renderer: exempt Powerline cells from minimum contrast requirements

With a minimum contrast set, the colored glyphs that Powerline uses
would sometimes be set to white or black while the surrounding background
colors remain unchanged, breaking up contiguous colors on segments of
the Powerline.

This no longer happens with this patch as Powerline glyphs are now
special-cased and exempt from the minimum contrast adjustment.

src/renderer/Metal.zig

@@ -2481,6 +2481,7 @@ fn updateCell(
             .normal => .fg,
             .color => .fg_color,
             .constrained => .fg_constrained,
+            .powerline => .fg_powerline,
         };
 
         try self.cells.add(self.alloc, .text, .{

src/renderer/OpenGL.zig

@@ -1253,11 +1253,7 @@ pub fn rebuildCells(
             const screen_cell = row.cells(.all)[screen.cursor.x];
             const x = screen.cursor.x - @intFromBool(screen_cell.wide == .spacer_tail);
             for (self.cells.items[start_i..]) |cell| {
-                if (cell.grid_col == x and
-                    (cell.mode == .fg or
-                    cell.mode == .fg_color or
-                    cell.mode == .fg_constrained))
-                {
+                if (cell.grid_col == x and cell.mode.isFg()) {
                     cursor_cell = cell;
                     break;
                 }
@@ -1382,7 +1378,7 @@ pub fn rebuildCells(
 
         _ = try self.addCursor(screen, cursor_style, cursor_color);
         if (cursor_cell) |*cell| {
-            if (cell.mode == .fg or cell.mode == .fg_constrained) {
+            if (cell.mode.isFg() and cell.mode != .fg_color) {
                 const cell_color = if (self.cursor_invert) blk: {
                     const sty = screen.cursor.page_pin.style(screen.cursor.page_cell);
                     break :blk sty.bg(screen.cursor.page_cell, color_palette) orelse self.background_color;
@@ -1708,6 +1704,7 @@ fn updateCell(
             .normal => .fg,
             .color => .fg_color,
             .constrained => .fg_constrained,
+            .powerline => .fg_powerline,
         };
 
         try self.cells.append(self.alloc, .{

src/renderer/cell.zig

@@ -14,6 +14,10 @@ pub const FgMode = enum {
     /// size. If a glyph is larger than the cell then it must be resized
     /// to fit.
     constrained,
+
+    /// Similar to normal, but the text consists of Powerline glyphs and is
+    /// optionally exempt from padding color extension and minimum contrast requirements.
+    powerline,
 };
 
 /// Returns the appropriate foreground mode for the given cell. This is
@@ -45,10 +49,9 @@ pub fn fgMode(
                 break :text .normal;
             }
 
-            // We exempt the Powerline range from this since they exhibit
-            // box-drawing behavior and should not be constrained.
+            // Special-case Powerline glyphs
             if (isPowerline(cp)) {
-                break :text .normal;
+                break :text .powerline;
             }
 
             // If we are at the end of the screen its definitely constrained

src/renderer/metal/shaders.zig

@@ -319,6 +319,7 @@ pub const CellText = extern struct {
         fg_constrained = 2,
         fg_color = 3,
         cursor = 4,
+        fg_powerline = 5,
     };
 };
 

src/renderer/opengl/CellProgram.zig

@@ -53,6 +53,7 @@ pub const CellMode = enum(u8) {
     fg = 2,
     fg_constrained = 3,
     fg_color = 7,
+    fg_powerline = 15,
 
