backend-infra-engineer: Release v0.3.3 snapshot
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docs/public/developer/palette-system-overview.md
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docs/public/developer/palette-system-overview.md
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# SNES Palette System Overview
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## Understanding SNES Color and Palette Organization
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### Core Concepts
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#### 1. SNES Color Format (15-bit BGR555)
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- **Storage**: 2 bytes per color (16 bits total, 15 bits used)
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- **Format**: `0BBB BBGG GGGR RRRR`
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- Bits 0-4: Red (5 bits, 0-31)
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- Bits 5-9: Green (5 bits, 0-31)
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- Bits 10-14: Blue (5 bits, 0-31)
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- Bit 15: Unused (always 0)
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- **Range**: Each channel has 32 levels (0-31)
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- **Total Colors**: 32,768 possible colors (2^15)
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#### 2. Palette Groups in Zelda 3
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Zelda 3 organizes palettes into logical groups for different game areas and entities:
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```cpp
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struct PaletteGroupMap {
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PaletteGroup overworld_main; // Main overworld graphics (35 colors each)
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PaletteGroup overworld_aux; // Auxiliary overworld (21 colors each)
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PaletteGroup overworld_animated; // Animated colors (7 colors each)
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PaletteGroup hud; // HUD graphics (32 colors each)
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PaletteGroup global_sprites; // Sprite palettes (60 colors each)
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PaletteGroup armors; // Armor colors (15 colors each)
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PaletteGroup swords; // Sword colors (3 colors each)
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PaletteGroup shields; // Shield colors (4 colors each)
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PaletteGroup sprites_aux1; // Auxiliary sprite palette 1 (7 colors each)
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PaletteGroup sprites_aux2; // Auxiliary sprite palette 2 (7 colors each)
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PaletteGroup sprites_aux3; // Auxiliary sprite palette 3 (7 colors each)
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PaletteGroup dungeon_main; // Dungeon palettes (90 colors each)
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PaletteGroup grass; // Grass colors (special handling)
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PaletteGroup object_3d; // 3D object palettes (8 colors each)
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PaletteGroup overworld_mini_map; // Mini-map palettes (128 colors each)
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};
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```
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#### 3. Color Representations in Code
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- **SNES 15-bit (`uint16_t`)**: On-disk format `0bbbbbgggggrrrrr`; store raw ROM
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words or write back with `ConvertRgbToSnes`.
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- **`gfx::snes_color` struct**: Expands each channel to 0-255 for arithmetic
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without floating point; use in converters and palette math.
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- **`gfx::SnesColor` class**: High-level wrapper retaining the original SNES
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value, a `snes_color`, and an ImVec4. Its `rgb()` accessor purposely returns
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0-255 components—run the helper converters (e.g., `ConvertSnesColorToImVec4`)
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before handing colors to ImGui widgets that expect 0.0-1.0 floats.
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### Dungeon Palette System
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#### Structure
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- **20 dungeon palettes** in the `dungeon_main` group
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- **90 colors per palette** (full SNES palette for BG layers)
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- **ROM Location**: `kDungeonMainPalettes` (check `snes_palette.cc` for exact address)
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#### Usage
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```cpp
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// Loading a dungeon palette
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auto& dungeon_pal_group = rom->palette_group().dungeon_main;
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int num_palettes = dungeon_pal_group.size(); // Should be 20
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int palette_id = room.palette; // Room's palette ID (0-19)
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// IMPORTANT: Use operator[] not palette() method!
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auto palette = dungeon_pal_group[palette_id]; // Returns reference
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// NOT: auto palette = dungeon_pal_group.palette(palette_id); // Returns copy!
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```
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#### Color Distribution (90 colors)
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The 90 colors are typically distributed as:
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- **BG1 Palette** (Background Layer 1): First 8-16 subpalettes
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- **BG2 Palette** (Background Layer 2): Next 8-16 subpalettes
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- **Sprite Palettes**: Remaining colors (handled separately)
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Each "subpalette" is 16 colors (one SNES palette unit).
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### Overworld Palette System
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#### Structure
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- **Main Overworld**: 35 colors per palette
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- **Auxiliary**: 21 colors per palette
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- **Animated**: 7 colors per palette (for water, lava effects)
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#### 3BPP Graphics and Left/Right Palettes
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Overworld graphics use 3BPP (3 bits per pixel) format:
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- **8 colors per tile** (2^3 = 8)
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- **Left Side**: Uses palette 0-7
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- **Right Side**: Uses palette 8-15
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When decompressing 3BPP graphics:
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```cpp
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// Palette assignment for 3BPP overworld tiles
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if (tile_position < half_screen_width) {
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// Left side of screen
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tile_palette_offset = 0; // Use colors 0-7
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} else {
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// Right side of screen
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tile_palette_offset = 8; // Use colors 8-15
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}
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```
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### Common Issues and Solutions
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#### Issue 1: Empty Palette
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**Symptom**: "Palette size: 0 colors"
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**Cause**: Using `palette()` method instead of `operator[]`
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**Solution**:
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```cpp
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// WRONG:
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auto palette = group.palette(id); // Returns copy, may be empty
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// CORRECT:
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auto palette = group[id]; // Returns reference
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```
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#### Issue 2: Bitmap Corruption
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**Symptom**: Graphics render only in top portion of image
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**Cause**: Wrong depth parameter in `CreateAndRenderBitmap`
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**Solution**:
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```cpp
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// WRONG:
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CreateAndRenderBitmap(0x200, 0x200, 0x200, data, bitmap, palette);
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// depth ^^^^ should be 8!
