sources/neuraloperator/examples/layers/plot_fourier_diff.py

"""
.. _fourier_diff :

Fourier Differentiation
======================

This tutorial demonstrates Fourier-based differentiation methods for computing
derivatives of periodic functions. Fourier differentiation is crucial for:

- Computing derivatives of functions with spectral accuracy
- Implementing physics-informed loss functions

The `FourierDiff` class provides efficient implementations of spectral
differentiation for periodic functions in 1D, 2D, and 3D domains.
"""

# %%
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#    <div style="margin-top: 3em;"></div>
#
# Import the library
# ------------------
# We first import our `neuralop` library and required dependencies.
import torch
import numpy as np
import matplotlib.pyplot as plt
from neuralop.losses.differentiation import FourierDiff

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")


# %%
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#    <div style="margin-top: 3em;"></div>
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# Creating an example of periodic 1D curve
# ----------------------------------------
# Here we consider sin(x) and cos(x), which are periodic on the interval [0, 2π]
L = 2 * torch.pi
x = torch.linspace(0, L, 101)[:-1]
f = torch.stack([torch.sin(x), torch.cos(x)], dim=0)
x_np = x.cpu().numpy()

# %%
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#    <div style="margin-top: 3em;"></div>
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# Differentiate the signal
# -----------------------------------------
# We use the FourierDiff class to differentiate the signal
fd1d = FourierDiff(dim=1, L=L, use_fc=False)
derivatives = fd1d.compute_multiple_derivatives(f, [1, 2, 3])
dfdx, df2dx2, df3dx3 = derivatives


# %%
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#    <div style="margin-top: 3em;"></div>
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# Plot the results for sin(x)
# ---------------------------
plt.figure()
plt.plot(x_np, dfdx[0].squeeze().cpu().numpy(), label="Fourier dfdx")
plt.plot(x_np, np.cos(x_np), "--", label="dfdx")
plt.plot(x_np, df2dx2[0].squeeze().cpu().numpy(), label="Fourier df2dx2")
plt.plot(x_np, -np.sin(x_np), "--", label="df2dx2")
plt.plot(x_np, df3dx3[0].squeeze().cpu().numpy(), label="Fourier df3dx3")
plt.plot(x_np, -np.cos(x_np), "--", label="df3dx3")
plt.xlabel("x")
plt.legend()
plt.show()

# %%
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#    <div style="margin-top: 3em;"></div>
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# Plot the results for cos(x)
# ---------------------------
plt.figure()
plt.plot(x_np, dfdx[1].squeeze().cpu().numpy(), label="Fourier dfdx")
plt.plot(x_np, -np.sin(x_np), "--", label="dfdx")
plt.plot(x_np, df2dx2[1].squeeze().cpu().numpy(), label="Fourier df2dx2")
plt.plot(x_np, -np.cos(x_np), "--", label="df2dx2")
plt.plot(x_np, df3dx3[1].squeeze().cpu().numpy(), label="Fourier df3dx3")
plt.plot(x_np, np.sin(x_np), "--", label="df3dx3")
plt.xlabel("x")
plt.legend()
plt.show()


# %%
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#    <div style="margin-top: 3em;"></div>
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# Creating an example of non-periodic 1D curve
# -------------------------------------------
# Here we consider sin(3x)-cos(x) and exp(-0.8x)+sin(x)
L = 2 * torch.pi
x = torch.linspace(0, L, 101)[:-1]
f = torch.stack(
    [torch.sin(3 * x) - torch.cos(x), torch.exp(-0.8 * x) + torch.sin(x)], dim=0
)
x_np = x.cpu().numpy()

# %%
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#    <div style="margin-top: 3em;"></div>
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# Differentiate the signal
# -----------------------------------------
# We use the FourierDiff class with Fourier continuation to differentiate the signal
fd1d = FourierDiff(dim=1, L=L, use_fc="Legendre", fc_degree=4, fc_n_additional_pts=50)
derivatives = fd1d.compute_multiple_derivatives(f, [1, 2])
dfdx, df2dx2 = derivatives


# %%
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#    <div style="margin-top: 3em;"></div>
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# Plot the results for sin(3x)-cos(x)
# --------------------------------------
plt.figure()
plt.plot(x_np, dfdx[0].squeeze().cpu().numpy(), label="Fourier dfdx")
plt.plot(x_np, 3 * torch.cos(3 * x) + torch.sin(x), "--", label="dfdx")
plt.plot(x_np, df2dx2[0].squeeze().cpu().numpy(), label="Fourier df2dx2")
plt.plot(x_np, -9 * torch.sin(3 * x) + torch.cos(x), "--", label="df2dx2")
plt.xlabel("x")
plt.legend()
plt.show()

