Lantern (stylized LANTERN) Engine is the internal name of the lighting system used in Sonic Adventure (Dreamcast). It is also the name of a mod that restores said lighting system in the PC version of Sonic Adventure DX.

Overview of the SA1 Lighting System

The Dreamcast version of Sonic Adventure has a lighting system (referred to internally as “LANTERN”) that consists of two modes:

Palette lighting using a series of 256-color palettes.
Lights set up by the Dreamcast’s Ninja library (Ninja Lights).

Lighting information for each level is stored in a pair of files: the PL file (PL_....BIN) contains the palettes, and the SL file (SL_....bin) contains light direction and data for setting up the Ninja Lights.

The Lantern Engine mod for Sonic Adventure DX PC is a reimplementation of palette lighting for the PC port of SADX. At the moment it does not implement the Ninja Lights.

Palette Lighting

Brightness Index Calculation

The palette-based system gives each vertex in the scene a “brightness index”, which is a value in the range of 0 to 255 used to select a color in a palette to set as vertex color. The brightness index is calculated using the following formula:

1 - ((dot(normal, light direction) + 1) / 2), where dot is dot product and normal is vertex normal.

The result is multiplied by 255 to get the color index in the palette. If we visualized brightness indices (from white being maximum brightness to black being completely unlit), it would look something like this:

Brightness indices visualized: from white (fullbright) to black (fully dark).

Diffuse and Specular Lighting

LANTERN palettes are applied in pairs to create diffuse and specular lighting.

Diffuse color is the color that is calculated by multiplying the original color with the color in the diffuse palette. It works like the “Multiply” blending mode in Photoshop.
Specular color is additive, so it works like the “Add” blending mode in Photoshop.

Making the diffuse color brighter makes the result closer to the original texture’s color. Making the specular color brighter adds to the brightness of the resulting color directly, so white diffuse + black specular is effectively the same as not applying any lighting at all, and white specular would make the vertex completely white regardless of the texture and diffuse color. Normally diffuse lighting is used to give the entire model or area a “base” color, while specular lighting is used to add gloss.

Diffuse palette samples (image by Speeps)

Palette Selection

The PL file can store up to eight pairs of diffuse/specular palettes, although only up to six pairs are used normally. There are several variables the game relies on to select which palettes to apply to a specific model:

1. Light type

This is a variable set in code before rendering the model. The light type value determines which diffuse palette is used by the model:

Value	Description	Diffuse Palette	Specular Palette
0	Used by the majority of models in the game except characters and bosses.	0	0 or 1
2	Used by characters and cutscene models.	2	2 or 3
4	Used by some bosses and for blending effects.	4	4 or 5

The majority of models in the game outside cutscenes use the level/object light type 0. These are the palettes you see on level pieces and most objects.

Character/NPC models and models used in cutscenes generally use the character/event light type 2, although there are some exceptions. For example, Amy’s Warrior Feather and some parts of Gamma’s body that are rendered as separate models (such as the Laser Scope) use the level/object light type 0 outside cutscenes. The gameplay models in Sky Chase use the level/object light type 0 despite being character-related; however, models involving the Tornado in Sky Chase cutscenes use the character light type 2. It happens the other way around as well: some non-character objects that would normally be using level/object palettes (such as floating meteors near the end of Twinkle Park Act 1) set the character light type so they also get character lighting, but this is uncommon.

The boss light type 4 is only used by a few models in the game. Diffuse 4 and Specular 5 are used by Chaos 2, Chaos 6 and Perfect Chaos. Diffuse 4/Specular 4 and Diffuse 5/Specular 5 are also used by the transition effect in Mystic Ruins Past when the Emerald Altar is on fire. The palettes blend between each other to create a burning fire effect.

2. Material flags

Material flags influence palette selection in the following ways:

If a mesh’s material has the “ignore lighting” flag (NJD_FLAG_IGNORE_LIGHT), the mesh is not palettized and the fullbright texture is used as-is. Neither the diffuse nor the specular palette is applied to the mesh, but the rest of the meshset may be palettized depending on whether or not the remaining materials have the “ignore lighting” flag.
If the model’s first material has the “ignore specular” flag (NJD_FLAG_IGNORE_SPECULAR), the entire model’s specular palette index is the same as the diffuse palette index (e.g. Diffuse 0, Specular 0).
If the model’s first material doesn’t have “ignore specular”, the specular palette index is the diffuse index + 1 (e.g. Diffuse 0, Specular 1).

