Handle negative z values correctly

src/empyre/vis/colors.py

@@ -78,7 +78,7 @@ class Colormap3D(colors.Colormap, metaclass=abc.ABCMeta):
         r, g, b = interpolate_color(sat, (0.5, 0.5, 0.5), np.stack((r, g, b), axis=-1))
         # THIRD color dimension: LUMINANCE (3D, color sphere)
         z_max = vmax if vmax is not None else np.abs(z).max() + 1E-30
-        z_clipped = np.minimum(np.abs(z), z_max) / z_max
+        z_clipped = np.minimum(np.abs(z), z_max) / z_max * np.sign(z)
         theta = np.arccos(z_clipped)
         lum = 1 - theta / np.pi  # goes from 0 (black) over 0.5 (grey) to 1 (white)!
         lum_target = np.where(lum < 0.5, 0, 1)  # Separate upper(white)/lower(black) hemispheres!