improve examples

Author: emmanuelle

doc/python/imshow.md

@@ -213,10 +213,10 @@ import plotly.express as px
 from skimage import data
 img = data.astronaut()
 # Increase contrast by clipping the data range between 50 and 200
-fig = px.imshow(img, zmin=50, zmax=200, binary_string=False)
+fig = px.imshow(img, zmin=50, zmax=200)
 # We customize the hovertemplate to show both the data and the color values
 # See https://plotly.com/python/hover-text-and-formatting/#customize-tooltip-text-with-a-hovertemplate
-fig.update_traces(hovertemplate="x: %{x} <br> y: %{y} <br> z: %{z} <br> color: %{color}")
+#fig.update_traces(hovertemplate="x: %{x} <br> y: %{y} <br> z: %{z} <br> color: %{color}")
 fig.show()
 ```
 
@@ -355,6 +355,36 @@ fig = px.imshow(img, binary_string=True)
 fig.show()
 ```
 
+### Contrast rescaling im imshow with binary string
+
+When the image is passed to the plotly figure as a binary string (which is the default mode for RGB images), and when the image is rescaled to adjust the contrast (for example when setting `zmin` and `zmax`), the original intensity values are not passed to the plotly figure and therefore no intensity value is displayed in the hover.
+
+```python
+import plotly.express as px
+from skimage import data
+import numpy as np
+img = np.arange(100).reshape((10, 10))
+fig = px.imshow(img, binary_string=True)
+# You can check that only x and y are displayed in the hover
+# You can use a hovertemplate to override the hover information
+# See https://plotly.com/python/hover-text-and-formatting/#customize-tooltip-text-with-a-hovertemplate
+fig.show()
+```
+
+You can set `binary_string=False` if you want the intensity value to appear in the hover even for a rescaled image. In the example below we also modify the hovertemplate to display both `z` (the data of the original image array) and `color` (the pixel value displayed in the figure).
+
+```python
+import plotly.express as px
+from skimage import data
+img = data.chelsea()
+# Increase contrast by clipping the data range between 50 and 200
+fig = px.imshow(img, binary_string=False, zmin=50, zmax=200)
+# We customize the hovertemplate to show both the data and the color values
+# See https://plotly.com/python/hover-text-and-formatting/#customize-tooltip-text-with-a-hovertemplate
+fig.update_traces(hovertemplate="x: %{x} <br> y: %{y} <br> z: %{z} <br> color: %{color}")
+fig.show()
+```
+
 ### Changing the level of compression of the binary string in `px.imshow`
 
 The `binary_compression_level` parameter controls the level of compression to be used by the backend creating the png string. Two different backends can be used, `pypng` (which is a dependency of `plotly` and is therefore always available), and `pil` for Pillow, which is often more performant. The compression level has to be between 0 (no compression) and 9 (highest compression), although increasing the compression above 4 and 5 usually only offers diminishing returns (no significant compression gain, at the cost of a longer execution time).

packages/python/plotly/plotly/express/_imshow.py

@@ -79,8 +79,8 @@ def _array_to_b64str(img, backend="pil", compression=4):
     return base64_string
 
 
-def _vectorize_zvalue(z, mode='max'):
-    alpha = 255 if mode == 'max' else 0
+def _vectorize_zvalue(z, mode="max"):
+    alpha = 255 if mode == "max" else 0
     if z is None:
         return z
     elif np.isscalar(z):
@@ -383,7 +383,10 @@ def imshow(
     elif img.ndim == 3 and img.shape[-1] in [3, 4] or (img.ndim == 2 and binary_string):
         rescale_image = True  # to check whether image has been modified
         if zmin is not None and zmax is not None:
-            zmin, zmax = _vectorize_zvalue(zmin, mode='min'), _vectorize_zvalue(zmax, mode='max')
+            zmin, zmax = (
+                _vectorize_zvalue(zmin, mode="min"),
+                _vectorize_zvalue(zmax, mode="max"),
+            )
         if binary_string:
             if zmin is None and zmax is None:  # no rescaling, faster
                 img_rescaled = img
@@ -393,7 +396,6 @@ def imshow(
                     img, in_range=(zmin[0], zmax[0]), out_range=np.uint8
                 )
             else:
-                print(zmin, zmax)
                 img_rescaled = np.dstack(
                     [
                         rescale_intensity(
@@ -433,7 +435,7 @@ def imshow(
     fig = go.Figure(data=trace, layout=layout)
     fig.update_layout(layout_patch)
     # Hover name, z or color
-    if binary_string and rescale_image and not (img_rescaled.max() == img.max()):
+    if binary_string and rescale_image and not np.all(img == img_rescaled):
         # we rescaled the image, hence z is not displayed in hover since it does
         # not correspond to img values
         hovertemplate = "%s: %%{x}<br>%s: %%{y}<extra></extra>" % (