Merge pull request matplotlib#30112 from trygvrad/colorbar-docs-colorizer

Update to docs with regards to colorbar and colorizer

Author: timhoffm

galleries/examples/images_contours_and_fields/multi_image.py

@@ -11,15 +11,17 @@
 value *x* in the image).
 
 If we want one colorbar to be representative for multiple images, we have
-to explicitly ensure consistent data coloring by using the same data
-normalization for all the images. We ensure this by explicitly creating a
-``norm`` object that we pass to all the image plotting methods.
+to explicitly ensure consistent data coloring by using the same
+data-to-color pipeline for all the images. We ensure this by explicitly
+creating a `matplotlib.colorizer.Colorizer` object that we pass to all
+the image plotting methods.
 """
 
 import matplotlib.pyplot as plt
 import numpy as np
 
-from matplotlib import colors
+import matplotlib.colorizer as mcolorizer
+import matplotlib.colors as mcolors
 
 np.random.seed(19680801)
 
@@ -31,12 +33,13 @@
 fig, axs = plt.subplots(2, 2)
 fig.suptitle('Multiple images')
 
-# create a single norm to be shared across all images
-norm = colors.Normalize(vmin=np.min(datasets), vmax=np.max(datasets))
+# create a colorizer with a predefined norm to be shared across all images
+norm = mcolors.Normalize(vmin=np.min(datasets), vmax=np.max(datasets))
+colorizer = mcolorizer.Colorizer(norm=norm)
 
 images = []
 for ax, data in zip(axs.flat, datasets):
-    images.append(ax.imshow(data, norm=norm))
+    images.append(ax.imshow(data, colorizer=colorizer))
 
 fig.colorbar(images[0], ax=axs, orientation='horizontal', fraction=.1)
 
@@ -45,30 +48,10 @@
 # %%
 # The colors are now kept consistent across all images when changing the
 # scaling, e.g. through zooming in the colorbar or via the "edit axis,
-# curves and images parameters" GUI of the Qt backend. This is sufficient
-# for most practical use cases.
-#
-# Advanced: Additionally sync the colormap
-# ----------------------------------------
-#
-# Sharing a common norm object guarantees synchronized scaling because scale
-# changes modify the norm object in-place and thus propagate to all images
-# that use this norm. This approach does not help with synchronizing colormaps
-# because changing the colormap of an image (e.g. through the "edit axis,
-# curves and images parameters" GUI of the Qt backend) results in the image
-# referencing the new colormap object. Thus, the other images are not updated.
-#
-# To update the other images, sync the
-# colormaps using the following code::
-#
-#     def sync_cmaps(changed_image):
-#         for im in images:
-#             if changed_image.get_cmap() != im.get_cmap():
-#                 im.set_cmap(changed_image.get_cmap())
-#
-#     for im in images:
-#         im.callbacks.connect('changed', sync_cmaps)
-#
+# curves and images parameters" GUI of the Qt backend. Additionally,
+# if the colormap of the colorizer is changed, (e.g. through the "edit
+# axis, curves and images parameters" GUI of the Qt backend) this change
+# propagates to the other plots and the colorbar.
 #
 # .. admonition:: References
 #
@@ -77,6 +60,5 @@
 #
 #    - `matplotlib.axes.Axes.imshow` / `matplotlib.pyplot.imshow`
 #    - `matplotlib.figure.Figure.colorbar` / `matplotlib.pyplot.colorbar`
+#    - `matplotlib.colorizer.Colorizer`
 #    - `matplotlib.colors.Normalize`
-#    - `matplotlib.cm.ScalarMappable.set_cmap`
-#    - `matplotlib.cbook.CallbackRegistry.connect`

galleries/users_explain/colors/colorbar_only.py

@@ -8,10 +8,12 @@
 This tutorial shows how to build and customize standalone colorbars, i.e.
 without an attached plot.
 
-A `~.Figure.colorbar` needs a "mappable" (`matplotlib.cm.ScalarMappable`)
-object (typically, an image) which indicates the colormap and the norm to be
-used.  In order to create a colorbar without an attached image, one can instead
-use a `.ScalarMappable` with no associated data.
+A `~.Figure.colorbar` requires a `matplotlib.colorizer.ColorizingArtist` which
+contains a `matplotlib.colorizer.Colorizer` that holds the data-to-color pipeline
+(norm and colormap). To create a colorbar without an attached plot one can
+directly instantiate the base class `.ColorizingArtist`, which has no associated
+data.
+
 """
 
 import matplotlib.pyplot as plt
@@ -23,17 +25,21 @@
 # -------------------------
 # Here, we create a basic continuous colorbar with ticks and labels.
 #
-# The arguments to the `~.Figure.colorbar` call are the `.ScalarMappable`
-# (constructed using the *norm* and *cmap* arguments), the axes where the
-# colorbar should be drawn, and the colorbar's orientation.
+# The arguments to the `~.Figure.colorbar` call are a `.ColorizingArtist`,
+# the axes where the colorbar should be drawn, and the colorbar's orientation.
+# To crate a `.ColorizingArtist` one must first make `.Colorizer` that holds the
+# desired *norm* and *cmap*.
+#
 #
 # For more information see the `~matplotlib.colorbar` API.
 
 fig, ax = plt.subplots(figsize=(6, 1), layout='constrained')
 
 norm = mpl.colors.Normalize(vmin=5, vmax=10)
 
-fig.colorbar(mpl.cm.ScalarMappable(norm=norm, cmap="cool"),
+colorizer = mpl.colorizer.Colorizer(norm=norm, cmap="cool")
+
+fig.colorbar(mpl.colorizer.ColorizingArtist(colorizer),
              cax=ax, orientation='horizontal', label='Some Units')
 