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// CORRECT:
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CreateAndRenderBitmap(0x200, 0x200, 8, data, bitmap, palette);
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// width, height, depth=8 bits
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```
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### Transparency and Conversion Best Practices
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- Preserve ROM palette words exactly as read; hardware enforces transparency on
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index 0 so we no longer call `set_transparent(true)` while loading.
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- Apply transparency only at render time via `SetPaletteWithTransparent()` for
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3BPP sub-palettes or `SetPalette()` for full 256-color assets.
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- `SnesColor::rgb()` yields components in 0-255 space; convert to ImGui’s
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expected 0.0-1.0 floats with the helper functions instead of manual divides.
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- Use the provided conversion helpers (`ConvertSnesToRgb`, `ImVec4ToSnesColor`,
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`SnesTo8bppColor`) to prevent rounding mistakes and alpha bugs.
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```cpp
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ImVec4 rgb_255 = snes_color.rgb();
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ImVec4 display = ConvertSnesColorToImVec4(snes_color);
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ImGui::ColorButton("color", display);
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```
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#### Issue 3: ROM Not Loaded in Preview
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**Symptom**: "ROM not loaded" error in emulator preview
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**Cause**: Initializing before ROM is set
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**Solution**:
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```cpp
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// Initialize emulator preview AFTER ROM is loaded and set
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void Load() {
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// ... load ROM data ...
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// ... set up other components ...
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// NOW initialize emulator preview with loaded ROM
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object_emulator_preview_.Initialize(rom_);
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}
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```
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### Palette Editor Integration
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#### Key Functions for UI
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```cpp
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// Reading a color from ROM
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absl::StatusOr<uint16_t> ReadColorFromRom(uint32_t address, const uint8_t* rom);
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// Converting SNES color to RGB
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SnesColor color(snes_value); // snes_value is uint16_t
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uint8_t r = color.red(); // 0-255 (converted from 0-31)
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uint8_t g = color.green(); // 0-255
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uint8_t b = color.blue(); // 0-255
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// Writing color back to ROM
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uint16_t snes_value = color.snes(); // Get 15-bit BGR555 value
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rom->WriteByte(address, snes_value & 0xFF); // Low byte
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rom->WriteByte(address + 1, (snes_value >> 8) & 0xFF); // High byte
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```
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#### Palette Widget Requirements
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1. **Display**: Show colors in organized grids (16 colors per row for SNES standard)
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2. **Selection**: Allow clicking to select a color
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3. **Editing**: Provide RGB sliders (0-255) or color picker
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4. **Conversion**: Auto-convert RGB (0-255) ↔ SNES (0-31) values
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5. **Preview**: Show before/after comparison
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6. **Save**: Write modified palette back to ROM
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#### Palette UI Helpers
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- `InlinePaletteSelector` renders a lightweight selection strip (no editing)
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ideal for 8- or 16-color sub-palettes.
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- `InlinePaletteEditor` supplies the full editing experience with ImGui color
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pickers, context menus, and optional live preview toggles.
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- `PopupPaletteEditor` fits in context menus or modals; it caps at 64 colors to
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keep popups manageable.
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- Legacy helpers such as `DisplayPalette()` remain for backward compatibility
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but inherit the 32-color limit—prefer the new helpers for new UI.
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-### Metadata-Driven Palette Application
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`gfx::BitmapMetadata` tracks the source BPP, palette format, type string, and
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expected color count. Set it immediately after creating a bitmap so later code
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can make the right choice automatically:
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```cpp
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bitmap.metadata() = BitmapMetadata{/*source_bpp=*/3,
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/*palette_format=*/1, // 0=full, 1=sub-palette
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/*source_type=*/"graphics_sheet",
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/*palette_colors=*/8};
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bitmap.ApplyPaletteByMetadata(palette);
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```
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- `palette_format == 0` routes to `SetPalette()` and preserves every color
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(Mode 7, HUD assets, etc.).
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- `palette_format == 1` routes to `SetPaletteWithTransparent()` and injects the
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transparent color 0 for 3BPP workflows.
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- Validation hooks help catch mismatched palette sizes before they hit SDL.
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### Graphics Manager Integration
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#### Sheet Palette Assignment
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```cpp
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// Assigning palette to graphics sheet
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if (sheet_id > 115) {
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// Sprite sheets use sprite palette
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graphics_sheet.SetPaletteWithTransparent(
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rom.palette_group().global_sprites[0], 0);
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} else {
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// Dungeon sheets use dungeon palette
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graphics_sheet.SetPaletteWithTransparent(
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rom.palette_group().dungeon_main[0], 0);
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}
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```
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### Texture Synchronization and Regression Notes
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- Call `bitmap.UpdateSurfacePixels()` after mutating `bitmap.mutable_data()` to
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copy rendered bytes into the SDL surface before queuing texture creation or
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updates.
