path: root/vertex_deformation.slang

#ifndef __CUSTOM31_INC
#define __CUSTOM31_INC

#define PI      3.14159265f
#define PI_RCP  0.31830988f
#define SQRT_2      1.41421356f
#define RCP_SQRT_2  0.70710678f
#define TAU     (2.0f * PI)
#define HALF_PI (0.5f * PI)
#define RCP_PI  (1.0f / PI)
#define RCP_TAU (1.0f / TAU)

#define glsl_mod(x,y) (((x)-(y)*floor((x)/(y))))

// Differentiable versions of common operators.
#define dabs(x) sqrt((x) * (x) + 1e-6f)
#define dmin(a, b) (0.5f * ((a) + (b) - dabs((a) - (b))))
#define dmax(a, b) (0.5f * ((a) + (b) + dabs((a) - (b))))
#define dlerp(x, y, t) ((x) * (1.0f-t) + (y) * t)
// This was derived using fourier analysis. See Scripts/approximate.py.
#define dfrac(x) \
  4.997559e-01 - \
  3.183096e-01 * sin(6.283185e+00*(x)) - \
  1.591544e-01 * sin(1.256637e+01*(x)) - \
  1.061025e-01 * sin(1.884956e+01*(x)) - \
  7.957647e-02 * sin(2.513274e+01*(x))

// Quintic interpolation for C2 continuity (smoother derivatives)
#define quintic(t) ((t) * (t) * (t) * ((t) * ((t) * 6.0f - 15.0f) + 10.0f))
#define d_quintic(t) (30.0f * (t) * (t) * ((t) * ((t) - 2.0f) + 1.0f))

// Macros for transforming normal and tangent using autodiff.
// r3r3 refers to "r3 to r3 transform", aka a mapping between 3d real-valued
// spaces.
#define R3R3_DECLARE_BASIS_VECTORS(xyz) \
  DifferentialPair<float3> dp_x = diffPair(xyz, float3(1, 0, 0)); \
  DifferentialPair<float3> dp_y = diffPair(xyz, float3(0, 1, 0)); \
  DifferentialPair<float3> dp_z = diffPair(xyz, float3(0, 0, 1))

#define R3R3_AUTODIFF_BASIS_VECTORS(fun, ...) \
  DifferentialPair<float3> dp_x_out = fwd_diff(fun)(dp_x, __VA_ARGS__); \
  DifferentialPair<float3> dp_y_out = fwd_diff(fun)(dp_y, __VA_ARGS__); \
  DifferentialPair<float3> dp_z_out = fwd_diff(fun)(dp_z, __VA_ARGS__)

#define R3R3_DEFORM_NORMAL_AND_TANGENT(normal, tangent)   \
  float3x3 jacobian = float3x3(                           \
    float3(dp_x_out.d.x, dp_y_out.d.x, dp_z_out.d.x),     \
    float3(dp_x_out.d.y, dp_y_out.d.y, dp_z_out.d.y),     \
    float3(dp_x_out.d.z, dp_y_out.d.z, dp_z_out.d.z)      \
  );                                                      \
  float jac_det = determinant(jacobian);                  \
  float3x3 itjac = inverse(transpose(jacobian), jac_det); \
  normal = mul(itjac, normal) * jac_det;            \
  tangent = mul(jacobian, tangent) * jac_det

// Syntactic sugar - wraps the previous three macros.
#define R3R3_NORMALS(xyz, normal, tangent, fun, ...)  \
  R3R3_DECLARE_BASIS_VECTORS(xyz);                    \
  R3R3_AUTODIFF_BASIS_VECTORS(fun, __VA_ARGS__);      \
  R3R3_DEFORM_NORMAL_AND_TANGENT(normal, tangent);    \
  xyz = fun(xyz, __VA_ARGS__)

