Juliet/AgentData/lighting_and_skybox_plan.md

# Lighting & Skybox Plan

## Current State

| Item | Current |
|------|---------|
| Vertex format | `float3 Position` + `float4 Color` (28 bytes, no normals) |
| Vertex shader | [Triangle.vert.hlsl](file:///w:/Classified/Juliet/Assets/source/Triangle.vert.hlsl) — bindless buffer read, `mul(VP, mul(Model, pos))` |
| Fragment shader | [SolidColor.frag.hlsl](file:///w:/Classified/Juliet/Assets/source/SolidColor.frag.hlsl) — passthrough `return Color` |
| Push constants | `ViewProjection` + `Model` + `BufferIndex` + extras |
| Texturing | None — no sampler usage, no texture reads |

---

## Phase 1: Add Normals to Vertex Data

Lighting requires normals. This is the foundation for everything else.

### Vertex Format Change

```diff
 struct Vertex
 {
     float Position[3];
+    float Normal[3];
     float Color[4];
 };
```

- **Stride**: 28 → 40 bytes
- Update `Triangle.vert.hlsl` stride constant: `uint stride = 40;`
- Load normal after position: `float3 normal = asfloat(buffer.Load3(offset + 12));`
- Load color shifts to: `float4 col = asfloat(buffer.Load4(offset + 24));`

### Files to modify

| File | Change |
|------|--------|
| [VertexData.h](file:///w:/Classified/Juliet/Juliet/include/Graphics/VertexData.h) | Add `float Normal[3]` to `Vertex` |
| [MeshRenderer.cpp](file:///w:/Classified/Juliet/Juliet/src/Graphics/MeshRenderer.cpp) | Update `AddCube()` to include per-face normals |
| [Triangle.vert.hlsl](file:///w:/Classified/Juliet/Assets/source/Triangle.vert.hlsl) | Update stride, load normal, pass to fragment |

---

## Phase 2: Basic Directional Light (Diffuse)

Simple single directional light with diffuse (Lambert) shading.

### Approach: Push light data through RootConstants

Add light direction and color to `RootConstants.hlsl` and the C++ `PushData`:

```hlsl
// RootConstants.hlsl additions
float3   LightDirection;  // Normalized, world-space
float    _LightPad;
float3   LightColor;
float    AmbientIntensity;
```

```cpp
// PushData additions
Vector3 LightDirection;
float   _LightPad;
Vector3 LightColor;
float   AmbientIntensity;
```

### Shader Changes

**Vertex shader** — transform normal to world space and pass it to fragment:

```hlsl
// Triangle.vert.hlsl output struct
struct Output
{
    float4 Color    : TEXCOORD0;
    float3 WorldNormal : TEXCOORD1;
    float4 Position : SV_Position;
};

// In main():
float3 worldNormal = mul((float3x3)Model, normal);
output.WorldNormal = worldNormal;
```

> [!NOTE]
> Using `(float3x3)Model` for normal transform is only correct for uniform-scale transforms. For non-uniform scale, you'd need the inverse-transpose. Fine for now with translation-only transforms.

**Fragment shader** — apply Lambert diffuse:

```hlsl
// SolidColor.frag.hlsl → rename to Lit.frag.hlsl
#include "RootConstants.hlsl"

float4 main(float4 Color : TEXCOORD0, float3 WorldNormal : TEXCOORD1) : SV_Target0
{
    float3 N = normalize(WorldNormal);
    float  NdotL = saturate(dot(N, -LightDirection));
    float3 diffuse = Color.rgb * LightColor * NdotL;
    float3 ambient = Color.rgb * AmbientIntensity;
    return float4(diffuse + ambient, Color.a);
}
```

### Files to modify

| File | Change |
|------|--------|
| [RootConstants.hlsl](file:///w:/Classified/Juliet/Assets/source/RootConstants.hlsl) | Add light params |
| [MeshRenderer.h](file:///w:/Classified/Juliet/Juliet/include/Graphics/MeshRenderer.h) | Add light params to `PushData` |
| [Triangle.vert.hlsl](file:///w:/Classified/Juliet/Assets/source/Triangle.vert.hlsl) | Pass world normal to fragment |
| [SolidColor.frag.hlsl](file:///w:/Classified/Juliet/Assets/source/SolidColor.frag.hlsl) | Lambert diffuse + ambient |
| [DebugDisplayRenderer.cpp](file:///w:/Classified/Juliet/Juliet/src/Graphics/DebugDisplayRenderer.cpp) | Update push data struct to match new layout |
| [main.cpp](file:///w:/Classified/Juliet/JulietApp/main.cpp) | Set `LightDirection`, `LightColor`, `AmbientIntensity` |

---

## Phase 3: Skybox

A skybox renders a cubemap texture behind all geometry, giving the scene a background.