 # %%
@@ -47,7 +53,9 @@
 
 fig, ax = plt.subplots(layout='constrained')
 
-fig.colorbar(mpl.cm.ScalarMappable(norm=mpl.colors.Normalize(0, 1), cmap='magma'),
+colorizer = mpl.colorizer.Colorizer(norm=mpl.colors.Normalize(0, 1), cmap='magma')
+
+fig.colorbar(mpl.colorizer.ColorizingArtist(colorizer),
              ax=ax, orientation='vertical', label='a colorbar label')
 
 # %%
@@ -65,7 +73,9 @@
 bounds = [-1, 2, 5, 7, 12, 15]
 norm = mpl.colors.BoundaryNorm(bounds, cmap.N, extend='both')
 
-fig.colorbar(mpl.cm.ScalarMappable(norm=norm, cmap="viridis"),
+colorizer = mpl.colorizer.Colorizer(norm=norm, cmap='viridis')
+
+fig.colorbar(mpl.colorizer.ColorizingArtist(colorizer),
              cax=ax, orientation='horizontal',
              label="Discrete intervals with extend='both' keyword")
 
@@ -94,8 +104,10 @@
 bounds = [1, 2, 4, 7, 8]
 norm = mpl.colors.BoundaryNorm(bounds, cmap.N)
 
+colorizer = mpl.colorizer.Colorizer(norm=norm, cmap=cmap)
+
 fig.colorbar(
-    mpl.cm.ScalarMappable(cmap=cmap, norm=norm),
+    mpl.colorizer.ColorizingArtist(colorizer),
     cax=ax, orientation='horizontal',
     extend='both',
     spacing='proportional',
@@ -116,8 +128,10 @@
 bounds = [-1.0, -0.5, 0.0, 0.5, 1.0]
 norm = mpl.colors.BoundaryNorm(bounds, cmap.N)
 
+colorizer = mpl.colorizer.Colorizer(norm=norm, cmap=cmap)
+
 fig.colorbar(
-    mpl.cm.ScalarMappable(cmap=cmap, norm=norm),
+    mpl.colorizer.ColorizingArtist(colorizer),
     cax=ax, orientation='horizontal',
     extend='both', extendfrac='auto',
     spacing='uniform',

lib/matplotlib/colorbar.py

@@ -16,8 +16,9 @@
 import numpy as np
 
 import matplotlib as mpl
-from matplotlib import _api, cbook, collections, cm, colors, contour, ticker
+from matplotlib import _api, cbook, collections, colors, contour, ticker
 import matplotlib.artist as martist
+import matplotlib.colorizer as mcolorizer
 import matplotlib.patches as mpatches
 import matplotlib.path as mpath
 import matplotlib.spines as mspines
@@ -199,12 +200,12 @@ class Colorbar:
     Draw a colorbar in an existing Axes.
 
     Typically, colorbars are created using `.Figure.colorbar` or
-    `.pyplot.colorbar` and associated with `.ScalarMappable`\s (such as an
+    `.pyplot.colorbar` and associated with `.ColorizingArtist`\s (such as an
     `.AxesImage` generated via `~.axes.Axes.imshow`).
 
     In order to draw a colorbar not associated with other elements in the
     figure, e.g. when showing a colormap by itself, one can create an empty
-    `.ScalarMappable`, or directly pass *cmap* and *norm* instead of *mappable*
+    `.ColorizingArtist`, or directly pass *cmap* and *norm* instead of *mappable*
     to `Colorbar`.
 
     Useful public methods are :meth:`set_label` and :meth:`add_lines`.
@@ -244,7 +245,7 @@ def __init__(
         ax : `~matplotlib.axes.Axes`
             The `~.axes.Axes` instance in which the colorbar is drawn.
 
-        mappable : `.ScalarMappable`
+        mappable : `.ColorizingArtist`
             The mappable whose colormap and norm will be used.
 
             To show the colors versus index instead of on a 0-1 scale, set the
@@ -288,7 +289,8 @@ def __init__(
             colorbar and at the right for a vertical.
         """
         if mappable is None:
-            mappable = cm.ScalarMappable(norm=norm, cmap=cmap)
+            colorizer = mcolorizer.Colorizer(norm=norm, cmap=cmap)
+            mappable = mcolorizer.ColorizingArtist(colorizer)
 
         self.mappable = mappable
         cmap = mappable.cmap

lib/matplotlib/figure.py

@@ -1200,17 +1200,18 @@ def colorbar(
         Parameters
         ----------
         mappable
-            The `matplotlib.cm.ScalarMappable` (i.e., `.AxesImage`,
+            The `matplotlib.colorizer.ColorizingArtist` (i.e., `.AxesImage`,
             `.ContourSet`, etc.) described by this colorbar.  This argument is
             mandatory for the `.Figure.colorbar` method but optional for the
             `.pyplot.colorbar` function, which sets the default to the current
             image.
 
-            Note that one can create a `.ScalarMappable` "on-the-fly" to
-            generate colorbars not attached to a previously drawn artist, e.g.
+            Note that one can create a `.colorizer.ColorizingArtist` "on-the-fly"
+            to generate colorbars not attached to a previously drawn artist, e.g.
             ::
 
-                fig.colorbar(cm.ScalarMappable(norm=norm, cmap=cmap), ax=ax)
+                cr = colorizer.Colorizer(norm=norm, cmap=cmap)
+                fig.colorbar(colorizer.ColorizingArtist(cr), ax=ax)
 
         cax : `~matplotlib.axes.Axes`, optional
             Axes into which the colorbar will be drawn.  If `None`, then a new