Note that only the first mesh’s material in the model affects specular palette selection, and all other meshes follow it regardless of whether they have “ignore specular” or not. Specular palette selection is consistent within a single model, but if the model has child or sibling models those are processed separately unlike the light type. So it is possible to have a model (e.g. Tails) that has child models with a different specular palette (Tails’ shoes). They would still use the same diffuse palette, however, as they would have the same light type as the parent model.

Sometimes the game forcibly adds or removes the “ignore specular” flag or sets a specific light type when rendering certain models. If you want to change palette selection in these cases, you would need to change/disable the code doing that, or use Lantern API to register those models’ materials for arbitrary palette selection.

3. White diffuse

In the Dreamcast version of SA1, if a mesh in a model has a material with the “ignore lighting” flag, the remaining meshes after it may get the fullbright diffuse palette (as if they were ignoring lighting), but keep the specular palette. The Lantern Engine mod does not do this by default. Dreamcast Conversion has an option to enable white diffuse.

4. Palette blending

There are cases when the game swaps or mixes different palettes for some special effects:

The Past fire effect creates the level/object palettes (Diffuse 0/Specular 0) by mixing Diffuse 4/Specular 4 and Diffuse 5/Specular 5 using a blending coefficient. The Lantern Engine mod blends between Diffuse 0/Specular 0 and Diffuse 5/Specular 5 instead, which are identical to Diffuse 0/Specular 0 in the PL file anyway.
Sky Deck Acts 1 and 2 swap palettes when the Egg Carrier changes altitude. Palettes in PL_60B are used normally, and palettes from PL_61B are used when the Egg Carrier descends into the clouds. While the original game only swaps the palettes, the Lantern Engine mod actually blends between them.
The Egg Viper boss fight blends palettes in PL_M0B.BIN with specific colors (white, red and cyan).
In Casinopolis Act 1, when Sonic goes to pinball areas the character specular palette blends with white, making character lighting brighter as Sonic is floating before getting teleported to the pinball field.

5. Palette selection example

Here is a simple example of palette selection based on Emerald Coast. Color-coded specular palettes are used to tell which light type is being used by the model:

Palette masks ingame

List of PL files

The game loads PL/SL files using a naming scheme that roughly matches the game’s internal level list. The first number/letter after “PL_” is the level ID, the number after that is usually act number, but there are some exceptions. Here is a list of PL files and stages that load them:

Action Stages

Filename	Description
PL_10B.BIN	Emerald Coast (all acts), Egg Carrier Outside when the Egg Carrier has sunk
PL_20B.BIN	Windy Valley Act 1
PL_21B.BIN	Windy Valley Act 2
PL_22B.BIN	Windy Valley Act 3
PL_30B.BIN	Twinkle Park Act 1
PL_31B.BIN	Twinkle Park Act 2
PL_32B.BIN	Twinkle Park Act 3
PL_40B.BIN	Speed Highway Act 1
PL_41B.BIN	Speed Highway Act 2
PL_42B.BIN	Speed Highway Act 3
PL_50B.BIN	Red Mountain Act 1
PL_51B.BIN	Red Mountain Act 2
PL_52B.BIN	Red Mountain Act 3 (Knuckles)
PL_60B.BIN	Character select screen, Sky Deck normal (Acts 1 and 2), Sky Deck Act 3
PL_61B.BIN	Sky Deck dark (Acts 1 and 2)
PL_70B.BIN	Lost World Act 1
PL_71B.BIN	Lost World Acts 2 and 3
PL_80B.BIN	Ice Cap Act 1
PL_81B.BIN	Ice Cap Act 2
PL_82B.BIN	Ice Cap Act 3
PL_83B.BIN	Ice Cap Act 4 (Big)
PL_90B.BIN	Casinopolis Act 1
PL_91B.BIN	Casinopolis Act 2 (Sewers)
PL_92B.BIN	Casinopolis Act 3 (Sonic pinball)
PL_93B.BIN	Casinopolis Act 4 (NiGHTS pinball)
PL_A0B.BIN	Final Egg Act 1
PL_A1B.BIN	Final Egg Act 2
PL_A2B.BIN	Final Egg Act 3 (Sonic/Gamma)
PL_C0B.BIN	Hot Shelter Act 1
PL_C1B.BIN	Hot Shelter Act 2
PL_C2B.BIN	Hot Shelter Act 3 (Gamma)

