
 
     /// font size
     font_size: font::Size,
+
+    /// glyph offset
+    glyph_offset: GlyphOffset,
 }
 
 impl GlyphCache {
@@ -166,6 +169,7 @@ impl GlyphCache {
     {
         let font = config.font();
         let size = font.size();
+        let glyph_offset = *font.glyph_offset();
 
         // Load regular font
         let regular_desc = if let Some(ref style) = font.normal.style {
@@ -223,6 +227,7 @@ impl GlyphCache {
             font_key: regular,
             bold_key: bold,
             italic_key: italic,
+            glyph_offset: glyph_offset,
         };
 
         macro_rules! load_glyphs_for_font {
@@ -253,9 +258,12 @@ impl GlyphCache {
     fn load_and_cache_glyph<L>(&mut self, glyph_key: GlyphKey, loader: &mut L)
         where L: LoadGlyph
     {
-        let rasterized = self.rasterizer.get_glyph(&glyph_key)
+        let mut rasterized = self.rasterizer.get_glyph(&glyph_key)
             .unwrap_or_else(|_| Default::default());
 
+        rasterized.left += self.glyph_offset.x as i32;
+        rasterized.top += self.glyph_offset.y as i32;
+
         let glyph = loader.load_glyph(&rasterized);
         self.cache.insert(glyph_key, glyph);
     }
@@ -263,12 +271,18 @@ impl GlyphCache {
     pub fn get<'a, L>(&'a mut self, glyph_key: &GlyphKey, loader: &mut L) -> &'a Glyph
         where L: LoadGlyph
     {
+        let glyph_offset = self.glyph_offset;
         let rasterizer = &mut self.rasterizer;
         self.cache
             .entry(*glyph_key)
             .or_insert_with(|| {
-                let rasterized = rasterizer.get_glyph(&glyph_key)
+                let mut rasterized = rasterizer.get_glyph(&glyph_key)
                     .unwrap_or_else(|_| Default::default());
+
+                // We need to apply the offset to glyphs that didn't get cached initially
+                rasterized.left += glyph_offset.x as i32;
+                rasterized.top += glyph_offset.y as i32;
+
                 loader.load_glyph(&rasterized)
             })
     }