Horizon fails with a single channel 4D volume

[pjsjongsung]

Description

The recent change of code for detecting unique values uses image shape for sampling random integers. This will throw an error when dealing with single channel 4D volumes since it would try to sample from a range of 0 to 0.

Way to reproduce

from dipy.viz.horizon.app import horizon
from dipy.io.image import load_nifti

image, affine = load_nifti(some_file_path_with_single_channel)

horizon(images=[(image, affine)])

[Garyfallidis]

Do you mean with data that have the following shape? For example (128, 128, 65, 1)?

[Garyfallidis]

People can currently use dipy_split to get the "first" volume. But indeed this should be fixed.