initial underline support, can be improved

TODO.md

@@ -1,5 +1,7 @@
 Bugs:
 
+* Underline should use freetype underline thickness hint
+
 Performance:
 
 * libuv allocates on every read, we should use a read buffer pool
@@ -19,8 +21,6 @@ Major Features:
 * Line wrap
 * Selection, highlighting
 * Copy (paste is done)
-* Bold
-* Underline
 * Strikethrough
 * Emoji
 * Ligatures

shaders/cell.f.glsl

@@ -28,6 +28,7 @@ const uint MODE_FG = 2u;
 const uint MODE_CURSOR_RECT = 3u;
 const uint MODE_CURSOR_RECT_HOLLOW = 4u;
 const uint MODE_CURSOR_BAR = 5u;
+const uint MODE_UNDERLINE = 6u;
 
 void main() {
     switch (mode) {
@@ -83,5 +84,9 @@ void main() {
     case MODE_CURSOR_BAR:
         out_FragColor = color;
         break;
+
+    case MODE_UNDERLINE:
+        out_FragColor = color;
+        break;
     }
 }

shaders/cell.v.glsl

@@ -9,6 +9,7 @@ const uint MODE_FG = 2u;
 const uint MODE_CURSOR_RECT = 3u;
 const uint MODE_CURSOR_RECT_HOLLOW = 4u;
 const uint MODE_CURSOR_BAR = 5u;
+const uint MODE_UNDERLINE = 6u;
 
 // The grid coordinates (x, y) where x < columns and y < rows
 layout (location = 0) in vec2 grid_coord;
@@ -158,5 +159,23 @@ void main() {
         gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
         color = bg_color_in / 255.0;
         break;
+
+    case MODE_UNDERLINE:
+        // Make the underline a smaller version of our cell
+        // TODO: use real font underline thickness
+        vec2 underline_size = vec2(cell_size.x, cell_size.y*0.05);
+
+        // Position our underline so that it is midway between the glyph
+        // baseline and the bottom of the cell.
+        vec2 underline_offset = vec2(cell_size.x, cell_size.y - (glyph_baseline / 2));
+
+        // Go to the bottom of the cell, take away the size of the
+        // underline, and that is our position. We also float it slightly
+        // above the bottom.
+        cell_pos = cell_pos + underline_offset - underline_size * position;
+
+        gl_Position = projection * vec4(cell_pos, 0.0, 1.0);
+        color = fg_color_in / 255.0;
+        break;
     }
 }

src/Grid.zig

@@ -271,9 +271,9 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
     try self.cells.ensureTotalCapacity(
         self.alloc,
 
-        // * 2 for background modes and cursor
+        // * 3 for background modes and cursor and underlines
         // + 1 for cursor
-        (term.screen.rows * term.screen.cols * 2) + 1,
+        (term.screen.rows * term.screen.cols * 3) + 1,
     );
 
     // Build each cell
@@ -308,9 +308,10 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
                 });
             }
 
-            // If the cell is empty then we draw nothing in the box.
+            const fg = cell.fg orelse self.foreground;
-            if (cell.empty()) continue;
 
+            // If the cell is empty then we draw nothing in the box.
+            if (!cell.empty()) {
                 // Determine our glyph styling
                 const style: font.Style = if (cell.attrs.bold == 1)
                     .bold
@@ -326,7 +327,6 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
                     break :glyph try self.font_atlas.addGlyph(self.alloc, cell.char, style);
                 };
 
-            const fg = cell.fg orelse self.foreground;
                 self.cells.appendAssumeCapacity(.{
                     .mode = 2,
                     .grid_col = @intCast(u16, x),
@@ -347,6 +347,29 @@ pub fn updateCells(self: *Grid, term: Terminal) !void {
                     .bg_a = 0,
                 });
             }
+
+            if (cell.attrs.underline == 1) {
+                self.cells.appendAssumeCapacity(.{
+                    .mode = 6, // underline
+                    .grid_col = @intCast(u16, x),
+                    .grid_row = @intCast(u16, y),
+                    .glyph_x = 0,
+                    .glyph_y = 0,
+                    .glyph_width = 0,
+                    .glyph_height = 0,
+                    .glyph_offset_x = 0,
+                    .glyph_offset_y = 0,
+                    .fg_r = fg.r,
+                    .fg_g = fg.g,
+                    .fg_b = fg.b,
+                    .fg_a = 255,
+                    .bg_r = 0,
+                    .bg_g = 0,
+                    .bg_b = 0,
+                    .bg_a = 0,
+                });
+            }
+        }
     }
 
     // Draw the cursor

src/terminal/Screen.zig

@@ -24,6 +24,7 @@ pub const Cell = struct {
     /// TODO: pack it
     attrs: struct {
         bold: u1 = 0,
+        underline: u1 = 0,
         inverse: u1 = 0,
     } = .{},
 

src/terminal/Terminal.zig

@@ -154,6 +154,10 @@ pub fn setAttribute(self: *Terminal, attr: sgr.Attribute) !void {
             self.cursor.pen.attrs.bold = 1;
         },
 
+        .underline => {
+            self.cursor.pen.attrs.underline = 1;
+        },
+
         .inverse => {
             self.cursor.pen.attrs.inverse = 1;
         },