### Approach: Fullscreen-triangle with inverse VP

Render a fullscreen triangle as the very last thing (or first with depth write off), sample a cubemap using the camera view direction reconstructed from screen coordinates.

### New Assets

- **Skybox cubemap texture** — a `.dds` or 6 `.png` face images loaded as a `TextureCube`
- **New shaders**: `Skybox.vert.hlsl` + `Skybox.frag.hlsl`

### Shader Design

**Vertex shader** — fullscreen triangle using `SV_VertexID`:

```hlsl
// Skybox.vert.hlsl
#include "RootConstants.hlsl"

struct Output
{
    float3 ViewDir  : TEXCOORD0;
    float4 Position : SV_Position;
};

Output main(uint vertexID : SV_VertexID)
{
    Output output;

    // Fullscreen triangle
    float2 uv = float2((vertexID << 1) & 2, vertexID & 2);
    float4 clipPos = float4(uv * 2.0 - 1.0, 1.0, 1.0);
    clipPos.y = -clipPos.y;

    output.Position = clipPos;

    // Reconstruct view direction from clip space
    // InverseViewProjection needs to be added to RootConstants
    float4 worldPos = mul(InverseViewProjection, clipPos);
    output.ViewDir = worldPos.xyz / worldPos.w;

    return output;
}
```

**Fragment shader** — sample cubemap:

```hlsl
// Skybox.frag.hlsl
#include "RootConstants.hlsl"

float4 main(float3 ViewDir : TEXCOORD0) : SV_Target0
{
    TextureCube skybox = ResourceDescriptorHeap[TextureIndex];
    SamplerState samp = SamplerDescriptorHeap[0]; // Linear clamp
    return skybox.Sample(samp, normalize(ViewDir));
}
```

### Pipeline Requirements

| Setting | Value |
|---------|-------|
| Depth write | **Off** (skybox is infinitely far) |
| Depth test | **LessEqual** or **Off** (render behind everything) |
| Cull mode | **None** (fullscreen triangle) |
| Draw call | `Draw(3, 0)` — no vertex buffer needed |

### New C++ Components

1. **Cubemap loading** — need to create `TextureCube` from 6 face images or a `.dds` cubemap file
2. **Skybox pipeline** — new `GraphicsPipeline` with the skybox shaders and the pipeline settings above
3. **Sampler** — need at least one linear sampler in the sampler heap (may already exist for ImGui)
4. **`InverseViewProjection`** — add to `RootConstants` and compute in C++ via a `MatrixInverse` function

> [!IMPORTANT]
> `MatrixInverse` needs to be implemented in `Matrix.h`. This is a non-trivial 4×4 matrix inversion (adjugate/determinant method or Gauss-Jordan).

### Files to modify/create

| File | Change |
|------|--------|
| [NEW] `Skybox.vert.hlsl` | Fullscreen triangle + view direction |
| [NEW] `Skybox.frag.hlsl` | Cubemap sample |
| [RootConstants.hlsl](file:///w:/Classified/Juliet/Assets/source/RootConstants.hlsl) | Add `InverseViewProjection` |
| [Matrix.h](file:///w:/Classified/Juliet/Juliet/include/Core/Math/Matrix.h) | Add `MatrixInverse()` |
| [MeshRenderer.h](file:///w:/Classified/Juliet/Juliet/include/Graphics/MeshRenderer.h) | Add `InverseViewProjection` to `PushData` |
| [main.cpp](file:///w:/Classified/Juliet/JulietApp/main.cpp) | Create skybox pipeline, load cubemap, render skybox |

---

## Recommended Implementation Order

```mermaid
graph LR
    A["Phase 1<br/>Add Normals"] --> B["Phase 2<br/>Directional Light"]
    B --> C["Phase 3<br/>Skybox"]
```

1. **Phase 1** (normals) is the smallest and unblocks Phase 2
2. **Phase 2** (lighting) gives the cubes visible 3D depth immediately
3. **Phase 3** (skybox) is independent of lighting but benefits from having `MatrixInverse` and sampler infrastructure

> [!TIP]
> Phases 1+2 together are a single session of work (~8 files). Phase 3 is larger due to cubemap loading and new pipeline creation.

---

## Open Questions

1. **Cubemap source** — procedural gradient skybox (no asset needed) vs actual HDR cubemap `.dds` file?
2. **Sampler heap** — does the engine already have a linear sampler registered, or does one need to be created?
3. **Specular** — want Blinn-Phong specular in Phase 2, or just diffuse + ambient for now?