feat: SubpixelLayout on PlatformProvider (ADR-024, v0.18.0)

CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.18.0] - 2026-05-09
+
+### Added
+
+- **SubpixelLayout on PlatformProvider** (ADR-024) — `SubpixelLayout()` method returns display subpixel arrangement (`SubpixelNone`, `SubpixelRGB`, `SubpixelBGR`, `SubpixelVRGB`, `SubpixelVBGR`). Enables LCD/ClearType font rendering in gg. Follows Qt6 `QPlatformScreen::SubpixelAntialiasingType` pattern — subpixel is a display/OS property, not GPU. `NullPlatformProvider` returns `SubpixelNone` (grayscale AA). Researched Qt6, GTK4/Wayland, FreeType, DRM/KMS — all treat subpixel as platform property.
+
+### Fixed
+
+- **Lint:** extracted `stringNone` reuse for SubpixelLayout.String().
+
 ## [0.17.0] - 2026-05-06
 
 ### Added

ROADMAP.md

@@ -4,8 +4,10 @@
 
 `gpucontext` is the shared foundation for the [gogpu](https://github.com/gogpu) ecosystem, providing interfaces and utilities for GPU resource sharing without circular dependencies.
 
-## Current: v0.16.0
+## Current: v0.18.0
 
+- SubpixelLayout on PlatformProvider (ADR-024, LCD/ClearType auto-detection)
+- AdapterInfo on DeviceProvider (ADR-020, render mode auto-selection)
 - WindowChrome.SetFullscreen / IsFullscreen (ADR-018, runtime fullscreen toggle)
 - CursorMode (Locked/Confined/Normal) for mouse grab / pointer lock
 - PointerEvent.DeltaX/DeltaY for relative mouse movement

platform.go

@@ -50,6 +50,55 @@ type PlatformProvider interface {
 	// FontScale returns the user's font size preference multiplier.
 	// 1.0 = default system font size. Used to scale Sp (scale-independent pixels).
 	FontScale() float32
+
+	// SubpixelLayout returns the display's subpixel arrangement for LCD text rendering.
+	// Used by 2D graphics libraries (gg) to enable ClearType-quality font rendering.
+	// Returns SubpixelNone when subpixel info is unavailable or on HiDPI displays
+	// where subpixels are too small to be visible.
+	SubpixelLayout() SubpixelLayout
+}
+
+// SubpixelLayout describes the physical arrangement of RGB subpixels on a display.
+// Used for LCD/ClearType font rendering to achieve sharper text by exploiting
+// the subpixel structure. Qt6 (QPlatformScreen), Wayland (wl_output.geometry),
+// and DRM/KMS (drmModeConnector) all expose this as a display property.
+type SubpixelLayout int
+
+const (
+	// SubpixelNone means no subpixel information is available or applicable.
+	// Text rendering falls back to grayscale anti-aliasing.
+	// Used on HiDPI displays where subpixels are too small to exploit.
+	SubpixelNone SubpixelLayout = iota
+
+	// SubpixelRGB is horizontal RGB ordering (most common: Windows LCD, most external monitors).
+	SubpixelRGB
+
+	// SubpixelBGR is horizontal BGR ordering (some Samsung and older displays).
+	SubpixelBGR
+
+	// SubpixelVRGB is vertical RGB ordering (rare, some rotated displays).
+	SubpixelVRGB
+
+	// SubpixelVBGR is vertical BGR ordering (rare).
+	SubpixelVBGR
+)
+
+// String returns the subpixel layout name for debugging.
+func (s SubpixelLayout) String() string {
+	switch s {
+	case SubpixelNone:
+		return stringNone
+	case SubpixelRGB:
+		return "RGB"
+	case SubpixelBGR:
+		return "BGR"
+	case SubpixelVRGB:
+		return "VRGB"
+	case SubpixelVBGR:
+		return "VBGR"
+	default:
+		return "Unknown"
+	}
 }
 
 // CursorShape represents the mouse cursor shape.
@@ -205,5 +254,8 @@ func (NullPlatformProvider) HighContrast() bool { return false }
 // FontScale returns 1.0.
 func (NullPlatformProvider) FontScale() float32 { return 1.0 }
 
+// SubpixelLayout returns SubpixelNone (grayscale AA).
+func (NullPlatformProvider) SubpixelLayout() SubpixelLayout { return SubpixelNone }
+
 // Ensure NullPlatformProvider implements PlatformProvider.
 var _ PlatformProvider = NullPlatformProvider{}

platform_test.go

@@ -141,11 +141,12 @@ func (m *mockPlatformProvider) ClipboardWrite(text string) error {
 	m.clipboard = text
 	return nil
 }
-func (m *mockPlatformProvider) SetCursor(cursor CursorShape) { m.cursor = cursor }
-func (m *mockPlatformProvider) DarkMode() bool               { return m.darkMode }
-func (m *mockPlatformProvider) ReduceMotion() bool           { return m.reduceMotion }
-func (m *mockPlatformProvider) HighContrast() bool           { return m.highContrast }
-func (m *mockPlatformProvider) FontScale() float32           { return m.fontScale }
+func (m *mockPlatformProvider) SetCursor(cursor CursorShape)   { m.cursor = cursor }
+func (m *mockPlatformProvider) DarkMode() bool                 { return m.darkMode }
+func (m *mockPlatformProvider) ReduceMotion() bool             { return m.reduceMotion }
+func (m *mockPlatformProvider) HighContrast() bool             { return m.highContrast }
+func (m *mockPlatformProvider) FontScale() float32             { return m.fontScale }
+func (m *mockPlatformProvider) SubpixelLayout() SubpixelLayout { return SubpixelRGB }
 
 // Ensure mockPlatformProvider implements PlatformProvider.
 var _ PlatformProvider = &mockPlatformProvider{}