#version 450 core
layout(location=0) out vec4 oColor;

layout(std140, binding=0) uniform PerFrame { mat4 uView; mat4 uProj; vec4 uLightDir; };
layout(std140, binding=2) uniform Material { vec4 uAlbedo; };

layout(location=0) in VS_OUT {
  vec3 wPos;
  vec2 uv;
} v;

// geometric normal from screen-space derivatives of the *world* position
vec3 normal_from_worldpos(vec3 w){
  vec3 dx = dFdx(w), dy = dFdy(w);
  vec3 n  = cross(dx,dy);
  float l2 = dot(n,n);
  return (l2 < 1e-12) ? vec3(0,0,1) : normalize(n);
}

void main(){
  vec3 N = normal_from_worldpos(v.wPos);
  vec3 L = normalize(-uLightDir.xyz);
  float ndl = max(dot(N,L), 0.0);

  // cheap color: height-based gradient + slight darkening by slope
  float h = v.wPos.z;                         // world height
  float h01 = clamp(h * 0.1 + 0.5, 0.0, 1.0); // remap height to 0..1
  vec3 low   = vec3(0.35, 0.38, 0.40);        // rock/soil
  vec3 high  = vec3(0.65, 0.75, 0.60);        // grass
  vec3 base  = mix(low, high, h01);

  float slope = 1.0 - abs(N.z);               // 0 = flat up, 1 = vertical
  base *= mix(1.0, 0.8, slope);               // darker on steep slopes

  vec3 ambient = vec3(0.08);
  vec3 color   = ambient + base * (uAlbedo.rgb * ndl);

  oColor = vec4(color, 1.0);
}