[Differentiable]
float3x3 inverse(no_diff float3x3 m, no_diff float det) {
  det = (det < 0.0f ? -1.0f : 1.0f) * max(1e-6f, abs(det));

  float invDet = 1.0f / det;
  float3x3 inv;

  inv._11 = (m._22 * m._33 - m._23 * m._32) * invDet;
  inv._12 = (m._13 * m._32 - m._12 * m._33) * invDet;
  inv._13 = (m._12 * m._23 - m._13 * m._22) * invDet;
  inv._21 = (m._23 * m._31 - m._21 * m._33) * invDet;
  inv._22 = (m._11 * m._33 - m._13 * m._31) * invDet;
  inv._23 = (m._13 * m._21 - m._11 * m._23) * invDet;
  inv._31 = (m._21 * m._32 - m._22 * m._31) * invDet;
  inv._32 = (m._31 * m._12 - m._11 * m._32) * invDet;
  inv._33 = (m._11 * m._22 - m._12 * m._21) * invDet;

  return inv;
}

float2 project_x_onto_y(float2 x, float2 y) {
  return (dot(x, y) / dot(y, y)) * y;
}

// Maps a 2x2 quad to a tube.
// `s_cart` is the axis along which the tube is rolled.
// `r_cart` is an axis along which points are not transformed. It is assumed to
// be orthogonal `s_cart`.
[Differentiable]
public float3 plane_to_tube(float3 xyz,
    no_diff float3 p, no_diff float3 r_cart, no_diff float3 s_cart,
    no_diff float t) {
  // Convert from cartesian to (r, s, r x s) space.
  // Ensure orthonormal basis vectors.
  // TODO remove normalize, do at higher level of stack.
  r_cart = normalize(r_cart);
  s_cart = normalize(s_cart);
  float3 rxs_cart = cross(s_cart, r_cart);
  float3x3 to_rsrxs = float3x3(r_cart, s_cart, rxs_cart);
  // Inverse of orthonormal matrix is just the transpose.
  float3x3 to_cart  = transpose(to_rsrxs);

  // Translate origin to `p` then change into (r, s, r x s) basis.
  xyz = mul(to_rsrxs, xyz - p);

  // Components in pivot basis: n (neutral axis), b (tangential), h (axial s).
  float epsilon = 1e-4f;
  float r = xyz.x;     // Lr
  float s = xyz.y;     // Lv0 x Lr
  float rxs = xyz.z;   // Lv0

  // Blend between two vectors:
  //  v0: vector of length ||Lr + Lv0|| at angle atan2(Lv0, Lr)
  //  v1: vector of length ||Lr|| at angle ||Lv0|| / ||Lr||
  // Interpolate in polar coordinates to make it wrap nicely.
  float r0 = length(float2(r, rxs));
  float r1 = max(abs(r), epsilon);

  float theta0 = atan2(rxs, r);
  float theta1 = rxs / r1;

  // Interpolate polar coordinates.
  float radius = dlerp(r0, r1, t);
  float theta = dlerp(theta0, theta1, t);

  // Map into (r, rxs) basis.
  float2 nb_t = float2(cos(theta), sin(theta)) * radius;

  // Un-project from (r, rxs) plane to full (r, s, rxs) basis.
  xyz = float3(nb_t.x, s, nb_t.y);

  // Map back to cartesian basis.
  xyz = mul(to_cart, xyz) + p;

  return xyz;
}

public void plane_to_tube_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 r, float3 s, float t) {
  R3R3_NORMALS(xyz, normal, tangent, plane_to_tube, p, r, s, t);
}

[Differentiable]
public float3 axis_align(float3 xyz, no_diff float3 po, no_diff float3 pp, no_diff float3 r, no_diff float t) {
  const float3 xyz0 = xyz;

  // We assume that `s` is orthogonal to `r`, and that `r` is normalized.
  float3 s = normalize(pp - po);
  float3 sxr = cross(s, r);

  float3x3 to_rsrxs = float3x3(r, s, sxr);
  // Inverse of orthonormal matrix is just the transpos.
  float3x3 to_cart  = transpose(to_rsrxs);

  // Move key vectors into (r, s, sxr) basis centered at `po`, per the derivation.
  float3 xyz_rsrxs = mul(to_rsrxs, xyz - po);

  float vr = xyz_rsrxs[0];
  float vs = xyz_rsrxs[1];
  float2 v0 = float2(vr, vs);
  float3 pp_rsrxs = mul(to_rsrxs, pp - po);
  float qr = pp_rsrxs[1] * vr / vs;  // TODO epsilon
  float2 q = float2(qr, pp_rsrxs[1]);

  // Translate to `q`.
  v0 -= q;

  // Project onto `s`.
  v0.x = 0;

  // Translate back to `po`.
  v0 += q;

  xyz_rsrxs[0] = v0[0];
  xyz_rsrxs[1] = v0[1];

