Add documentation section and example for using as_mpl_selector with callback function

docs/masks.rst

@@ -273,3 +273,54 @@ statistics::
     >>> import numpy as np
     >>> np.average(data, weights=mask)  # doctest: +FLOAT_CMP
     9364.012674888021
+
+
+.. _interactive-masks:
+
+Interactive Mask Control
+------------------------
+
+In the last example we will show how to use a
+:ref:`Matplotlib selector<regions-as_mpl_selector>` widget with a custom
+``callback`` function for creating a mask and updating it interactively through
+the selector.
+We first create an :class:`~regions.EllipsePixelRegion` and add an ``as_mpl_selector``
+property linked to the Matplotlib axes. This can be moved around to
+position it on different sources, and resized just like its Rectangle
+counterpart, using the handles of the bounding box. 
+
+The user-defined callback function here generates a mask from this region and overlays
+it on the image as an alpha filter (keeping the areas outside shaded).
+We will use this mask as an aperture as well to calculate integrated
+and averaged flux, which is updated live in the text field of the plot as well. 
+
+.. plot::
+   :context:
+   :include-source:
+   :align: center
+
+    from astropy import units as u
+    from regions import PixCoord, EllipsePixelRegion
+
+    hdulist = fits.open(filename)
+    hdu = hdulist[0]
+
+    plt.clf()
+    ax = plt.subplot(1, 1, 1)
+    im = ax.imshow(hdu.data, cmap=plt.cm.viridis, interpolation='nearest', origin='lower')
+    text = ax.text(122, 1002, '', size='small', color='yellow')
+    ax.set_xlim(120, 180)
+    ax.set_ylim(1000, 1059)
+
+    def update_sel(region):
+        mask = region.to_mask(mode='subpixels', subpixels=10)
+        im.set_alpha((mask.to_image(hdu.data.shape) + 1) / 2)
+        total = mask.multiply(hdu.data).sum()
+        mean = np.average(hdu.data, weights=mask.to_image(hdu.data.shape))
+        text.set_text(f'Total: {total:g}\nMean: {mean:g}')
+
+    ellipse = EllipsePixelRegion(center=PixCoord(x=126, y=1031), width=8, height=4,
+                                 angle=-0*u.deg, visual={'color': 'yellow'})
+    selector = ellipse.as_mpl_selector(ax, callback=update_sel, use_data_coordinates=True)
+
+    hdulist.close()

docs/shapes.rst

@@ -272,6 +272,52 @@ to a :class:`~regions.SkyRegion`, call the
     radius: 18.55481729935556 arcsec
 
 
+.. _regions-as_mpl_selector:
+
+Selectors for Regions
+---------------------
+
+Several geometric regions (at this time, :class:`~regions.RectanglePixelRegion`,
+:class:`~regions.EllipsePixelRegion` and :class:`~regions.PolygonPixelRegion`)
+provide a method :meth:`~regions.RectanglePixelRegion.as_mpl_selector` to
+create an interactive editable matplotlib widget that will be
+connected to its parent region.
+
+.. plot::
+   :context:
+   :include-source:
+
+    import matplotlib.pyplot as plt
+    import numpy as np
+    from regions import PixCoord, RectanglePixelRegion
+
+    x, y = np.mgrid[-15:16, -10:21]
+    z = np.exp(-(x / 4)**2 - (y / 6)**2)
+    ax = plt.subplot()
+    img = ax.imshow(z)
+
+    rectangle = RectanglePixelRegion(center=PixCoord(x=12, y=15), width=14, height=10)
+    selector = rectangle.as_mpl_selector(ax)
+
+The ``selector`` creates and establishes a link to a matplotlib ``Selector``
+widget that allows manually positioning the ``rectangle`` region at the
+central point, and scaling it by dragging its corner points.
+Several modifier keys as specified by the ``state_modifier_keys`` parameter to
+:class:`matplotlib.widgets.RectangleSelector` provide further control of the
+manipulation of this widget, with the following operations available:
+
+- "move": Move the existing shape from anywhere, default: "space".
+- "clear": Clear the current shape, default: "escape".
+- "square": Make the shape square, default: "shift".
+- "center": Change the shape around its center, default: "ctrl".
+- "rotate": Rotate the shape around its center, default: "r" (toggles, requires Matplotlib 3.6+).
+
+Via the optional ``callback`` parameter this method can be passed a
+custom function that will be called on every update of the region,
+i.e. after every move or resize of the selector.
+For an example of its usage see :ref:`Interactive Mask Control<interactive-masks>`.
+
+
 Multiple Regions
 ----------------