#version 430 core
// Sky mesh vertex shader — computes the per-vertex lighting tint that
// gives retail its time-of-day variation:
//
//     tint = clamp(emissive + ambient + max(dot(N, -sunDir), 0) * sunColor,
//                  0.0, 1.0)
//
// This is the retail-verbatim AdjustPlanes formula ported from the
// decompiled D3D fixed-function lighting. The `emissive` scalar is the
// Surface.Luminosity FLOAT field (NOT the SurfaceType.Luminous flag bit) —
// for Dereth's sky meshes, the DOME + SUN/MOON have emissive=1.0
// (texture-passthrough regardless of lighting), while CLOUDS have
// emissive=0.0 (lit normally, so they pick up the ambient tint that
// produces retail's purple-haze night / warm-tan dusk / pale-cool noon).
//
// See docs/research/2026-04-23-sky-retail-verbatim.md §6 for the full
// decompile trail and field citations.

layout(location = 0) in vec3 aPos;
layout(location = 1) in vec3 aNormal;
layout(location = 2) in vec2 aTex;

uniform mat4 uModel;          // per-object arc transform
uniform mat4 uSkyView;        // camera view with M41..M43 = 0
uniform mat4 uSkyProjection;  // near=0.1, far=1e6
uniform vec2 uUvScroll;       // cumulative TexVelocityX/Y * time

// Per-frame lighting — keyframe-interpolated values pushed before each
// Render() call. AmbColor × AmbBright and DirColor × DirBright are
// pre-multiplied by SkyDescLoader so the shader feeds them straight in.
uniform vec3  uAmbientColor;  // AmbColor × AmbBright
uniform vec3  uSunColor;      // DirColor × DirBright
uniform vec3  uSunDir;        // unit vector FROM surface TO sun

// Per-submesh: Surface.Luminosity (0..1 self-illumination scalar).
uniform float uEmissive;

out vec2 vTex;
out vec3 vTint;

void main() {
    vTex = aTex + uUvScroll;
    gl_Position = uSkyProjection * uSkyView * uModel * vec4(aPos, 1.0);

    // uModel for sky is scale * RotZ(-heading) * RotY(-rotation) — pure
    // orthonormal rotation, so mat3(uModel) correctly transforms normals
    // without needing transpose(inverse(...)).
    vec3 worldNormal = normalize(mat3(uModel) * aNormal);

    // Per-vertex lighting. `emissive` is broadcast scalar → vec3. For
    // emissive=1.0 the clamp saturates to white regardless of ambient/sun
    // (the retail "unlit" mesh). For emissive=0.0 only the ambient + sun
    // term drives `tint` (the retail "lit" mesh, e.g. clouds).
    float diff = max(dot(worldNormal, uSunDir), 0.0);
    vec3 lit = vec3(uEmissive) + uAmbientColor + diff * uSunColor;
    vTint = clamp(lit, 0.0, 1.0);
}