yaze/docs/overworld_loading_guide.md

Overworld Loading Guide: ZScream vs Yaze

This document provides a comprehensive guide to understanding how overworld loading works in both ZScream (C#) and yaze (C++), including the differences between vanilla ROMs and ZSCustomOverworld v2/v3 ROMs.

Table of Contents

1. Overview
2. ROM Types and Versions
3. Overworld Map Structure
4. Loading Process
5. ZScream Implementation
6. Yaze Implementation
7. Key Differences
8. Common Issues and Solutions

Overview

Both ZScream and yaze are Zelda 3 ROM editors that support editing overworld maps. They handle three main types of ROMs:

• Vanilla ROMs: Original Zelda 3 ROMs without modifications
• ZSCustomOverworld v2: ROMs with expanded overworld features
• ZSCustomOverworld v3: ROMs with additional features like overlays and custom background colors

ROM Types and Versions

Version Detection

Both editors detect the ROM version using the same constant:

// Address: 0x140145
constexpr int OverworldCustomASMHasBeenApplied = 0x140145;

// Version values:
// 0xFF = Vanilla ROM
// 0x02 = ZSCustomOverworld v2
// 0x03 = ZSCustomOverworld v3

Feature Support by Version

Feature	Vanilla	v2	v3
Basic Overworld Maps	✅	✅	✅
Area Size Enum	❌	❌	✅
Main Palette	❌	✅	✅
Custom Background Colors	❌	✅	✅
Subscreen Overlays	✅	✅	✅
Animated GFX	❌	❌	✅
Custom Tile Graphics	❌	❌	✅
Vanilla Overlays	✅	✅	✅

Note: Subscreen overlays are a shared concept from vanilla ROMs. ZSCustomOverworld v2+ expands on this by adding support for special overworld areas and custom overlay configurations.

Overworld Map Structure

Core Properties

Each overworld map contains the following core properties:

class OverworldMap {
  // Basic properties
  uint8_t index_;                    // Map index (0-159)
  uint8_t parent_;                   // Parent map ID
  uint8_t world_;                    // World type (0=LW, 1=DW, 2=SW)
  uint8_t game_state_;               // Game state (0=Beginning, 1=Zelda, 2=Agahnim)
  
  // Graphics and palettes
  uint8_t area_graphics_;            // Area graphics ID
  uint8_t area_palette_;             // Area palette ID
  uint8_t main_palette_;             // Main palette ID (v2+)
  std::array<uint8_t, 3> sprite_graphics_;  // Sprite graphics IDs
  std::array<uint8_t, 3> sprite_palette_;   // Sprite palette IDs
  
  // Map properties
  uint16_t message_id_;              // Message ID
  bool mosaic_;                      // Mosaic effect enabled
  bool large_map_;                   // Is large map (vanilla)
  AreaSizeEnum area_size_;           // Area size (v3)
  
  // Custom features (v2/v3)
  uint16_t area_specific_bg_color_;  // Custom background color
  uint16_t subscreen_overlay_;       // Subscreen overlay ID (references special area maps)
  uint8_t animated_gfx_;             // Animated graphics ID
  std::array<uint8_t, 8> custom_gfx_ids_;  // Custom tile graphics
  
  // Overlay support (vanilla and custom)
  uint16_t vanilla_overlay_id_;      // Vanilla overlay ID
  bool has_vanilla_overlay_;         // Has vanilla overlay data
  std::vector<uint8_t> vanilla_overlay_data_;  // Raw overlay data
};

Overlays and Special Area Maps

Understanding Overlays

Overlays in Zelda 3 are overworld maps from special areas (maps 0x80-0x9F) that are displayed semi-transparently on top of other maps. This is a shared concept from vanilla ROMs that ZSCustomOverworld expands upon.

Special Area Maps (0x80-0x9F)

Special area maps are the overworld maps past the dark world (maps 0x80-0x9F). These include:

• 0x80-0x8A: Various special areas (Lost Woods, Skull Woods, etc.)
• 0x8B-0x8F: Additional special areas
• 0x90-0x9F: More special areas including Death Mountain variants

Overlay ID Mappings

Common overlay IDs reference specific special area maps:

Overlay ID	Special Area Map	Description
0x009D	0x8D	Fog 2 (Lost Woods/Skull Woods)
0x0095	0x85	Sky Background (Death Mountain)
0x009C	0x8C	Lava (Dark World Death Mountain)
0x0096	0x86	Pyramid Background
0x0097	0x87	Fog 1 (Master Sword Area)
0x0093	0x83	Triforce Room Curtains

Vanilla Overlay Loading

absl::Status LoadVanillaOverlay() {
  uint8_t asm_version = (*rom_)[OverworldCustomASMHasBeenApplied];
  
  // Only load vanilla overlays for vanilla ROMs
  if (asm_version != 0xFF) {
    has_vanilla_overlay_ = false;
    return absl::OkStatus();
  }
  
