#version 140

#define INTENSITY_CORRECTION 0.6

// normalized values for (-0.6/1.31, 0.6/1.31, 1./1.31)
const vec3 LIGHT_TOP_DIR = vec3(-0.4574957, 0.4574957, 0.7624929);
#define LIGHT_TOP_DIFFUSE    (0.8 * INTENSITY_CORRECTION)
#define LIGHT_TOP_SPECULAR   (0.125 * INTENSITY_CORRECTION)
#define LIGHT_TOP_SHININESS  20.0

// normalized values for (1./1.43, 0.2/1.43, 1./1.43)
const vec3 LIGHT_FRONT_DIR = vec3(0.6985074, 0.1397015, 0.6985074);
#define LIGHT_FRONT_DIFFUSE  (0.3 * INTENSITY_CORRECTION)

#define INTENSITY_AMBIENT    0.3

// vertex attributes
in vec3 v_position;
in vec3 v_normal;
// instance attributes
in vec3 i_offset;
in vec2 i_scales;

in mat4 instanceMatrix;
uniform mat4 view_matrix;
uniform mat4 projection_matrix;

// x = tainted, y = specular;
out vec2 intensity;

mat3 inverse_mat3(mat3 m) {
  float a00 = m[0][0], a01 = m[0][1], a02 = m[0][2];
  float a10 = m[1][0], a11 = m[1][1], a12 = m[1][2];
  float a20 = m[2][0], a21 = m[2][1], a22 = m[2][2];

  float b01 = a22 * a11 - a12 * a21;
  float b11 = -a22 * a10 + a12 * a20;
  float b21 = a21 * a10 - a11 * a20;

  float det = a00 * b01 + a01 * b11 + a02 * b21;

  return mat3(b01, (-a22 * a01 + a02 * a21), (a12 * a01 - a02 * a11),
              b11, (a22 * a00 - a02 * a20), (-a12 * a00 + a02 * a10),
              b21, (-a21 * a00 + a01 * a20), (a11 * a00 - a01 * a10)) / det;
}

void main()
{
    mat4 view_model_matrix = view_matrix * instanceMatrix;

    // First transform the normal into camera space and normalize the result.
    vec3 normal = normalize(transpose(inverse_mat3(mat3(view_model_matrix))) * v_normal);

    // Compute the cos of the angle between the normal and lights direction. The light is directional so the direction is constant for every vertex.
    // Since these two are normalized the cosine is the dot product. We also need to clamp the result to the [0,1] range.
    float NdotL = max(dot(normal, LIGHT_TOP_DIR), 0.0);

    intensity.x = INTENSITY_AMBIENT + NdotL * LIGHT_TOP_DIFFUSE;
    vec4 position = view_model_matrix * vec4(v_position, 1.0);
    intensity.y = LIGHT_TOP_SPECULAR * pow(max(dot(-normalize(position.xyz), reflect(-LIGHT_TOP_DIR, normal)), 0.0), LIGHT_TOP_SHININESS);

    // Perform the same lighting calculation for the 2nd light source (no specular applied).
    NdotL = max(dot(normal, LIGHT_FRONT_DIR), 0.0);
    intensity.x += NdotL * LIGHT_FRONT_DIFFUSE;

    gl_Position = projection_matrix * position;
}