Reuse png metadata handling of imsave() in FigureCanvasAgg.print_png().

anntzer · anntzer · commit 7aad47a379bf · 2020-05-24T00:34:37.000+02:00
This avoids duplicating the conversion of metadata to PngInfo and
revealed a bug in the priority between `metadata` and `pil_kwargs` in
imsave().

Note that because `np.asarray(self.buffer_rgba())` is already a RGBA
uint8 array, there is no colormapping step happening in imsave().

Ideally mplcairo should also be able to use imsave() for saving to png.
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -33,7 +33,6 @@
 
 import numpy as np
 from PIL import Image
-from PIL.PngImagePlugin import PngInfo
 
 import matplotlib as mpl
 from matplotlib import cbook
@@ -502,24 +501,16 @@ def print_png(self, filename_or_obj, *args,
 
         if metadata is None:
             metadata = {}
-        if pil_kwargs is None:
-            pil_kwargs = {}
         metadata = {
             "Software":
                 f"matplotlib version{mpl.__version__}, http://matplotlib.org/",
             **metadata,
         }
         FigureCanvasAgg.draw(self)
-        # Only use the metadata kwarg if pnginfo is not set, because the
-        # semantics of duplicate keys in pnginfo is unclear.
-        if "pnginfo" not in pil_kwargs:
-            pnginfo = PngInfo()
-            for k, v in metadata.items():
-                pnginfo.add_text(k, v)
-            pil_kwargs["pnginfo"] = pnginfo
-        pil_kwargs.setdefault("dpi", (self.figure.dpi, self.figure.dpi))
-        (Image.fromarray(np.asarray(self.buffer_rgba()))
-         .save(filename_or_obj, format="png", **pil_kwargs))
+        mpl.image.imsave(
+            filename_or_obj, np.asarray(self.buffer_rgba()), format="png",
+            origin="upper", dpi=self.figure.dpi,
+            metadata=metadata, pil_kwargs=pil_kwargs)
 
     def print_to_buffer(self):
         FigureCanvasAgg.draw(self)
diff --git a/lib/matplotlib/image.py b/lib/matplotlib/image.py
@@ -1520,8 +1520,10 @@ def imsave(fname, arr, vmin=None, vmax=None, cmap=None, format=None,
         pil_shape = (rgba.shape[1], rgba.shape[0])
         image = PIL.Image.frombuffer(
             "RGBA", pil_shape, rgba, "raw", "RGBA", 0, 1)
-        if format == "png" and metadata is not None:
-            # cf. backend_agg's print_png.
+        if (format == "png"
+                and metadata is not None and "pnginfo" not in pil_kwargs):
+            # Only use the metadata kwarg if pnginfo is not set, because the
+            # semantics of duplicate keys in pnginfo is unclear.
             pnginfo = PIL.PngImagePlugin.PngInfo()
             for k, v in metadata.items():
                 pnginfo.add_text(k, v)