  // Move back into cartesian space.
  xyz = mul(to_cart, xyz_rsrxs) + po;

  return dlerp(xyz0, xyz, t);
}

public void axis_align_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 po, float3 pp, float3 r, float t) {
  R3R3_NORMALS(xyz, normal, tangent, axis_align, po, pp, r, t);
}

// Maps a tube with circular cross section on the xz plane to a quad on the xy
// plane.
[Differentiable]
public float3 tube_to_plane(float3 xyz,
    no_diff float3 p, no_diff float3 r_cart, no_diff float3 s_cart,
    no_diff float t) {
  // Convert from cartesian to (r, s, r x s) space.
  // Ensure orthonormal basis vectors.
  // TODO remove normalize, do at higher level of stack.
  r_cart = normalize(r_cart);
  s_cart = normalize(s_cart);
  float3 rxs_cart = cross(s_cart, r_cart);
  float3x3 to_rsrxs = float3x3(r_cart, s_cart, rxs_cart);
  float3x3 to_cart  = inverse(to_rsrxs, determinant(to_rsrxs));

  // Translate origin to `p` then change into (r, s, r x s) basis.
  xyz = mul(to_rsrxs, xyz - p);

  // Components in pivot basis: n (neutral axis), b (tangential), h (axial s).
  float epsilon = 1e-4f;
  float r = xyz.x;     // Lr
  float s = xyz.y;     // Lv0 x Lr
  float rxs = xyz.z;   // Lv0
  float2 v0 = float2(r, rxs);

  float theta0 = atan2(rxs, r);
  float Lr  = length(v0);
  float Lv0 = theta0 * Lr;
  float phi = atan2(Lv0, Lr);
  float rr = length(float2(Lr, Lv0));

  // Blend between two vectors:
  //  v0: vector of length ||Lr + Lv0|| at angle atan2(Lv0, Lr)
  //  v1: vector of length ||Lr|| at angle ||Lv0|| / ||Lr||
  // Interpolate in polar coordinates to make it wrap nicely.
  float r0 = Lr;
  float r1 = rr;

  float theta1 = phi;

  // Interpolate polar coordinates.
  float radius = dlerp(r0, r1, t);
  float theta = dlerp(theta0, theta1, t);

  // Map into (r, rxs) basis.
  float2 nb_t = float2(cos(theta), sin(theta)) * radius;

  // Un-project from (r, rxs) plane to full (r, s, rxs) basis.
  xyz = float3(nb_t.x, s, nb_t.y);

  // Map back to cartesian basis.
  xyz = mul(to_cart, xyz) + p;

  return xyz;
}

public void tube_to_plane_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 r, float3 s, float t) {
  R3R3_NORMALS(xyz, normal, tangent, tube_to_plane, p, r, s, t);
}

[Differentiable]
public float3 point_align(float3 xyz, no_diff float3 po, no_diff float3 pp, no_diff float3 r, no_diff float t) {
  const float3 xyz0 = xyz;

  // We assume that `s` is orthogonal to `r`, and that `r` is normalized.
  float3 s = normalize(pp - po);
  float3 sxr = cross(s, r);

  float3x3 to_rsrxs = float3x3(r, s, sxr);
  // Inverse of orthonormal matrix is just the transpose.
  float3x3 to_cart  = transpose(to_rsrxs);

  // Move key vectors into (r, s, sxr) basis centered at `po`, per the derivation.
  float3 xyz_rsrxs = mul(to_rsrxs, xyz - po);
  float3 pp_rsrxs  = mul(to_rsrxs, pp - po);

  float2 v1 = xyz_rsrxs.xy;
  float v1r = v1.x;
  float v1s = v1.y;

  float vs = v1.y;
  float qs = pp_rsrxs.y;
  float qr = v1.x;
  float2 q = float2(qr, qs);
  q *= vs / qs;

  xyz_rsrxs.xy = q;

  // Move back into cartesian space.
  xyz = mul(to_cart, xyz_rsrxs) + po;

  return dlerp(xyz0, xyz, t);
}

public void point_align_normal(inout float3 xyz, inout float3 normal, inout float3 tangent, float3 po, float3 pp, float3 r, float t) {
  R3R3_NORMALS(xyz, normal, tangent, point_align, po, pp, r, t);
}