# %%
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#    <div style="margin-top: 3em;"></div>
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# Plot the results for exp(-0.8x)+sin(x)
# ---------------------------------------
plt.figure()
plt.plot(x_np, dfdx[1].squeeze().cpu().numpy(), label="Fourier dfdx")
plt.plot(x_np, -0.8 * torch.exp(-0.8 * x) + torch.cos(x), "--", label="dfdx")
plt.plot(x_np, df2dx2[1].squeeze().cpu().numpy(), label="Fourier df2dx2")
plt.plot(x_np, 0.64 * torch.exp(-0.8 * x) - torch.sin(x), "--", label="df2dx2")
plt.xlabel("x")
plt.legend()
plt.show()


# %%
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# 2D Fourier Differentiation Examples
# ===================================
# Here we demonstrate the FourierDiff class for 2D functions

# %%
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#    <div style="margin-top: 3em;"></div>
#
# Creating an example of periodic 2D function
# -----------------------------------------
# Here we consider f(x,y) = sin(x) * cos(y), which is periodic on the interval [0, 2π] × [0, 2π]
L_x, L_y = 2 * torch.pi, 2 * torch.pi
nx, ny = 180, 186
x = torch.linspace(0, L_x, nx, dtype=torch.float64)
y = torch.linspace(0, L_y, ny, dtype=torch.float64)
X, Y = torch.meshgrid(x, y, indexing="ij")

# Test function: f(x,y) = sin(x) * cos(y)
f_2d = torch.sin(X) * torch.cos(Y)

# %%
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# Differentiate the 2D signal
# -----------------------------------------
# We use the FourierDiff class to compute derivatives
fd2d = FourierDiff(dim=2, L=(L_x, L_y))

# Compute derivatives
df_dx = fd2d.dx(f_2d)
df_dy = fd2d.dy(f_2d)
laplacian = fd2d.laplacian(f_2d)

# Expected analytical results for f(x,y) = sin(x) * cos(y)
df_dx_expected = torch.cos(X) * torch.cos(Y)
df_dy_expected = -torch.sin(X) * torch.sin(Y)
laplacian_expected = -2 * torch.sin(X) * torch.cos(Y)

# %%
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# Plot the 2D results
# ----------------------
fig, axes = plt.subplots(2, 3, figsize=(15, 10))
fig.suptitle("2D Fourier Differentiation Results: f(x,y) = sin(x) * cos(y)")

# Compute consistent colorbar limits for each derivative pair
df_dx_min = min(df_dx.min().item(), df_dx_expected.min().item())
df_dx_max = max(df_dx.max().item(), df_dx_expected.max().item())
df_dy_min = min(df_dy.min().item(), df_dy_expected.min().item())
df_dy_max = max(df_dy.max().item(), df_dy_expected.max().item())

# Original function
im0 = axes[0, 0].imshow(f_2d.cpu().numpy())
axes[0, 0].set_title("Original: sin(x) * cos(y)")
plt.colorbar(im0, ax=axes[0, 0], shrink=0.57)

# ∂f/∂x computed
im1 = axes[0, 1].imshow(df_dx.cpu().numpy(), vmin=df_dx_min, vmax=df_dx_max)
axes[0, 1].set_title("∂f/∂x (computed)")
plt.colorbar(im1, ax=axes[0, 1], shrink=0.57)

# ∂f/∂x expected
im2 = axes[0, 2].imshow(df_dx_expected.cpu().numpy(), vmin=df_dx_min, vmax=df_dx_max)
axes[0, 2].set_title("∂f/∂x (expected: cos(x) * cos(y))")
plt.colorbar(im2, ax=axes[0, 2], shrink=0.57)

# ∂f/∂y computed
im3 = axes[1, 0].imshow(df_dy.cpu().numpy(), vmin=df_dy_min, vmax=df_dy_max)
axes[1, 0].set_title("∂f/∂y (computed)")
plt.colorbar(im3, ax=axes[1, 0], shrink=0.57)

# ∂f/∂y expected
im4 = axes[1, 1].imshow(df_dy_expected.cpu().numpy(), vmin=df_dy_min, vmax=df_dy_max)
axes[1, 1].set_title("∂f/∂y (expected: -sin(x) * sin(y))")
plt.colorbar(im4, ax=axes[1, 1], shrink=0.57)

# Laplacian
im5 = axes[1, 2].imshow(laplacian.cpu().numpy())
axes[1, 2].set_title("∇²f (computed)")
plt.colorbar(im5, ax=axes[1, 2], shrink=0.57)

plt.tight_layout()
plt.show()


# %%
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# 3D Fourier Differentiation Examples
# ===================================
# Here we demonstrate the FourierDiff class for 3D functions

# %%
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#
# Creating an example of periodic 3D function
# -----------------------------------------
# Here we consider f(x,y,z) = sin(x) * cos(y) * sin(z), which is periodic on [0, 2π]³
L_x, L_y, L_z = 2 * torch.pi, 2 * torch.pi, 2 * torch.pi
nx, ny, nz = 176, 180, 192
x = torch.linspace(0, L_x, nx, dtype=torch.float64)
y = torch.linspace(0, L_y, ny, dtype=torch.float64)
z = torch.linspace(0, L_z, nz, dtype=torch.float64)
X, Y, Z = torch.meshgrid(x, y, z, indexing="ij")

