3dtransform.rst

3D transform

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

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    from matplotlib.pylab import *
    import dtcwt

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

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    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 :py:class:`dtcwt.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

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    :context:

    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 :ref:`3d-directional-example`.