[Differentiable]
public float3 seal(float3 xyz, no_diff float A, no_diff float k, no_diff float t) {
  float x = xyz.x;
  float y = xyz.y;
  float z = xyz.z;

  float x0 = x + sin(y * k) * 0.1;
  float y0 = y + sin(x * k * 2) * 0.5 + cos(z);
  float z0 = (z + sin(x * y * k * PI + t)) * A * (1.0 + sin(y * k) * sin(x * k));

  x0 += z0 * 0.1 * sin(z0 * PI + 1.5);
  y0 += z0 * 0.1 * sin(z0 * PI + 1.5);

  x0 -= 0.0;
  y0 -= 1.2;

  return float3(
      x0, y0, z0
  );
}

public void seal_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float A, float k, float t) {
  R3R3_NORMALS(xyz, normal, tangent, seal, A, k, t);
}

[Differentiable]
public float3 sine_wave(float3 xyz,
    no_diff float3 amplitude,
    no_diff float3 direction,
    no_diff float3 k,
    no_diff float3 omega,
    no_diff float t) {
  xyz += amplitude * sin(k * dot(xyz, direction) + omega * t);
  return xyz;
}

public void sine_wave_normal(inout float3 xyz, inout float3 normal, inout float3 tangent,
    float3 amplitude, float3 direction, float3 k, float3 omega, float t) {
  R3R3_NORMALS(xyz, normal, tangent, sine_wave, amplitude, direction, k, omega, t);
}

[Differentiable]
public float3 norm_conversion(float3 xyz, no_diff float input_k, no_diff float output_k, no_diff float t) {
  float3 xyz_abs = dabs(xyz)+1e-4f;
  float lin = pow(pow(xyz_abs.x, input_k) + pow(xyz_abs.y, input_k) + pow(xyz_abs.z, input_k), 1.0f / input_k);
  float lout = pow(pow(xyz_abs.x, output_k) + pow(xyz_abs.y, output_k) + pow(xyz_abs.z, output_k), 1.0f / output_k);
  float scale = dlerp(1.0f, lout / lin, t);
  return xyz * scale;
}

public void norm_conversion_normal(inout float3 xyz, inout float3 normal, inout float3 tangent, float input_k, float output_k, float t) {
  R3R3_NORMALS(xyz, normal, tangent, norm_conversion, input_k, output_k, t);
}

[Differentiable]
float3 rand3_hash3(float3 p)
{
    // Improved Murmurhash3 by Squirrel Eiserloh (GDC 2017)
    p = float3(dot(p, float3(127.1, 311.7, 74.7)),
               dot(p, float3(269.5, 183.3, 246.1)),
               dot(p, float3(113.5, 271.9, 124.6)));
    return frac(sin(p) * 43758.5453123);
}

[Differentiable]
float rand3_hash1(float3 p)
{
    // Improved Murmurhash3 by Squirrel Eiserloh (GDC 2017)
    float p0 = dot(p, float3(127.1, 311.7, 74.7));
    return frac(sin(p0) * 43758.5453123);
}

// Calculate value noise and jacobian in one shot.
// Based on https://iquilezles.org/articles/morenoise/
float3 value_noise(float3 xyz, out float3 dx, out float3 dy, out float3 dz) {
  float3 cell = floor(xyz);
  float3 f = xyz - cell;

  float ux = quintic(f.x);
  float uy = quintic(f.y);
  float uz = quintic(f.z);

  float dux = d_quintic(f.x);
  float duy = d_quintic(f.y);
  float duz = d_quintic(f.z);

  float3 n000 = rand3_hash3(cell + float3(0.0f, 0.0f, 0.0f));
  float3 n001 = rand3_hash3(cell + float3(0.0f, 0.0f, 1.0f));
  float3 n010 = rand3_hash3(cell + float3(0.0f, 1.0f, 0.0f));
  float3 n011 = rand3_hash3(cell + float3(0.0f, 1.0f, 1.0f));
  float3 n100 = rand3_hash3(cell + float3(1.0f, 0.0f, 0.0f));
  float3 n101 = rand3_hash3(cell + float3(1.0f, 0.0f, 1.0f));
  float3 n110 = rand3_hash3(cell + float3(1.0f, 1.0f, 0.0f));
  float3 n111 = rand3_hash3(cell + float3(1.0f, 1.0f, 1.0f));