# Test function: f(x,y,z) = sin(x) * cos(y) * sin(z)
f_3d = torch.sin(X) * torch.cos(Y) * torch.sin(Z)

# Alternative: create tensor directly like in the test
f_3d_alt = torch.randn(nx, ny, nz, dtype=torch.float64)

# %%
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# Differentiate the 3D signal
# -----------------------------------------
# We use the FourierDiff class to compute derivatives
fd3d = FourierDiff(dim=3, L=(L_x, L_y, L_z))

# Compute derivatives
df_dx_3d = fd3d.dx(f_3d)
df_dy_3d = fd3d.dy(f_3d)
df_dz_3d = fd3d.dz(f_3d)
laplacian_3d = fd3d.laplacian(f_3d)

# Expected analytical results for f(x,y,z) = sin(x) * cos(y) * sin(z)
df_dx_expected_3d = torch.cos(X) * torch.cos(Y) * torch.sin(Z)
df_dy_expected_3d = -torch.sin(X) * torch.sin(Y) * torch.sin(Z)
df_dz_expected_3d = torch.sin(X) * torch.cos(Y) * torch.cos(Z)
laplacian_expected_3d = -3 * torch.sin(X) * torch.cos(Y) * torch.sin(Z)

# %%
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# Plot a slice of the 3D results (z=0 plane)
# ------------------------------------------
z_slice_idx = nz // 2
fig, axes = plt.subplots(2, 3, figsize=(18, 12))
fig.suptitle("3D Fourier Differentiation Results (z=0 slice): f(x,y,z) = sin(x) * cos(y) * sin(z)")

# Compute consistent colorbar limits for each derivative pair at the z-slice
df_dx_3d_slice = df_dx_3d[:, :, z_slice_idx]
df_dx_expected_3d_slice = df_dx_expected_3d[:, :, z_slice_idx]
df_dy_3d_slice = df_dy_3d[:, :, z_slice_idx]
df_dy_expected_3d_slice = df_dy_expected_3d[:, :, z_slice_idx]

df_dx_3d_min = min(df_dx_3d_slice.min().item(), df_dx_expected_3d_slice.min().item())
df_dx_3d_max = max(df_dx_3d_slice.max().item(), df_dx_expected_3d_slice.max().item())
df_dy_3d_min = min(df_dy_3d_slice.min().item(), df_dy_expected_3d_slice.min().item())
df_dy_3d_max = max(df_dy_3d_slice.max().item(), df_dy_expected_3d_slice.max().item())

# Original function slice
im0 = axes[0, 0].imshow(f_3d[:, :, z_slice_idx].cpu().numpy())
axes[0, 0].set_title("Original: sin(x) * cos(y) * sin(z)")
plt.colorbar(im0, ax=axes[0, 0], shrink=0.57)

# ∂f/∂x slice
im1 = axes[0, 1].imshow(df_dx_3d_slice.cpu().numpy(), vmin=df_dx_3d_min, vmax=df_dx_3d_max)
axes[0, 1].set_title("∂f/∂x (computed)")
plt.colorbar(im1, ax=axes[0, 1], shrink=0.57)

# ∂f/∂x expected slice
im2 = axes[0, 2].imshow(df_dx_expected_3d_slice.cpu().numpy(), vmin=df_dx_3d_min, vmax=df_dx_3d_max)
axes[0, 2].set_title("∂f/∂x (expected: cos(x) * cos(y) * sin(z))")
plt.colorbar(im2, ax=axes[0, 2], shrink=0.57)

# ∂f/∂y slice
im3 = axes[1, 0].imshow(df_dy_3d_slice.cpu().numpy(), vmin=df_dy_3d_min, vmax=df_dy_3d_max)
axes[1, 0].set_title("∂f/∂y (computed)")
plt.colorbar(im3, ax=axes[1, 0], shrink=0.57)

# ∂f/∂y expected slice
im4 = axes[1, 1].imshow(df_dy_expected_3d_slice.cpu().numpy(), vmin=df_dy_3d_min, vmax=df_dy_3d_max)
axes[1, 1].set_title("∂f/∂y (expected: -sin(x) * sin(y) * sin(z))")
plt.colorbar(im4, ax=axes[1, 1], shrink=0.57)

# ∂f/∂z slice
im5 = axes[1, 2].imshow(df_dz_3d[:, :, z_slice_idx].cpu().numpy())
axes[1, 2].set_title("∂f/∂z (computed)")
plt.colorbar(im5, ax=axes[1, 2], shrink=0.57)

plt.tight_layout()
plt.show()