fix: inherit fill color into Form XObjects; reset fill color on cs (#1688)

Address final review findings: a mask painted inside a Form XObject now
inherits the fill color in effect at the form's Do operator (previously it
reset to black, missing gray/color promotion one indirection deep). The cs
operator now resets the fill color to black per PDF spec, so a stale color
set before cs cannot leak to a subsequently drawn mask.

src/ocrmypdf/pdfinfo/_contentstream.py

@@ -134,7 +134,9 @@ def _normalize_stack(graphobjs):
             yield (operands, operator)
 
 
-def _interpret_contents(contentstream: Object, initial_shorthand=UNIT_SQUARE):
+def _interpret_contents(
+    contentstream: Object, initial_shorthand=UNIT_SQUARE, initial_fill_ink=Ink.mono
+):
     """Interpret the PDF content stream.
 
     The stack represents the state of the PDF graphics stack.  We track the
@@ -163,7 +165,7 @@ def _interpret_contents(contentstream: Object, initial_shorthand=UNIT_SQUARE):
     """
     stack = []
     ctm = Matrix(initial_shorthand)
-    fill_ink = Ink.mono  # PDF default fill color is black
+    fill_ink = initial_fill_ink  # PDF default fill color is black
     fill_space = '/DeviceGray'  # current fill colorspace name (for sc/scn)
     xobject_settings: list[XobjectSettings] = []
     inline_images: list[InlineSettings] = []
@@ -216,6 +218,9 @@ def _interpret_contents(contentstream: Object, initial_shorthand=UNIT_SQUARE):
                 fill_ink = _ink_from_components('cmyk', vals)
                 fill_space = '/DeviceCMYK'
         elif operator == 'cs':
+            # Selecting a colorspace resets the fill color to that space's
+            # initial value, which is black for all device colorspaces.
+            fill_ink = Ink.mono
             if operands:
                 fill_space = str(operands[0])
         elif operator in ('sc', 'scn'):

src/ocrmypdf/pdfinfo/_image.py

@@ -352,13 +352,19 @@ def _find_form_xobject_images(pdf: Pdf, container: Object, contentsinfo: Content
             # but in practice both Form XObjects and multiple drawing of the
             # same object are both very rare.
             ctm_shorthand = settings.shorthand
+            # A Form XObject inherits the graphics state (including fill color)
+            # in effect at the Do that draws it, so a mask painted with an
+            # inherited gray/color fill must carry that classification inward.
             yield from _process_content_streams(
-                pdf=pdf, container=form_xobject, shorthand=ctm_shorthand
+                pdf=pdf,
+                container=form_xobject,
+                shorthand=ctm_shorthand,
+                initial_fill_ink=settings.fill_ink,
             )
 
 
 def _process_content_streams(
-    *, pdf: Pdf, container: Object, shorthand=None
+    *, pdf: Pdf, container: Object, shorthand=None, initial_fill_ink=Ink.mono
 ) -> Iterator[VectorMarker | TextMarker | ImageInfo]:
     """Find all individual instances of images drawn in the container.
 
@@ -399,7 +405,7 @@ def _process_content_streams(
     else:
         return
 
-    contentsinfo = _interpret_contents(container, initial_shorthand)
+    contentsinfo = _interpret_contents(container, initial_shorthand, initial_fill_ink)
 
     if contentsinfo.found_vector:
         yield VectorMarker()

tests/test_pdfinfo.py

@@ -440,3 +440,42 @@ def test_imageinfo_ink_black(mask_black_pdf):
 def test_imageinfo_ink_none_for_regular_image(eight_by_eight_regular_image):
     image = pdfinfo.PdfInfo(eight_by_eight_regular_image)[0].images[0]
     assert image.ink is None
+
+
+def test_fill_ink_cs_resets_color_to_black():
+    # `cs` resets the fill color to the colorspace's initial value (black),
+    # so a stale color set before `cs` must not leak to the drawn mask.
+    assert _ink_of_first_xobject(b"0.8 0.2 0.2 rg /DeviceGray cs /Im0 Do") is Ink.mono
+
+
+def test_imageinfo_ink_inherited_in_form_xobject(outdir):
+    # A mask drawn inside a Form XObject inherits the fill color set before the
+    # Do that paints the form; the gray classification must reach the mask.
+    pdf = pikepdf.Pdf.new()
+    pdf.add_blank_page(page_size=(72, 72))
+
+    mask = pikepdf.Stream(pdf, bytes([0x7E] * 8))
+    mask.Type = pikepdf.Name.XObject
+    mask.Subtype = pikepdf.Name.Image
+    mask.Width = 8
+    mask.Height = 8
+    mask.ImageMask = True
+    mask.BitsPerComponent = 1
+
+    # Form draws the mask with no color of its own, inheriting the caller's.
+    form = pikepdf.Stream(pdf, b"q 72 0 0 72 0 0 cm /Im0 Do Q")
+    form.Type = pikepdf.Name.XObject
+    form.Subtype = pikepdf.Name.Form
+    form.BBox = [0, 0, 72, 72]
+    form.Resources = pikepdf.Dictionary(XObject=pikepdf.Dictionary(Im0=mask))
+
+    fname = pdf.pages[0].add_resource(form, pikepdf.Name.XObject)
+    pdf.pages[0].Contents = pikepdf.Stream(
+        pdf, b"0.263 0.263 0.263 rg %s Do" % bytes(fname)
+    )
+    out = outdir / 'form_mask.pdf'
+    pdf.save(out)
+
+    image = pdfinfo.PdfInfo(out)[0].images[0]
+    assert image.type_ == 'stencil'
+    assert image.ink is Ink.gray