Overhaul imshow to clarify flow

skimage/io/_plugins/matplotlib_plugin.py

@@ -1,3 +1,4 @@
+from collections import namedtuple
 import numpy as np
 import warnings
 import matplotlib.pyplot as plt
@@ -9,6 +10,106 @@ _nonstandard_colormap = 'cubehelix'
 _diverging_colormap = 'RdBu'
 
 
+ImageProperties = namedtuple('ImageProperties',
+                             ['signed', 'out_of_range_float',
+                              'low_dynamic_range', 'unsupported_dtype'])
+
+
+def _get_image_properties(image):
+    """Determine nonstandard properties of an input image.
+
+    Parameters
+    ----------
+    image : array
+        The input image.
+
+    Returns
+    -------
+    ip : ImageProperties named tuple
+        The properties of the image:
+
+        - signed: whether the image has negative values.
+        - out_of_range_float: if the image has floating point data
+          outside of [-1, 1].
+        - low_dynamic_range: if the image is in the standard image
+          range (e.g. [0, 1] for a floating point image) but its
+          dynamic range would be too small to display with standard
+          image ranges.
+        - unsupported_dtype: if the image data type is not a
+          standard skimage type, e.g. ``numpy.uint64``.
+    """
+    immin, immax = np.min(image), np.max(image)
+    imtype = image.dtype.type
+    try:
+        lo, hi = dtypes.dtype_range[imtype]
+    except KeyError:
+        lo, hi = immin, immax
+
+    signed = immin < 0
+    out_of_range_float = (np.issubdtype(image.dtype, np.float) and
+                          (immin < lo or immax > hi))
+    low_dynamic_range = (immin != immax and
+                         (float(immax - immin) / (hi - lo)) < (1. / 255))
+    unsupported_dtype = image.dtype not in dtypes._supported_types
+
+    ip = ImageProperties(signed, out_of_range_float,
+                         low_dynamic_range, unsupported_dtype)
+    return ip
+
+
+def _raise_warnings(image_properties):
+    """Raise the appropriate warning for each nonstandard image type.
+
+    Parameters
+    ----------
+    image_properties : ImageProperties named tuple
+        The properties of the considered image.
+    """
+    ip = image_properties
+    if ip.unsupported_dtype:
+        warnings.warn("Non-standard image type; displaying image with "
+                      "stretched contrast.")
+    if ip.low_dynamic_range:
+        warnings.warn("Low image dynamic range; displaying image with "
+                      "stretched contrast.")
+    if ip.out_of_range_float:
+        warnings.warn("Float image out of standard range; displaying image "
+                      "with stretched contrast.")
+
+
+def _get_display_range(image):
+    """Return the display range for a given set of image properties.
+
+    Parameters
+    ----------
+    image : array
+        The input image.
+
+    Returns
+    -------
+    lo, hi : same type as immin, immax
+        The display range to be used for the input image.
+    cmap : string
+        The name of the colormap to use.
+    """
+    ip = _get_image_properties(image)
+    immin, immax = np.min(image), np.max(image)
+    if ip.signed:
+        magnitude = max(abs(immin), abs(immax))
+        lo, hi = -magnitude, magnitude
+        cmap = _diverging_colormap
+    elif any(ip):
+        _raise_warnings(ip)
+        lo, hi = immin, immax
+        cmap = _nonstandard_colormap
+    else:
+        lo = 0
+        imtype = image.dtype.type
+        hi = dtypes.dtype_range[imtype][1]
+        cmap = _default_colormap
+    return lo, hi, cmap
+
+
 def imshow(im, *args, **kwargs):
     """Show the input image and return the current axes.
 
@@ -37,41 +138,13 @@ def imshow(im, *args, **kwargs):
     """
     if plt.gca().has_data():
         plt.figure()
-    immin, immax = np.min(im), np.max(im)
-    lo, hi = immin, immax
-    cmap = _default_colormap
-    supported_dtype = im.dtype in dtypes._supported_types
-    if supported_dtype:
-        lo, hi = dtypes.dtype_range[im.dtype.type]
-        # for nonnegative float images, ensure lower bound is 0, not -1
-        if immin > 0:
-            lo = 0
-        # for signed images, show diverging colormap centered at 0
-        else:
-            cmap = _diverging_colormap
-            magnitude = max(-immin, abs(immax))
-            lo, hi = -magnitude, magnitude
-    else:
-        warnings.warn("Non-standard image type; displaying image with "
-                      "stretched contrast.")
-    out_of_range_float = (np.issubdtype(im.dtype, np.float) and
-                          (immin < lo or immax > hi))
-    if out_of_range_float:
-        warnings.warn("Float image out of standard range; displaying image "
-                      "with stretched contrast.")
-    low_dynamic_range = (immin != immax and
-                         (float(immax - immin) / (hi - lo)) < (1. / 255))
-    if low_dynamic_range:
-        warnings.warn("Low image dynamic range; displaying image with "
-                      "stretched contrast.")
-    if not supported_dtype or out_of_range_float or low_dynamic_range:
-        cmap = _nonstandard_colormap
+    lo, hi, cmap = _get_display_range(im)
     kwargs.setdefault('interpolation', 'nearest')
     kwargs.setdefault('cmap', cmap)
     kwargs.setdefault('vmin', lo)
     kwargs.setdefault('vmax', hi)
     ax_im = plt.imshow(im, *args, **kwargs)
-    if not supported_dtype or out_of_range_float or low_dynamic_range:
+    if cmap != _default_colormap:
         plt.colorbar()
     return ax_im