ENH: Add option to relpace text with boxes in image_comparision test

This should be the right level of simplification when text is relevant
in image comparision tests but the exact characters do not matter. The
drawn bboxes only rely on font metrics and thus give accurate sizes,
while being insensitive to rendering details.

This is a minimal proof-of-concept. Exact implementation may be improved
.

Author: timhoffm

lib/matplotlib/testing/decorators.py

@@ -15,6 +15,7 @@
 import matplotlib.units
 import matplotlib.testing
 from matplotlib import _pylab_helpers, cbook, ft2font, pyplot as plt, ticker
+import matplotlib.text as mtext
 from .compare import comparable_formats, compare_images, make_test_filename
 from .exceptions import ImageComparisonFailure
 
@@ -117,11 +118,12 @@ class _ImageComparisonBase:
     any code that would be specific to any testing framework.
     """
 
-    def __init__(self, func, tol, remove_text, savefig_kwargs):
+    def __init__(self, func, tol, remove_text, replace_text, savefig_kwargs):
         self.func = func
         self.baseline_dir, self.result_dir = _image_directories(func)
         self.tol = tol
         self.remove_text = remove_text
+        self.replace_text = replace_text
         self.savefig_kwargs = savefig_kwargs
 
     def copy_baseline(self, baseline, extension):
@@ -164,19 +166,23 @@ def compare(self, fig, baseline, extension, *, _lock=False):
         lock = (cbook._lock_path(actual_path)
                 if _lock else contextlib.nullcontext())
         with lock:
+            if self.replace_text:
+                mtext.Text._draw_bbox_only = True
             try:
                 fig.savefig(actual_path, **kwargs)
             finally:
                 # Matplotlib has an autouse fixture to close figures, but this
                 # makes things more convenient for third-party users.
                 plt.close(fig)
+            if self.replace_text:
+                mtext.Text._draw_bbox_only = False
             expected_path = self.copy_baseline(baseline, extension)
             _raise_on_image_difference(expected_path, actual_path, self.tol)
 
 
 def _pytest_image_comparison(baseline_images, extensions, tol,
-                             freetype_version, remove_text, savefig_kwargs,
-                             style):
+                             freetype_version, remove_text, replace_text,
+                             savefig_kwargs, style):
     """
     Decorate function with image comparison for pytest.
 
@@ -212,6 +218,7 @@ def wrapper(*args, extension, request, **kwargs):
                 pytest.skip(f"Cannot compare {extension} files {reason}")
 
             img = _ImageComparisonBase(func, tol=tol, remove_text=remove_text,
+                                       replace_text=replace_text,
                                        savefig_kwargs=savefig_kwargs)
             matplotlib.testing.set_font_settings_for_testing()
 
@@ -259,6 +266,7 @@ def wrapper(*args, extension, request, **kwargs):
 
 def image_comparison(baseline_images, extensions=None, tol=0,
                      freetype_version=None, remove_text=False,
+                     replace_text=False,
                      savefig_kwarg=None,
                      # Default of mpl_test_settings fixture and cleanup too.
                      style=("classic", "_classic_test_patch")):
@@ -344,6 +352,7 @@ def image_comparison(baseline_images, extensions=None, tol=0,
     return _pytest_image_comparison(
         baseline_images=baseline_images, extensions=extensions, tol=tol,
         freetype_version=freetype_version, remove_text=remove_text,
+        replace_text=replace_text,
         savefig_kwargs=savefig_kwarg, style=style)
 
 

lib/matplotlib/tests/baseline_images/test_text/replace_text.png

@@ -1190,3 +1190,15 @@ def test_ytick_rotation_mode():
         tick.set_rotation(angle)
 
     plt.subplots_adjust(left=0.4, right=0.6, top=.99, bottom=.01)
+
+
+@image_comparison(baseline_images=['replace_text'],
+                  replace_text=True, extensions=['png'], style='mpl20')
+def test_replace_text():
+    fig, ax = plt.subplots()
+    ax.plot([2, 1], label='line 1')
+    ax.plot([3, 2], label='line 2')
+    ax.legend(title='mylegend')
+    ax.set_title('testing text replacement')
+    ax.set_xlabel('xlabel')
+    ax.set_ylabel('ylabel')

lib/matplotlib/tests/test_text.py

@@ -97,6 +97,13 @@ class Text(Artist):
 
     zorder = 3
     _charsize_cache = dict()
+    _draw_bbox_only = False
+    """
+    A helper flag to replace texts by a bounding box. This is primarily
+    intended for use in figure comparison tests.
+    For sipmlicity, the flag is evaluated at draw time and thus applies
+    to all drawn text.
+    """
 
     def __repr__(self):
         return f"Text({self._x}, {self._y}, {self._text!r})"
@@ -778,6 +785,9 @@ def draw(self, renderer):
 
             # Update the location and size of the bbox
             # (`.patches.FancyBboxPatch`), and draw it.
+            if self._draw_bbox_only:
+                self.set_bbox(dict(facecolor='none', edgecolor='red', pad=0))
+
             if self._bbox_patch:
                 self.update_bbox_position_size(renderer)
                 self._bbox_patch.draw(renderer)
@@ -793,6 +803,9 @@ def draw(self, renderer):
 
             for line, wh, x, y in info:
 
+                if self._draw_bbox_only:
+                    break
+
                 mtext = self if len(info) == 1 else None
                 x = x + posx
                 y = y + posy