  // Load overlay pointer for this map
  int address = (kOverlayPointersBank << 16) +
                ((*rom_)[kOverlayPointers + (index_ * 2) + 1] << 8) +
                (*rom_)[kOverlayPointers + (index_ * 2)];
  
  // Parse overlay commands (SNES assembly-like)
  // Commands like LDA, LDX, STA, JMP, END (0x60)
  // These commands configure which special area map to use as overlay
  
  return absl::OkStatus();
}

Special Area Graphics Loading

Special area maps require special handling for graphics loading:

void LoadAreaInfo() {
  if (parent_ >= kSpecialWorldMapIdStart) {
    // Special World (SW) areas
    if (asm_version >= 3 && asm_version != 0xFF) {
      // Use expanded sprite tables for v3
      sprite_graphics_[0] = (*rom_)[kOverworldSpecialSpriteGfxGroupExpandedTemp + 
                                     parent_ - kSpecialWorldMapIdStart];
    } else {
      // Use original sprite tables for v2/vanilla
      sprite_graphics_[0] = (*rom_)[kOverworldSpecialGfxGroup + 
                                     parent_ - kSpecialWorldMapIdStart];
    }
    
    // Handle special cases for specific maps
    if (index_ == 0x88 || index_ == 0x93) {
      area_graphics_ = 0x51;
      area_palette_ = 0x00;
    } else if (index_ == 0x95) {
      // Make this the same GFX as LW death mountain areas
      area_graphics_ = (*rom_)[kAreaGfxIdPtr + 0x03];
      area_palette_ = (*rom_)[kOverworldMapPaletteIds + 0x03];
    } else if (index_ == 0x96) {
      // Make this the same GFX as pyramid areas
      area_graphics_ = (*rom_)[kAreaGfxIdPtr + 0x5B];
      area_palette_ = (*rom_)[kOverworldMapPaletteIds + 0x5B];
    } else if (index_ == 0x9C) {
      // Make this the same GFX as DW death mountain areas
      area_graphics_ = (*rom_)[kAreaGfxIdPtr + 0x43];
      area_palette_ = (*rom_)[kOverworldMapPaletteIds + 0x43];
    }
  }
}

Loading Process

1. Version Detection

Both editors first detect the ROM version:

uint8_t asm_version = rom[OverworldCustomASMHasBeenApplied];

2. Map Initialization

For each of the 160 overworld maps (0x00-0x9F):

// ZScream
var map = new OverworldMap(index, overworld);

// Yaze
OverworldMap map(index, rom);

3. Property Loading

The loading process varies by ROM version:

Vanilla ROMs (asm_version == 0xFF)

void LoadAreaInfo() {
  // Load from vanilla tables
  message_id_ = rom[kOverworldMessageIds + index_ * 2];
  area_graphics_ = rom[kOverworldMapGfx + index_];
  area_palette_ = rom[kOverworldMapPaletteIds + index_];
  
  // Determine large map status
  large_map_ = (rom[kOverworldMapSize + index_] != 0);
  
  // Load vanilla overlay
  LoadVanillaOverlay();
}

ZSCustomOverworld v2/v3

void LoadAreaInfo() {
  // Use expanded tables for v3
  if (asm_version >= 3) {
    message_id_ = rom[kOverworldMessagesExpanded + index_ * 2];
    area_size_ = static_cast<AreaSizeEnum>(rom[kOverworldScreenSize + index_]);
  } else {
    message_id_ = rom[kOverworldMessageIds + index_ * 2];
    area_size_ = large_map_ ? LargeArea : SmallArea;
  }
  
  // Load custom overworld data
  LoadCustomOverworldData();
}

4. Custom Data Loading

For ZSCustomOverworld ROMs:

void LoadCustomOverworldData() {
  // Load main palette
  main_palette_ = rom[OverworldCustomMainPaletteArray + index_];
  
  // Load custom background color
  if (rom[OverworldCustomAreaSpecificBGEnabled] != 0) {
    area_specific_bg_color_ = rom[OverworldCustomAreaSpecificBGPalette + index_ * 2];
  }
  
  // Load v3 features
  if (asm_version >= 3) {
    subscreen_overlay_ = rom[OverworldCustomSubscreenOverlayArray + index_ * 2];
    animated_gfx_ = rom[OverworldCustomAnimatedGFXArray + index_];
    
    // Load custom tile graphics (8 sheets)
    for (int i = 0; i < 8; i++) {
      custom_gfx_ids_[i] = rom[OverworldCustomTileGFXGroupArray + index_ * 8 + i];
    }
  }
}

ZScream Implementation

OverworldMap Constructor

public OverworldMap(byte index, Overworld overworld) {
    Index = index;
    this.overworld = overworld;
    
    // Load area info
    LoadAreaInfo();
    