  float3 n00 = lerp(n000, n001, uz);
  float3 n01 = lerp(n010, n011, uz);
  float3 n10 = lerp(n100, n101, uz);
  float3 n11 = lerp(n110, n111, uz);

  float3 n0 = lerp(n00, n01, uy);
  float3 n1 = lerp(n10, n11, uy);

  float oneMinusUx = 1.0f - ux;
  float oneMinusUy = 1.0f - uy;

  float3 noise_half = lerp(n0, n1, ux);

  float3 dnoise_half_dx = (n1 - n0) * dux;

  float3 dn0_dy = (n01 - n00) * duy;
  float3 dn1_dy = (n11 - n10) * duy;
  float3 dnoise_half_dy = dn0_dy * oneMinusUx + dn1_dy * ux;

  float3 dn00_dz = (n001 - n000) * duz;
  float3 dn01_dz = (n011 - n010) * duz;
  float3 dn10_dz = (n101 - n100) * duz;
  float3 dn11_dz = (n111 - n110) * duz;
  float3 dn0_dz = dn00_dz * oneMinusUy + dn01_dz * uy;
  float3 dn1_dz = dn10_dz * oneMinusUy + dn11_dz * uy;
  float3 dnoise_half_dz = dn0_dz * oneMinusUx + dn1_dz * ux;

  dx = 2.0f * dnoise_half_dx;
  dy = 2.0f * dnoise_half_dy;
  dz = 2.0f * dnoise_half_dz;

  return -1.0f + 2.0f * noise_half;
}

float3 fbm_with_derivatives(float3 xyz,
    no_diff float t,
    no_diff float3 amplitude,
    no_diff float gain,
    no_diff float lacunarity,
    no_diff float3 period,
    no_diff float octaves,
    no_diff float3 velocity,
    out float3 dx,
    out float3 dy,
    out float3 dz) {
  float3 noise = float3(0.0f, 0.0f, 0.0f);
  float3 gain_i = amplitude;
  float3 freq_i = 1.0f / period;

  dx = float3(0.0f, 0.0f, 0.0f);
  dy = float3(0.0f, 0.0f, 0.0f);
  dz = float3(0.0f, 0.0f, 0.0f);

  for (uint i = 0; i < octaves; ++i) {
    float3 dx_i, dy_i, dz_i;
    float3 octave_noise = value_noise((xyz - velocity * t) * freq_i, dx_i, dy_i, dz_i);
    noise += gain_i * octave_noise;
    dx += gain_i * freq_i.x * dx_i;
    dy += gain_i * freq_i.y * dy_i;
    dz += gain_i * freq_i.z * dz_i;
    freq_i *= lacunarity;
    gain_i *= gain;
  }

  dx += float3(1.0f, 0.0f, 0.0f);
  dy += float3(0.0f, 1.0f, 0.0f);
  dz += float3(0.0f, 0.0f, 1.0f);

  return xyz + noise;
}

public float3 fbm(float3 xyz,
    no_diff float t,
    no_diff float3 amplitude,
    no_diff float gain,
    no_diff float lacunarity,
    no_diff float3 period,
    no_diff float octaves,
    no_diff float3 velocity) {
  float3 dx_unused, dy_unused, dz_unused;
  return fbm_with_derivatives(xyz, t, amplitude, gain, lacunarity, period, octaves,
      velocity, dx_unused, dy_unused, dz_unused);
}

public void fbm_normal(inout float3 xyz, inout float3 normal, inout float3 tangent,
    no_diff float t,
    no_diff float3 amplitude, no_diff float gain, no_diff float lacunarity,
    no_diff float3 period, no_diff float octaves, no_diff float3 velocity) {
  float3 dx, dy, dz;
  xyz = fbm_with_derivatives(xyz, t, amplitude, gain, lacunarity, period, octaves,
      velocity, dx, dy, dz);
  float3x3 jac = float3x3(
      dx.x, dy.x, dz.x,
      dx.y, dy.y, dz.y,
      dx.z, dy.z, dz.z);
  float jac_det = determinant(jac);
  float3x3 itjac = inverse(transpose(jac), jac_det);
  normal = mul(itjac, normal) * jac_det;
  tangent = mul(jac, tangent);
}

