fonts: synthesize dim when a light weight font is unavailable

docs/changelog.md

@@ -25,7 +25,7 @@ As features stabilize some brief notes about them will accumulate here.
 * Fixed: errors loading custom color schemes are now logged to the error log [#794](https://github.com/wez/wezterm/issues/794)
 * Fixed: OSC 7 (current working directory) now works with paths that contain spaces and other special characters. Thanks to [@Arvedui](https://github.com/Arvedui)! [#799](https://github.com/wez/wezterm/pull/799)
 * Changed: the homebrew tap is now a Cask that installs to the /Applications directory on macOS. Thanks to [@laggardkernel](https://github.com/laggardkernel)!
-* New: bold and/or italics are now synthesized for fonts when the matching font is not actually italic or doesn't match the requested weight. [#815](https://github.com/wez/wezterm/issues/815)
+* New: bold/dim and/or italics are now synthesized for fonts when the matching font is not actually italic or doesn't match the requested weight. [#815](https://github.com/wez/wezterm/issues/815)
 * Updated: conpty.dll to v1.9.1445.0; fixes color bar artifacts when resizing window and allows win32 console applications to use mouse events
 * Fixed: Windows: pane could linger after the process has died, closing only when a new pane/tab event occurs
 * Fixed: Windows: first character after `wezterm ssh` keyboard authention was swallowed [#771](https://github.com/wez/wezterm/issues/771)

wezterm-font/src/lib.rs

@@ -123,6 +123,20 @@ impl LoadedFont {
             .metrics_for_idx(font_idx, self.font_size, self.dpi)
     }
 
+    pub fn brightness_adjust(&self, font_idx: usize) -> f32 {
+        let synthesize_dim = self
+            .handles
+            .borrow()
+            .get(font_idx)
+            .map(|p| p.synthesize_dim)
+            .unwrap_or(false);
+        if synthesize_dim {
+            0.5
+        } else {
+            1.0
+        }
+    }
+
     pub fn rasterize_glyph(
         &self,
         glyph_pos: u32,

wezterm-font/src/parser.rs

@@ -23,6 +23,7 @@ pub struct ParsedFont {
     coverage: Mutex<RangeSet<u32>>,
     pub synthesize_italic: bool,
     pub synthesize_bold: bool,
+    pub synthesize_dim: bool,
 }
 
 impl std::fmt::Debug for ParsedFont {
@@ -47,6 +48,7 @@ impl Clone for ParsedFont {
             italic: self.italic,
             synthesize_italic: self.synthesize_italic,
             synthesize_bold: self.synthesize_bold,
+            synthesize_dim: self.synthesize_dim,
             handle: self.handle.clone(),
             cap_height: self.cap_height.clone(),
             coverage: Mutex::new(self.coverage.lock().unwrap().clone()),
@@ -139,6 +141,8 @@ impl ParsedFont {
             }
             if p.synthesize_bold {
                 code.push_str("  -- Will synthesize bold\n");
+            } else if p.synthesize_dim {
+                code.push_str("  -- Will synthesize dim\n");
             }
 
             if p.weight == FontWeight::Regular && p.stretch == FontStretch::Normal && !p.italic {
@@ -176,6 +180,7 @@ impl ParsedFont {
             italic,
             synthesize_italic: false,
             synthesize_bold: false,
+            synthesize_dim: false,
             handle,
             coverage: Mutex::new(RangeSet::new()),
             cap_height,
@@ -384,6 +389,9 @@ impl ParsedFont {
         self.synthesize_bold = attr.weight > FontWeight::Regular
             && attr.weight > self.weight
             && self.weight <= FontWeight::Regular;
+        self.synthesize_dim = attr.weight < FontWeight::Regular
+            && attr.weight < self.weight
+            && self.weight >= FontWeight::Regular;
         self
     }
 }

wezterm-gui/src/glyphcache.rs

@@ -99,6 +99,7 @@ impl<'a> std::hash::Hash for (dyn GlyphKeyTrait + 'a) {
 /// The image data may be None for whitespace glyphs.
 pub struct CachedGlyph<T: Texture2d> {
     pub has_color: bool,
+    pub brightness_adjust: f32,
     pub x_offset: PixelLength,
     pub y_offset: PixelLength,
     pub bearing_x: PixelLength,
@@ -431,6 +432,7 @@ impl<T: Texture2d> GlyphCache<T> {
                     style
                 );
                 Rc::new(CachedGlyph {
+                    brightness_adjust: 1.0,
                     has_color: false,
                     texture: None,
                     x_offset: PixelLength::zero(),
@@ -455,6 +457,7 @@ impl<T: Texture2d> GlyphCache<T> {
     ) -> anyhow::Result<Rc<CachedGlyph<T>>> {
         let base_metrics;
         let idx_metrics;
+        let brightness_adjust;
         let glyph;
 
         {
@@ -463,6 +466,7 @@ impl<T: Texture2d> GlyphCache<T> {
             glyph = font.rasterize_glyph(info.glyph_pos, info.font_idx)?;
 
             idx_metrics = font.metrics_for_idx(info.font_idx)?;
+            brightness_adjust = font.brightness_adjust(info.font_idx);
         }
 
         let aspect = (idx_metrics.cell_width / idx_metrics.cell_height).get();
@@ -558,6 +562,7 @@ impl<T: Texture2d> GlyphCache<T> {
         let glyph = if glyph.width == 0 || glyph.height == 0 {
             // a whitespace glyph
             CachedGlyph {
+                brightness_adjust: 1.0,
                 has_color: glyph.has_color,
                 texture: None,
                 x_offset: info.x_offset * scale,
@@ -598,6 +603,7 @@ impl<T: Texture2d> GlyphCache<T> {
             let tex = self.atlas.allocate(&raw_im)?;
 
             let g = CachedGlyph {
+                brightness_adjust,
                 has_color: glyph.has_color,
                 texture: Some(tex),
                 x_offset,

wezterm-gui/src/termwindow/render.rs

@@ -13,8 +13,7 @@ use ::window::glium::uniforms::{
 use ::window::glium::{uniform, BlendingFunction, LinearBlendingFactor, Surface};
 use ::window::WindowOps;
 use anyhow::anyhow;
-use config::ConfigHandle;
-use config::TextStyle;
+use config::{ConfigHandle, HsbTransform, TextStyle};
 use mux::pane::Pane;
 use mux::renderable::{RenderableDimensions, StableCursorPosition};
 use mux::tab::{PositionedPane, PositionedSplit, SplitDirection};
@@ -1002,7 +1001,15 @@ impl super::TermWindow {
                     quad.set_texture_adjust(left, top, right, bottom);
                     quad.set_underline(underline_tex_rect);
                     quad.set_underline_color(underline_color);
-                    quad.set_hsv(hsv);
+                    quad.set_hsv(if glyph.brightness_adjust != 1.0 {
+                        let hsv = hsv.unwrap_or_else(|| HsbTransform::default());
+                        Some(HsbTransform {
+                            brightness: hsv.brightness * glyph.brightness_adjust,
+                            ..hsv
+                        })
+                    } else {
+                        hsv
+                    });
                     quad.set_has_color(glyph.has_color);
                     quad.set_cursor(
                         gl_state