ENH: Let CannyPlugin adapt to input image

Add `dtype_limits` helper function.

skimage/util/dtype.py

@@ -2,7 +2,7 @@ from __future__ import division
 import numpy as np
 
 __all__ = ['img_as_float', 'img_as_int', 'img_as_uint', 'img_as_ubyte',
-           'img_as_bool']
+           'img_as_bool', 'dtype_limits']
 
 from .. import get_log
 log = get_log('dtype_converter')
@@ -28,6 +28,23 @@ if np.__version__ >= "1.6.0":
     _supported_types += (np.float16, )
 
 
+def dtype_limits(image, auto_clip=True):
+    """Return intensity limits, i.e. (min, max) tuple, of the image's dtype.
+
+    Parameters
+    ----------
+    image : ndarray
+        Input image.
+    auto_clip : bool
+        If True, clip the negative range (i.e. return 0 for min intensity)
+        if the input image has no negative values.
+    """
+    imin, imax = dtype_range[image.dtype.type]
+    if auto_clip and imin < 0 and image.min() >= 0:
+        imin = 0
+    return imin, imax
+
+
 def convert(image, dtype, force_copy=False, uniform=False):
     """
     Convert an image to the requested data-type.

skimage/viewer/plugins/canny.py

@@ -1,5 +1,7 @@
-from skimage.filter import canny
+import numpy as np
 
+import skimage
+from skimage.filter import canny
 from .overlayplugin import OverlayPlugin
 from ..widgets import Slider, ComboBox
 
@@ -12,7 +14,17 @@ class CannyPlugin(OverlayPlugin):
     def __init__(self, *args, **kwargs):
         super(CannyPlugin, self).__init__(image_filter=canny, **kwargs)
 
+    def attach(self, image_viewer):
+        image = image_viewer.image
+        imin, imax = skimage.dtype_limits(image)
+        itype = 'float' if np.issubdtype(image.dtype, float) else 'int'
         self.add_widget(Slider('sigma', 0, 5, update_on='release'))
-        self.add_widget(Slider('low threshold', 0, 255, update_on='release'))
-        self.add_widget(Slider('high threshold', 0, 255, update_on='release'))
+        self.add_widget(Slider('low threshold', imin, imax, value_type=itype,
+                        update_on='release'))
+        self.add_widget(Slider('high threshold', imin, imax, value_type=itype,
+                        update_on='release'))
         self.add_widget(ComboBox('color', self.color_names, ptype='plugin'))
+        # Call parent method at end b/c it calls `filter_image`, which needs
+        # the values specified by the widgets. Alternatively, move call to
+        # parent method to beginning and add a call to `self.filter_image()`
+        super(CannyPlugin,self).attach(image_viewer)