Bosses

Filename	Description
PL_F0B.BIN	Chaos 0
PL_G0B.BIN	Chaos 2
PL_H0B.BIN	Chaos 4
PL_I0B.BIN	Chaos 6
PL_J0B.BIN	Perfect Chaos (first 3 hits)
PL_J1B.BIN	Perfect Chaos 2nd phase
PL_K0B.BIN	Egg Hornet
PL_M0B.BIN	Egg Viper
PL_N0B.BIN	Zero
PL_O0B.BIN	E-101 Beta
PL_P0B.BIN	E-101R

Adventure Fields

Filename	Description
PL_Q1B.BIN	Station Square Evening (All acts)
PL_Q3B.BIN	Egg Walker, Station Square Night (All acts)
PL_Q4B.BIN	Station Square Day (All acts)
PL_T0B.BIN	Egg Carrier Outside (before the Egg Carrier sinks)
PL_T2B.BIN	Egg Carrier Private Room
PL_T3B.BIN	Egg Carrier Captain’s Room
PL_T5B.BIN	Egg Carrier Pool
PL_W0B.BIN	Egg Carrier Ammunition Room
PL_W1B.BIN	Egg Carrier Bridge
PL_W2B.BIN	Egg Carrier Hedgehog Hammer Room
PL_W3B.BIN	Egg Carrier Prison
PL_W4B.BIN	Egg Carrier Reservoir Room
PL_W5B.BIN	Egg Carrier Chao Garden transporter
PL_X0B.BIN	Mystic Ruins Day (Station, Angel Island and Jungle)
PL_X1B.BIN	Mystic Ruins Evening (Station, Angel Island and Jungle)
PL_X2B.BIN	Mystic Ruins Night (Station, Angel Island and Jungle)
PL_X3B.BIN	Mystic Ruins Eggman’s Base
PL_Y0B.BIN	Past Act 1 (Echidna City)
PL_Y1B.BIN	Past Act 2 (Emerald Altar)
PL_Y2B.BIN	Past Act 3 (Emerald Altar on fire)

Subgames

Filename	Description
PL_Z0B.BIN	Twinkle Circuit
PL1A0B.BIN	Sky Chase Act 1
PL1B0B.BIN	Sky Chase Act 2
PL1C0B.BIN	Sand Hill

Chao

Filename	Description
PL1D0B.BIN	Station Square Chao Garden
PL1E0B.BIN	Egg Carrier Chao Garden
PL1F0B.BIN	Mystic Ruins Chao Garden Day
PL1F1B.BIN	Mystic Ruins Chao Garden Evening (unused in the original game, used in SADX Lantern Engine)
PL1F2B.BIN	Mystic Ruins Chao Garden Night (unused in the original game, used in SADX Lantern Engine)
PL1G0B.BIN	Chao Stadium
PL1G1B.BIN	Chao Race

Unused

The following files are present on the disk, but not used by the game:

Filename	Description
PL_9MB.BIN	An alternative version of the Casinopolis sewers palette that is never applied. It was originally supposed to be used by an object called MUD. The object would gradually change character palettes to make them darker.
PL_MRD.BIN	Possibly alternative Mystic Ruins Day palette
PL_MRE.BIN	Possibly alternative Mystic Ruins Evening palette
PL_MRN.BIN	Possibly alternative Mystic Ruins Night palette
PL_T1B.BIN	Egg Carrier related, might have been used in the early transformation cutscene
PL_T4B.BIN	Egg Carrier related, possibly the lighting used for some endgame credits images
PL1D1B.BIN	Possibly Station Square Chao Garden Night
PL1E1B.BIN	Egg Carrier Chao Garden Evening
PL1E2B.BIN	Egg Carrier Chao Garden Night

SL Files

SL files use the same naming scheme as the PL files. They store the following data:

Light rotation: Y and Z rotation values that are applied to the default light direction, which is a downward facing vector (0, -1, 0).
Lighting configuration for models that use Ninja Lights instead of palette lighting.