// Maps a plane on [-1, 1] on xz plane to a hemi-octahedron with radius 1.
[Differentiable]
public float3 plane_to_hemi_octahedron(float3 xyz,
    no_diff float3 p, no_diff float3 r_cart, no_diff float3 s_cart,
    no_diff float t) {
  // Convert from cartesian to (r, s, r x s) space.
  r_cart = normalize(r_cart);
  s_cart = normalize(s_cart);
  float3 rxs_cart = cross(s_cart, r_cart);
  float3x3 to_rsrxs = float3x3(r_cart, s_cart, rxs_cart);
  float3x3 to_cart = transpose(to_rsrxs);

  // Translate origin to `p` then change into (r, s, r x s) basis.
  xyz = mul(to_rsrxs, xyz - p);

  float3 xyz0 = xyz;

  // Extract planar coordinates: x and z form the 2D plane
  float x = xyz.x;
  float z = xyz.z;

  // Rotate 45° and scale to fit square into diamond
  float x_rot = (x + z) * 0.5;
  float z_rot = (z - x) * 0.5;

  // Octahedral decode: y = 1 - |x| - |z|, clamped to hemisphere
  // Use differentiable abs and max for smooth autodiff
  float y = dmax(0.0f, 1.0f - dabs(x_rot) - dabs(z_rot));

  float3 oct_pos = float3(x_rot, y, z_rot);
  float len = dot(oct_pos, oct_pos);
  oct_pos = oct_pos * (1.0f + xyz.y);

  // Rotate back by -45° around y to undo input rotation
  float x_unrot = (oct_pos.x - oct_pos.z) * RCP_SQRT_2;
  float z_unrot = (oct_pos.x + oct_pos.z) * RCP_SQRT_2;
  oct_pos = float3(x_unrot, oct_pos.y, z_unrot);

  // Interpolate between original position and sphere position
  float3 result = dlerp(xyz0, oct_pos, t);

  // Map back to cartesian basis
  xyz = mul(to_cart, result) + p;

  return xyz;
}

public void plane_to_hemi_octahedron_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 r, float3 s, float t) {
  R3R3_NORMALS(xyz, normal, tangent, plane_to_hemi_octahedron, p, r, s, t);
}

// Maps a hemi-octahedron with raidus 1 to a quad on [-1, 1] on the (r, rxs) plane.
[Differentiable]
public float3 hemi_octahedron_to_plane(float3 xyz,
    no_diff float3 p, no_diff float3 r_cart, no_diff float3 s_cart,
    no_diff float t) {
  // Convert from cartesian to (r, s, r x s) space.
  r_cart = normalize(r_cart);
  s_cart = normalize(s_cart);
  float3 rxs_cart = cross(s_cart, r_cart);
  float3x3 to_rsrxs = float3x3(r_cart, s_cart, rxs_cart);
  float3x3 to_cart = transpose(to_rsrxs);

  // Translate origin to `p` then change into (r, s, r x s) basis.
  xyz = mul(to_rsrxs, xyz - p);

  float3 xyz0 = xyz;

  // Undo the -45° unrotation from forward pass (rotate by +45°)
  float x_rot = (xyz.x + xyz.z) * RCP_SQRT_2;
  float z_rot = (xyz.z - xyz.x) * RCP_SQRT_2;

  // The forward pass scales by (1 + s_original), and the unscaled hemi-octahedron
  // has L1 norm = 1, so L1 of the scaled point recovers s_original.
  float L1_scaled = dabs(x_rot) + dabs(xyz.y) + dabs(z_rot);
  float s_original = L1_scaled - 1.0f;

  // Remove (1 + s) scale to get the unit hemi-octahedron point, then decode.
  float L1_inv = 1.0f / L1_scaled;
  float x_oct = x_rot * L1_inv;
  float z_oct = z_rot * L1_inv;

  // Undo the initial 45° rotation (x_rot = (x+z)*0.5, z_rot = (z-x)*0.5)
  // Inverse: x = x_rot - z_rot, z = x_rot + z_rot
  float x_plane = x_oct - z_oct;
  float z_plane = x_oct + z_oct;

  float3 plane_pos = float3(x_plane, s_original, z_plane);

  // Interpolate between original position and plane position
  float3 result = dlerp(xyz0, plane_pos, t);

  // Map back to cartesian basis
  xyz = mul(to_cart, result) + p;

  return xyz;
}

public void hemi_octahedron_to_plane_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 r, float3 s, float t) {
  R3R3_NORMALS(xyz, normal, tangent, hemi_octahedron_to_plane, p, r, s, t);
}

