#version 460
// Flat shading: the per-face normal is reconstructed from screen-space
// derivatives of the model-space position, so the vertex stream needs no
// normals. color.a selects shaded (1) vs constant colour (0, for wireframe).

layout(location = 0) in vec3 vModelPos;
layout(location = 0) out vec4 outColor;

layout(push_constant) uniform PC {
    mat4 mvp;
    vec4 color;
} pc;

void main() {
    vec3 dx = dFdx(vModelPos);
    vec3 dy = dFdy(vModelPos);
    vec3 c  = cross(dx, dy);
    float len = length(c);
    vec3 n  = (len > 1e-8) ? c / len : vec3(0.0, 0.0, 1.0);

    const vec3 L = normalize(vec3(0.4, 0.85, 0.55));
    float diff   = abs(dot(n, L));            // two-sided
    float shade  = clamp(0.25 + 0.8 * diff, 0.0, 1.0);

    vec3 base = pc.color.rgb;
    vec3 rgb  = mix(base, base * shade, pc.color.a);
    outColor  = vec4(rgb, 1.0);
}