     // Non-exhaustive because masks change it
     _,
@@ -61,6 +62,10 @@ pub const CellMode = enum(u8) {
     pub fn mask(self: CellMode, m: CellMode) CellMode {
         return @enumFromInt(@intFromEnum(self) | @intFromEnum(m));
     }
+
+    pub fn isFg(self: CellMode) bool {
+        return @intFromEnum(self) & @intFromEnum(@as(CellMode, .fg)) != 0;
+    }
 };
 
 pub fn init() !CellProgram {

src/renderer/shaders/cell.f.glsl

@@ -28,6 +28,7 @@ const uint MODE_BG = 1u;
 const uint MODE_FG = 2u;
 const uint MODE_FG_CONSTRAINED = 3u;
 const uint MODE_FG_COLOR = 7u;
+const uint MODE_FG_POWERLINE = 15u;
 
 void main() {
     float a;
@@ -39,6 +40,7 @@ void main() {
 
     case MODE_FG:
     case MODE_FG_CONSTRAINED:
+    case MODE_FG_POWERLINE:
         a = texture(text, glyph_tex_coords).r;
         vec3 premult = color.rgb * color.a;
         out_FragColor = vec4(premult.rgb*a, a);

src/renderer/shaders/cell.metal

@@ -162,6 +162,7 @@ enum CellTextMode : uint8_t {
   MODE_TEXT_CONSTRAINED = 2u,
   MODE_TEXT_COLOR = 3u,
   MODE_TEXT_CURSOR = 4u,
+  MODE_TEXT_POWERLINE = 4u,
 };
 
 struct CellTextVertexIn {
@@ -263,6 +264,10 @@ vertex CellTextVertexOut cell_text_vertex(unsigned int vid [[vertex_id]],
   // If we have a minimum contrast, we need to check if we need to
   // change the color of the text to ensure it has enough contrast
   // with the background.
+  // We only apply this adjustment to "normal" text with MODE_TEXT,
+  // since we want color glyphs to appear in their original color
+  // and Powerline glyphs to be unaffected (else parts of the line would
+  // have different colors as some parts are displayed via background colors).
   if (uniforms.min_contrast > 1.0f && input.mode == MODE_TEXT) {
     float4 bg_color = float4(input.bg_color) / 255.0f;
     out.color = contrasted_color(uniforms.min_contrast, out.color, bg_color);
@@ -288,6 +293,7 @@ fragment float4 cell_text_fragment(CellTextVertexOut in [[stage_in]],
   switch (in.mode) {
     case MODE_TEXT_CURSOR:
     case MODE_TEXT_CONSTRAINED:
+    case MODE_TEXT_POWERLINE:
     case MODE_TEXT: {
       // Normalize the texture coordinates to [0,1]
       float2 size =

src/renderer/shaders/cell.v.glsl

@@ -8,6 +8,7 @@ const uint MODE_BG = 1u;
 const uint MODE_FG = 2u;
 const uint MODE_FG_CONSTRAINED = 3u;
 const uint MODE_FG_COLOR = 7u;
+const uint MODE_FG_POWERLINE = 15u;
 
 // The grid coordinates (x, y) where x < columns and y < rows
 layout (location = 0) in vec2 grid_coord;
@@ -198,6 +199,7 @@ void main() {
     case MODE_FG:
     case MODE_FG_CONSTRAINED:
     case MODE_FG_COLOR:
+    case MODE_FG_POWERLINE:
         vec2 glyph_offset_calc = glyph_offset;
 
         // The glyph_offset.y is the y bearing, a y value that when added
@@ -227,6 +229,7 @@ void main() {
         ivec2 text_size;
         switch(mode) {
         case MODE_FG_CONSTRAINED:
+        case MODE_FG_POWERLINE:
         case MODE_FG:
             text_size = textureSize(text, 0);
             break;
@@ -242,6 +245,10 @@ void main() {
         // If we have a minimum contrast, we need to check if we need to
         // change the color of the text to ensure it has enough contrast
         // with the background.
+        // We only apply this adjustment to "normal" text with MODE_FG,
+        // since we want color glyphs to appear in their original color
+        // and Powerline glyphs to be unaffected (else parts of the line would
+        // have different colors as some parts are displayed via background colors).
         vec4 color_final = color_in / 255.0;
         if (min_contrast > 1.0 && mode == MODE_FG) {
             vec4 bg_color = bg_color_in / 255.0;