Add new method Colormap.with_alpha()

We currently do not have a way of adjusting only the transparency of the
 mapped colors of the artist, e.g. the fill in contourf(). The
 Artist alpha parameter also affects lines and hatches.

The simplest solution is to support creating semi-transparent
colormaps, which can then be used via
`contourf(..., cmap=plt.colormaps['jet'].with_alpha(0.5))`. See also
matplotlib#29044 (comment).

Author: timhoffm

doc/users/next_whats_new/colormap_with_alpha

@@ -0,0 +1,5 @@
+Tuning transparency of colormaps
+--------------------------------
+The new method `.Colormap.with_alpha` allows to create a new colormap with the same
+color values but a new uniform alpha value. This is handy if you want to modify only
+the transparency of mapped colors for an Artist.

lib/matplotlib/colors.py

@@ -943,6 +943,26 @@ def _set_extremes(self):
             self._lut[self._i_over] = self._lut[self.N - 1]
         self._lut[self._i_bad] = self._rgba_bad
 
+    def with_alpha(self, alpha):
+        """
+        Return a copy of the colormap with a new uniform transparency.
+
+        Parameters
+        ----------
+        alpha : float
+             The alpha blending value, between 0 (transparent) and 1 (opaque).
+        """
+        if not isinstance(alpha, Real):
+            raise TypeError(
+                f"'alpha' must be numeric or None, not {type(alpha)}")
+        if not 0 <= alpha <= 1:
+            ValueError("'alpha' must be between 0 and 1, inclusive")
+        new_cm = self.copy()
+        if not new_cm._isinit:
+            new_cm._init()
+        new_cm._lut[:, 3] = alpha
+        return new_cm
+
     def _init(self):
         """Generate the lookup table, ``self._lut``."""
         raise NotImplementedError("Abstract class only")

lib/matplotlib/colors.pyi

@@ -111,6 +111,7 @@ class Colormap:
         under: ColorType | None = ...,
         over: ColorType | None = ...
     ) -> Colormap: ...
+    def with_alpha(self, alpha: float) -> Colormap: ...
     def is_gray(self) -> bool: ...
     def resampled(self, lutsize: int) -> Colormap: ...
     def reversed(self, name: str | None = ...) -> Colormap: ...

lib/matplotlib/tests/test_colors.py

@@ -20,7 +20,7 @@
 import matplotlib.scale as mscale
 from matplotlib.rcsetup import cycler
 from matplotlib.testing.decorators import image_comparison, check_figures_equal
-from matplotlib.colors import is_color_like, to_rgba_array
+from matplotlib.colors import is_color_like, to_rgba_array, ListedColormap
 
 
 @pytest.mark.parametrize('N, result', [
@@ -248,6 +248,17 @@ def test_LinearSegmentedColormap_from_list_bad_under_over():
     assert mcolors.same_color(cmap.get_over(), "y")
 
 
+def test_colormap_with_alpha():
+    cmap = ListedColormap(["red", "green", ("blue", 0.8)])
+    cmap2 = cmap.with_alpha(0.5)
+    # color is the same:
+    vals = [0, 0.5, 1]  # numeric positions that map to the listed colors
+    assert_array_equal(cmap(vals)[:, :2], cmap2(vals)[:, :2])
+    # alpha of cmap2 is changed:
+    assert_array_equal(cmap(vals)[:, 3], [1, 1, 0.8])
+    assert_array_equal(cmap2(vals)[:, 3], [0.5, 0.5, 0.5])
+
+
 def test_BoundaryNorm():
     """
     GitHub issue #1258: interpolation was failing with numpy