Support sketch_params in pgf backend

takimata · takimata · commit 38ab792dc412 · 2021-08-29T22:46:43.000+02:00
Fixes #20516 PGF's `random steps` decoration seems to be the most similar, but does not exactly match the behaviour described in matplotlib's docs. Therefore I repurposed the `randomness` argument as a seed to give control on how the line looks afterwards.
diff --git a/lib/matplotlib/artist.py b/lib/matplotlib/artist.py
@@ -677,6 +677,17 @@ def set_sketch_params(self, scale=None, length=None, randomness=None):
         """
         Set the sketch parameters.
 
+        Requires the following preamble when using the PGF backend:
+
+            \\usepackage{pgf}
+            \\usepgfmodule{decorations}
+            \\usepgflibrary{decorations.pathmorphing}
+
+        This also applies to PGF backend + PDF output, where this must be added
+        to `pgf.preamble` manually. The PGF backend uses the `randomness`
+        argument as a seed and not as described below. Pass the same seed to
+        obtain the same random shape.
+
         Parameters
         ----------
         scale : float, optional
diff --git a/lib/matplotlib/backends/backend_pgf.py b/lib/matplotlib/backends/backend_pgf.py
@@ -600,6 +600,21 @@ def _print_pgf_path(self, gc, path, transform, rgbFace=None):
                         r"{\pgfqpoint{%fin}{%fin}}"
                         % coords)
 
+        # apply pgf decorators
+        sketch_params = gc.get_sketch_params() if gc else None
+        if sketch_params is not None:
+            # Only "length" directly maps to "segment length" in PGF's API
+            # The others are combined in "amplitude" -> Use "randomness" as
+            # PRNG seed to allow the user to force the same shape on multiple
+            # sketched lines
+            scale, length, randomness = sketch_params
+            if scale is not None:
+                writeln(self.fh, r"\pgfkeys{/pgf/decoration/.cd, "
+                        f"segment length = {(length * f):f}in, "
+                        f"amplitude = {(scale * f):f}in}}")
+                writeln(self.fh, f"\\pgfmathsetseed{{{int(randomness)}}}")
+                writeln(self.fh, r"\pgfdecoratecurrentpath{random steps}")
+
     def _pgf_path_draw(self, stroke=True, fill=False):
         actions = []
         if stroke:
