renderer: fix color glyph rendering under OpenGL

Also changes color atlas to always use an sRGB internal format so that
the texture reads automatically linearize the colors.

Renames the misleading `rgba` atlas format to `bgra`, since both
FreeType and CoreText are set up to draw color glyphs in bgra.

src/font/Atlas.zig

@@ -50,15 +50,18 @@ modified: std.atomic.Value(usize) = .{ .raw = 0 },
 resized: std.atomic.Value(usize) = .{ .raw = 0 },
 
 pub const Format = enum(u8) {
+    /// 1 byte per pixel grayscale.
     grayscale = 0,
-    rgb = 1,
-    rgba = 2,
+    /// 3 bytes per pixel BGR.
+    bgr = 1,
+    /// 4 bytes per pixel BGRA.
+    bgra = 2,
 
     pub fn depth(self: Format) u8 {
         return switch (self) {
             .grayscale => 1,
-            .rgb => 3,
-            .rgba => 4,
+            .bgr => 3,
+            .bgra => 4,
         };
     }
 };

src/font/SharedGrid.zig

@@ -79,7 +79,7 @@ pub fn init(
 
     var atlas_grayscale = try Atlas.init(alloc, 512, .grayscale);
     errdefer atlas_grayscale.deinit(alloc);
-    var atlas_color = try Atlas.init(alloc, 512, .rgba);
+    var atlas_color = try Atlas.init(alloc, 512, .bgra);
     errdefer atlas_color.deinit(alloc);
 
     var result: SharedGrid = .{

src/font/face/freetype.zig

@@ -391,7 +391,7 @@ pub const Face = struct {
         const format: ?font.Atlas.Format = switch (bitmap_ft.pixel_mode) {
             freetype.c.FT_PIXEL_MODE_MONO => null,
             freetype.c.FT_PIXEL_MODE_GRAY => .grayscale,
-            freetype.c.FT_PIXEL_MODE_BGRA => .rgba,
+            freetype.c.FT_PIXEL_MODE_BGRA => .bgra,
             else => {
                 log.warn("glyph={} pixel mode={}", .{ glyph_index, bitmap_ft.pixel_mode });
                 @panic("unsupported pixel mode");

src/renderer/Metal.zig

@@ -347,7 +347,7 @@ pub fn initAtlasTexture(
 ) Texture.Error!Texture {
     const pixel_format: mtl.MTLPixelFormat = switch (atlas.format) {
         .grayscale => .r8unorm,
-        .rgba => .bgra8unorm,
+        .bgra => .bgra8unorm_srgb,
         else => @panic("unsupported atlas format for Metal texture"),
     };
 

src/renderer/OpenGL.zig

@@ -440,7 +440,7 @@ pub fn initAtlasTexture(
     const format: gl.Texture.Format, const internal_format: gl.Texture.InternalFormat =
         switch (atlas.format) {
             .grayscale => .{ .red, .red },
-            .rgba => .{ .rgba, .srgba },
+            .bgra => .{ .bgra, .srgba },
             else => @panic("unsupported atlas format for OpenGL texture"),
         };
 

src/renderer/generic.zig

@@ -336,7 +336,7 @@ pub fn Renderer(comptime GraphicsAPI: type) type {
                 const color = try api.initAtlasTexture(&.{
                     .data = undefined,
                     .size = 1,
-                    .format = .rgba,
+                    .format = .bgra,
                 });
                 errdefer color.deinit();
 

src/renderer/shaders/glsl/cell_text.f.glsl

@@ -87,19 +87,19 @@ void main() {
         case MODE_TEXT_COLOR:
         {
             // For now, we assume that color glyphs
-            // are already premultiplied sRGB colors.
+            // are already premultiplied linear colors.
             vec4 color = texture(atlas_color, in_data.tex_coord);
 
-            // If we aren't doing linear blending, we can return this right away.
-            if (!use_linear_blending) {
+            // If we are doing linear blending, we can return this right away.
+            if (use_linear_blending) {
                 out_FragColor = color;
                 return;
             }
 
-            // Otherwise we need to linearize the color. Since the alpha is
-            // premultiplied, we need to divide it out before linearizing.
+            // Otherwise we need to unlinearize the color. Since the alpha is
+            // premultiplied, we need to divide it out before unlinearizing.
             color.rgb /= vec3(color.a);
-            color = linearize(color);
+            color = unlinearize(color);
             color.rgb *= vec3(color.a);
 
             out_FragColor = color;

src/renderer/shaders/shaders.metal

@@ -553,19 +553,19 @@ fragment float4 cell_text_fragment(
     }
 
     case MODE_TEXT_COLOR: {
-      // For now, we assume that color glyphs are
-      // already premultiplied Display P3 colors.
+      // For now, we assume that color glyphs
+      // are already premultiplied linear colors.
       float4 color = textureColor.sample(textureSampler, in.tex_coord);
 
-      // If we aren't doing linear blending, we can return this right away.
-      if (!uniforms.use_linear_blending) {
+      // If we're doing linear blending, we can return this right away.
+      if (uniforms.use_linear_blending) {
         return color;
       }
 
-      // Otherwise we need to linearize the color. Since the alpha is
-      // premultiplied, we need to divide it out before linearizing.
+      // Otherwise we need to unlinearize the color. Since the alpha is
+      // premultiplied, we need to divide it out before unlinearizing.
       color.rgb /= color.a;
-      color = linearize(color);
+      color = unlinearize(color);
       color.rgb *= color.a;
 
       return color;