3dtransform.rst

3D transform

In the examples below I assume you've imported pyplot and numpy and, of course, the dtcwt library itself

from matplotlib.pylab import * import dtcwt

We can demonstrate the 3D transform by generating a 64x64x64 array which contains the image of a sphere

GRID_SIZE = 64 SPHERE_RAD = int(0.45 * GRID_SIZE) + 0.5

grid = np.arange(-(GRID_SIZE>>1), GRID_SIZE>>1) X, Y, Z = np.meshgrid(grid, grid, grid) r = np.sqrt(X*X + Y*Y + Z*Z)

sphere = 0.5 + 0.5 * np.clip(SPHERE_RAD-r, -1, 1)

trans = dtcwt.Transform3d() sphere_t = trans.forward(sphere, nlevels=2)

The function returns a :pydtcwt.Pyramid instance containing the lowpass image and a tuple of complex highpass coefficients

>>> print(sphere_t.lowpass.shape)
(16, 16, 16)
>>> for highpasses in sphere_t.highpasses:
...     print(highpasses.shape)
(32, 32, 32, 28)
(16, 16, 16, 28)
(8, 8, 8, 28)

Performing the inverse transform should result in perfect reconstruction

>>> Z = trans.inverse(sphere_t)
>>> print(np.abs(Z - sphere).max()) # Should be < 1e-12
8.881784197e-15

If you plot the locations of the large complex coefficients, you can see the directional sensitivity of the transform

from mpl_toolkits.mplot3d import Axes3D

figure() imshow(sphere[:,:,GRID_SIZE>>1], interpolation='none', cmap=cm.gray) title('2d slice from input sphere')

# Plot large magnitude wavelet coefficients' position in 3D.

figure(figsize=(16,9)) Yh = sphere_t.highpasses nplts = Yh[-1].shape[3] nrows = np.ceil(np.sqrt(nplts)) ncols = np.ceil(nplts / nrows) W = np.max(Yh[-1].shape[:3]) for idx in range(Yh[-1].shape[3]): C = np.abs(Yh[-1][:,:,:,idx]) ax = gcf().add_subplot(nrows, ncols, idx+1, projection='3d') ax.set_aspect('equal') good = C > 0.2*C.max() x,y,z = np.nonzero(good) ax.scatter(x, y, z, c=C[good].ravel()) ax.auto_scale_xyz((0,W), (0,W), (0,W))

tight_layout()

For a further directional sensitivity example, see 3d-directional-example.