Merge pull request #21557 from anntzer/pdfpngtransparency

Fix transparency when exporting to png via pgf backend.

lib/matplotlib/__init__.py

@@ -302,8 +302,8 @@ def _get_executable_info(name):
     ----------
     name : str
         The executable to query.  The following values are currently supported:
-        "dvipng", "gs", "inkscape", "magick", "pdftops".  This list is subject
-        to change without notice.
+        "dvipng", "gs", "inkscape", "magick", "pdftocairo", "pdftops".  This
+        list is subject to change without notice.
 
     Returns
     -------
@@ -315,7 +315,10 @@ def _get_executable_info(name):
     ------
     ExecutableNotFoundError
         If the executable is not found or older than the oldest version
-        supported by Matplotlib.
+        supported by Matplotlib.  For debugging purposes, it is also
+        possible to "hide" an executable from Matplotlib by adding it to the
+        :envvar:`_MPLHIDEEXECUTABLES` environment variable (a comma-separated
+        list), which must be set prior to any calls to this function.
     ValueError
         If the executable is not one that we know how to query.
     """
@@ -351,6 +354,9 @@ def impl(args, regex, min_ver=None, ignore_exit_code=False):
                 f"Failed to determine the version of {args[0]} from "
                 f"{' '.join(args)}, which output {output}")
 
+    if name in os.environ.get("_MPLHIDEEXECUTABLES", "").split(","):
+        raise ExecutableNotFoundError(f"{name} was hidden")
+
     if name == "dvipng":
         return impl(["dvipng", "-version"], "(?m)^dvipng(?: .*)? (.+)", "1.6")
     elif name == "gs":
@@ -407,6 +413,8 @@ def impl(args, regex, min_ver=None, ignore_exit_code=False):
             raise ExecutableNotFoundError(
                 f"You have ImageMagick {info.version}, which is unsupported")
         return info
+    elif name == "pdftocairo":
+        return impl(["pdftocairo", "-v"], "pdftocairo version (.*)")
     elif name == "pdftops":
         info = impl(["pdftops", "-v"], "^pdftops version (.*)",
                     ignore_exit_code=True)

lib/matplotlib/backends/backend_pgf.py

@@ -168,26 +168,28 @@ def _metadata_to_str(key, value):
 
 def make_pdf_to_png_converter():
     """Return a function that converts a pdf file to a png file."""
-    if shutil.which("pdftocairo"):
-        def cairo_convert(pdffile, pngfile, dpi):
-            cmd = ["pdftocairo", "-singlefile", "-png", "-r", "%d" % dpi,
-                   pdffile, os.path.splitext(pngfile)[0]]
-            subprocess.check_output(cmd, stderr=subprocess.STDOUT)
-        return cairo_convert
+    try:
+        mpl._get_executable_info("pdftocairo")
+    except mpl.ExecutableNotFoundError:
+        pass
+    else:
+        return lambda pdffile, pngfile, dpi: subprocess.check_output(
+            ["pdftocairo", "-singlefile", "-transp", "-png", "-r", "%d" % dpi,
+             pdffile, os.path.splitext(pngfile)[0]],
+            stderr=subprocess.STDOUT)
     try:
         gs_info = mpl._get_executable_info("gs")
     except mpl.ExecutableNotFoundError:
         pass
     else:
-        def gs_convert(pdffile, pngfile, dpi):
-            cmd = [gs_info.executable,
-                   '-dQUIET', '-dSAFER', '-dBATCH', '-dNOPAUSE', '-dNOPROMPT',
-                   '-dUseCIEColor', '-dTextAlphaBits=4',
-                   '-dGraphicsAlphaBits=4', '-dDOINTERPOLATE',
-                   '-sDEVICE=png16m', '-sOutputFile=%s' % pngfile,
-                   '-r%d' % dpi, pdffile]
-            subprocess.check_output(cmd, stderr=subprocess.STDOUT)
-        return gs_convert
+        return lambda pdffile, pngfile, dpi: subprocess.check_output(
+            [gs_info.executable,
+             '-dQUIET', '-dSAFER', '-dBATCH', '-dNOPAUSE', '-dNOPROMPT',
+             '-dUseCIEColor', '-dTextAlphaBits=4',
+             '-dGraphicsAlphaBits=4', '-dDOINTERPOLATE',
+             '-sDEVICE=pngalpha', '-sOutputFile=%s' % pngfile,
+             '-r%d' % dpi, pdffile],
+            stderr=subprocess.STDOUT)
     raise RuntimeError("No suitable pdf to png renderer found.")
 
 

lib/matplotlib/tests/test_backend_pgf.py

@@ -312,10 +312,12 @@ def test_bbox_inches_tight():
 
 @needs_xelatex
 @needs_ghostscript
-def test_png():
-    # Just a smoketest.
-    fig, ax = plt.subplots()
-    fig.savefig(BytesIO(), format="png", backend="pgf")
+def test_png_transparency():  # Actually, also just testing that png works.
+    buf = BytesIO()
+    plt.figure().savefig(buf, format="png", backend="pgf", transparent=True)
+    buf.seek(0)
+    t = plt.imread(buf)
+    assert (t[..., 3] == 0).all()  # fully transparent.
 
 
 @needs_xelatex