avoid converting palette PNG with alpha to RGB (closes: #158)

src/img2pdf.py

@@ -1432,8 +1432,10 @@ def get_imgmetadata(
     iccp = None
     if "icc_profile" in imgdata.info:
         iccp = imgdata.info.get("icc_profile")
-    # GIMP saves bilevel tiff images with an RGB ICC profile which
+    # GIMP saves bilevel tiff images with an RGB ICC profile which is useless
-    # is useless and produces an error in Adobe Acrobat -- ignore it
+    # and produces an error in Adobe Acrobat, so we ignore it with a warning.
+    # imagemagick also used to (wrongly) include an RGB ICC profile for bilevel
+    # images: https://github.com/ImageMagick/ImageMagick/issues/2070
     if iccp is not None and color == Colorspace["1"] and imgformat == ImageFormat.TIFF:
         with io.BytesIO(iccp) as f:
             prf = ImageCms.ImageCmsProfile(f)
@@ -1442,10 +1444,8 @@ def get_imgmetadata(
             and prf.profile.manufacturer == "GIMP"
             and prf.profile.profile_description == "GIMP built-in sRGB"
         ):
-            logger.warning(
+            logger.warning("Ignoring RGB ICC profile in bilevel TIFF produced by GIMP.")
-                "Ignoring RGB ICC profile in bilevel TIFF produced by GIMP."
+            logger.warning("https://gitlab.gnome.org/GNOME/gimp/-/issues/3438")
-            )
-            logger.warning("https://gitlab.gnome.org/GNOME/gimp/-/issues/9518")
             iccp = None
 
     logger.debug("width x height = %dpx x %dpx", imgwidthpx, imgheightpx)
@@ -2101,7 +2101,16 @@ def read_images(rawdata, colorspace, first_frame_only=False, rot=None):
                 )
             )
         else:
-            if (
+            if color in [Colorspace.P, Colorspace.PA] and iccp is not None:
+                # PDF does not support palette images with icc profile
+                if color == Colorspace.P:
+                    newcolor = Colorspace.RGB
+                    newimg = newimg.convert(mode="RGB")
+                elif color == Colorspace.PA:
+                    newcolor = Colorspace.RGBA
+                    newimg = newimg.convert(mode="RGBA")
+                smaskidat = None
+            elif (
                 color == Colorspace.RGBA
                 or color == Colorspace.LA
                 or color == Colorspace.PA
@@ -2115,6 +2124,11 @@ def read_images(rawdata, colorspace, first_frame_only=False, rot=None):
                     newcolor = color
                     l, a = newimg.split()
                     newimg = l
+                elif color == Colorspace.PA or (
+                    color == Colorspace.P and "transparency" in newimg.info
+                ):
+                    newcolor = color
+                    a = newimg.convert(mode="RGBA").split()[-1]
                 else:
                     newcolor = Colorspace.RGBA
                     r, g, b, a = newimg.convert(mode="RGBA").split()
@@ -2125,15 +2139,6 @@ def read_images(rawdata, colorspace, first_frame_only=False, rot=None):
                     "Image contains an alpha channel. Computing a separate "
                     "soft mask (/SMask) image to store transparency in PDF."
                 )
-            elif color in [Colorspace.P, Colorspace.PA] and iccp is not None:
-                # PDF does not support palette images with icc profile
-                if color == Colorspace.P:
-                    newcolor = Colorspace.RGB
-                    newimg = newimg.convert(mode="RGB")
-                elif color == Colorspace.PA:
-                    newcolor = Colorspace.RGBA
-                    newimg = newimg.convert(mode="RGBA")
-                smaskidat = None
             else:
                 newcolor = color
                 smaskidat = None

src/img2pdf_test.py

@@ -4276,9 +4276,10 @@ def gif_transparent_pdf(tmp_path_factory, gif_transparent_img, request):
             == b"q\n45.0000 0 0 45.0000 0.0000 0.0000 cm\n/Im0 Do\nQ"
         )
         assert p.pages[0].Resources.XObject.Im0.BitsPerComponent == 8
-        assert p.pages[0].Resources.XObject.Im0.ColorSpace == "/DeviceRGB"
+        assert p.pages[0].Resources.XObject.Im0.ColorSpace[0] == "/Indexed"
+        assert p.pages[0].Resources.XObject.Im0.ColorSpace[1] == "/DeviceRGB"
         assert p.pages[0].Resources.XObject.Im0.DecodeParms.BitsPerComponent == 8
-        assert p.pages[0].Resources.XObject.Im0.DecodeParms.Colors == 3
+        assert p.pages[0].Resources.XObject.Im0.DecodeParms.Colors == 1
         assert p.pages[0].Resources.XObject.Im0.DecodeParms.Predictor == 15
         assert p.pages[0].Resources.XObject.Im0.Filter == "/FlateDecode"
         assert p.pages[0].Resources.XObject.Im0.Height == 60