// Maps [-1, 1] on (r, rxs) plane to a unit sphere using octahedral mapping.
[Differentiable]
public float3 plane_to_octahedron(float3 xyz,
    no_diff float3 p, no_diff float3 r_cart, no_diff float3 s_cart,
    no_diff float t) {
  r_cart = normalize(r_cart);
  s_cart = normalize(s_cart);
  float3 rxs_cart = cross(s_cart, r_cart);
  float3x3 to_rsrxs = float3x3(r_cart, s_cart, rxs_cart);
  float3x3 to_cart = transpose(to_rsrxs);

  xyz = mul(to_rsrxs, xyz - p);
  float3 xyz0 = xyz;

  float l1_norm = dabs(xyz.x) + dabs(xyz.z);
  if (l1_norm > 1) {
    xyz.x = sign(xyz0.x) * (1 - dabs(xyz0.z));
    xyz.z = sign(xyz0.z) * (1 - dabs(xyz0.x));
  }
  xyz.y = 1 - l1_norm;

  xyz *= (1 + xyz0.y);

  float3 result = dlerp(xyz0, xyz, t);

  xyz = mul(to_cart, result) + p;
  return xyz;
}

public void plane_to_octahedron_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 r, float3 s, float t) {
  R3R3_NORMALS(xyz, normal, tangent, plane_to_octahedron, p, r, s, t);
}

// Maps a unit sphere to a plane using octahedral mapping.
[Differentiable]
public float3 octahedron_to_plane(float3 xyz,
    no_diff float3 p, no_diff float3 r_cart, no_diff float3 s_cart,
    no_diff float t) {
  r_cart = normalize(r_cart);
  s_cart = normalize(s_cart);
  float3 rxs_cart = cross(s_cart, r_cart);
  float3x3 to_rsrxs = float3x3(r_cart, s_cart, rxs_cart);
  float3x3 to_cart = transpose(to_rsrxs);

  xyz = mul(to_rsrxs, xyz - p);
  float3 xyz0 = xyz;

  // The forward pass scales by (1 + s_original), and the unscaled octahedron
  // has L1 norm = 1, so L1 of the scaled point recovers s_original.
  float l1_norm = dabs(xyz.x) + dabs(xyz.y) + dabs(xyz.z);
  float s_original = l1_norm - 1.0f;
  xyz /= l1_norm;
  float2 xz_tmp = xyz.xz;
  if (xyz.y < 0) {
    xyz.x = sign(xz_tmp[0]) * (1 - dabs(xz_tmp[1]));
    xyz.z = sign(xz_tmp[1]) * (1 - dabs(xz_tmp[0]));
  }
  xyz.y = s_original;

  float3 result = dlerp(xyz0, xyz, t);
  xyz = mul(to_cart, result) + p;
  return xyz;
}

public void octahedron_to_plane_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 r, float3 s, float t) {
  R3R3_NORMALS(xyz, normal, tangent, octahedron_to_plane, p, r, s, t);
}

[Differentiable]
public float3 scale(float3 xyz,
    no_diff float3 k, no_diff float t) {
  return dlerp(xyz, xyz * k, t);
}

public void scale_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 k, float t) {
  R3R3_NORMALS(xyz, normal, tangent, scale, k, t);
}

[Differentiable]
public float3 translate(float3 xyz,
    no_diff float3 offset, no_diff float t) {
  return xyz + offset * t;
}

public void translate_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 offset, float t) {
  R3R3_NORMALS(xyz, normal, tangent, translate, offset, t);
}

[Differentiable]
public float3 rotate(float3 xyz,
    no_diff float3 p, no_diff float3 axis, no_diff float angle, no_diff float t) {
  float theta = angle * t;
  float3 a = normalize(axis);
  float c = cos(theta);
  float s = sin(theta);
  float3 v = xyz - p;
  // Rodrigues' rotation formula
  float3 rotated = v * c + cross(a, v) * s + a * dot(a, v) * (1.0f - c);
  return rotated + p;
}

public void rotate_normal(inout float3 xyz, inout float3 normal,
    inout float3 tangent, float3 p, float3 axis, float angle, float t) {
  R3R3_NORMALS(xyz, normal, tangent, rotate, p, axis, angle, t);
}

#endif  // __CUSTOM31_INC