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- `Bitmap::ApplyStoredPalette()` now rebuilds an `SDL_Color` array sized to the
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actual palette instead of forcing 256 entries—this fixes regressions where
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8- or 16-color palettes were padded with opaque black.
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- When updating SDL palette data yourself, mirror that pattern:
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```cpp
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std::vector<SDL_Color> colors(palette.size());
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for (size_t i = 0; i < palette.size(); ++i) {
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const auto& c = palette[i];
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const ImVec4 rgb = c.rgb(); // 0-255 components
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colors[i] = SDL_Color{static_cast<Uint8>(rgb.x),
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static_cast<Uint8>(rgb.y),
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static_cast<Uint8>(rgb.z),
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c.is_transparent() ? 0 : 255};
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}
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SDL_SetPaletteColors(surface->format->palette, colors.data(), 0,
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static_cast<int>(colors.size()));
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```
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### Best Practices
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1. **Always use `operator[]` for palette access** - returns reference, not copy
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2. **Validate palette IDs** before accessing:
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```cpp
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if (palette_id >= 0 && palette_id < group.size()) {
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auto palette = group[palette_id];
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}
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```
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3. **Use correct depth parameter** when creating bitmaps (usually 8 for indexed color)
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4. **Initialize ROM-dependent components** only after ROM is fully loaded
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5. **Cache palettes** when repeatedly accessing the same palette
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6. **Update textures** after changing palettes (textures don't auto-update)
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### User Workflow Tips
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- Choose the widget that matches the task: selectors for choosing colors,
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editors for full control, popups for contextual tweaks.
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- The live preview toggle trades responsiveness for performance; disable it
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while batch-editing large (64+ color) palettes.
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- Right-click any swatch in the editor to copy the color as SNES hex, RGB
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tuples, or HTML hex—useful when coordinating with external art tools.
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- Remember hardware rules: palette index 0 is always transparent and will not
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display even if the stored value is non-zero.
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- Keep ROM backups when performing large palette sweeps; palette groups are
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shared across screens so a change can have multiple downstream effects.
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### ROM Addresses (for reference)
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```cpp
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// From snes_palette.cc
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constexpr uint32_t kOverworldPaletteMain = 0xDE6C8;
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constexpr uint32_t kOverworldPaletteAux = 0xDE86C;
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constexpr uint32_t kOverworldPaletteAnimated = 0xDE604;
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constexpr uint32_t kHudPalettes = 0xDD218;
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constexpr uint32_t kGlobalSpritesLW = 0xDD308;
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constexpr uint32_t kArmorPalettes = 0xDD630;
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constexpr uint32_t kSwordPalettes = 0xDD630;
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constexpr uint32_t kShieldPalettes = 0xDD648;
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constexpr uint32_t kSpritesPalettesAux1 = 0xDD39E;
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constexpr uint32_t kSpritesPalettesAux2 = 0xDD446;
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constexpr uint32_t kSpritesPalettesAux3 = 0xDD4E0;
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constexpr uint32_t kDungeonMainPalettes = 0xDD734;
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constexpr uint32_t kHardcodedGrassLW = 0x5FEA9;
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constexpr uint32_t kTriforcePalette = 0xF4CD0;
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constexpr uint32_t kOverworldMiniMapPalettes = 0x55B27;
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```
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## Graphics Sheet Palette Application
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### Default Palette Assignment
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Graphics sheets receive default palettes during ROM loading based on their index:
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```cpp
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// In LoadAllGraphicsData() - rom.cc
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if (i < 113) {
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// Sheets 0-112: Overworld/Dungeon graphics
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graphics_sheets[i].SetPalette(rom.palette_group().dungeon_main[0]);
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} else if (i < 128) {
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// Sheets 113-127: Sprite graphics
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graphics_sheets[i].SetPalette(rom.palette_group().sprites_aux1[0]);
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} else {
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// Sheets 128-222: Auxiliary/HUD graphics
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graphics_sheets[i].SetPalette(rom.palette_group().hud.palette(0));
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}
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```
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This ensures graphics are visible immediately after loading rather than appearing white.
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### Palette Update Workflow
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When changing a palette in any editor:
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1. Apply the palette: `bitmap.SetPalette(new_palette)`
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2. Notify Arena: `gfx::Arena::Get().NotifySheetModified(sheet_index)`
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3. Changes propagate to all editors automatically
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### Common Pitfalls
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**Wrong Palette Access**:
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```cpp
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// WRONG - Returns copy, may be empty
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auto palette = group.palette(id);
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// CORRECT - Returns reference
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auto palette = group[id];
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```
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**Missing Surface Update**:
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```cpp
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// WRONG - Only updates vector, not SDL surface
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bitmap.mutable_data() = new_data;
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// CORRECT - Updates both vector and surface
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bitmap.set_data(new_data);
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```
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