    // Load custom data if available
    if (ROM.DATA[Constants.OverworldCustomASMHasBeenApplied] != 0xFF) {
        LoadCustomOverworldData();
    }
    
    // Build graphics and palette
    BuildMap();
}

Key Methods

• LoadAreaInfo(): Loads basic map properties from ROM
• LoadCustomOverworldData(): Loads ZSCustomOverworld features
• LoadPalette(): Loads and processes palette data
• BuildMap(): Constructs the final map bitmap

Yaze Implementation

OverworldMap Constructor

OverworldMap::OverworldMap(int index, Rom* rom) : index_(index), rom_(rom) {
  LoadAreaInfo();
  LoadCustomOverworldData();
  SetupCustomTileset(asm_version);
}

Key Methods

• LoadAreaInfo(): Loads basic map properties
• LoadCustomOverworldData(): Loads ZSCustomOverworld features
• LoadVanillaOverlay(): Loads vanilla overlay data
• LoadPalette(): Loads and processes palette data
• BuildTileset(): Constructs graphics tileset
• BuildBitmap(): Creates the final map bitmap

Key Differences

1. Language and Architecture

Aspect	ZScream	Yaze
Language	C#	C++
Memory Management	Garbage Collected	Manual (RAII)
Graphics	System.Drawing	Custom OpenGL
UI Framework	WinForms	ImGui

2. Data Structures

ZScream:

public class OverworldMap {
    public byte Index { get; set; }
    public AreaSizeEnum AreaSize { get; set; }
    public Bitmap GFXBitmap { get; set; }
    // ... other properties
}

Yaze:

class OverworldMap {
  uint8_t index_;
  AreaSizeEnum area_size_;
  std::vector<uint8_t> bitmap_data_;
  // ... other member variables
};

3. Error Handling

ZScream: Uses exceptions and try-catch blocks Yaze: Uses absl::Status return values and RETURN_IF_ERROR macros

4. Graphics Processing

ZScream: Uses .NET's Bitmap class and GDI+ Yaze: Uses custom gfx::Bitmap class with OpenGL textures

Common Issues and Solutions

1. Version Detection Issues

Problem: ROM not recognized as ZSCustomOverworld Solution: Check that OverworldCustomASMHasBeenApplied is set correctly

2. Palette Loading Errors

Problem: Maps appear with wrong colors Solution: Verify palette group addresses and 0xFF fallback handling

3. Graphics Not Loading

Problem: Blank textures or missing graphics Solution: Check graphics buffer bounds and ProcessGraphicsBuffer implementation

4. Overlay Issues

Problem: Vanilla overlays not displaying Solution:

• Verify overlay pointer addresses and SNES-to-PC conversion
• Ensure special area maps (0x80-0x9F) are properly loaded with correct graphics
• Check that overlay ID mappings are correct (e.g., 0x009D → map 0x8D)
• Verify that overlay preview shows the actual bitmap of the referenced special area map

Problem: Overlay preview showing hex data instead of bitmap Solution: Implement proper overlay preview that displays the bitmap of the referenced special area map, not just the overlay command data

5. Large Map Problems

Problem: Large maps not rendering correctly Solution: Check parent-child relationships and large map detection logic

6. Special Area Graphics Issues

Problem: Special area maps (0x80-0x9F) showing blank or incorrect graphics Solution:

• Verify special area graphics loading in LoadAreaInfo()
• Check that special cases for maps like 0x88, 0x93, 0x95, 0x96, 0x9C are handled correctly
• Ensure proper sprite graphics table selection for v2 vs v3 ROMs
• Verify that special area maps use the correct graphics from referenced LW/DW maps

Best Practices

1. Version-Specific Code

Always check the ASM version before accessing version-specific features:

uint8_t asm_version = (*rom_)[OverworldCustomASMHasBeenApplied];
if (asm_version >= 3) {
    // v3 features
} else if (asm_version == 0xFF) {
    // Vanilla features
}

2. Error Handling

Use proper error handling for ROM operations:

absl::Status LoadPalette() {
    RETURN_IF_ERROR(LoadPaletteData());
    RETURN_IF_ERROR(ProcessPalette());
    return absl::OkStatus();
}

3. Memory Management

Be careful with memory management in C++:

// Good: RAII and smart pointers
std::vector<uint8_t> data;
std::unique_ptr<OverworldMap> map;

// Bad: Raw pointers without cleanup
uint8_t* raw_data = new uint8_t[size];
OverworldMap* map = new OverworldMap();

4. Thread Safety

Both editors use threading for performance:

// Yaze: Use std::async for parallel processing
auto future = std::async(std::launch::async, [this](int map_index) {
    RefreshChildMap(map_index);
}, map_index);

Conclusion

Understanding the differences between ZScream and yaze implementations is crucial for maintaining compatibility and adding new features. Both editors follow similar patterns but use different approaches due to their respective languages and architectures.

The key is to maintain the same ROM data structure understanding while adapting to each editor's specific implementation patterns.