Handle null Form resources without losing inline measurements

README.md

@@ -62,6 +62,11 @@ invocations. A Form's own resources take precedence over inherited page
 resources. If reuse under different inherited resources produces different
 estimates, the definition's estimate becomes unknown with reason
 `context_dependent_color_space`; invocation counting still includes every use.
+Version 0.2.1 treats a Form's direct or indirect `/Resources null` like an omitted
+entry, preserving the tool's existing legacy resource fallback. An explicitly
+empty dictionary remains empty. PDF 32000-1 sections 7.3.7 and 7.3.9 specify
+null-entry equivalence; section 7.8.3 describes legacy Form resource inheritance
+as obsolete. This compatibility handling is not a modern PDF conformance claim.
 Cycles, excessive nesting/invocations, page limits and image limits
 produce explicit partial results. Uninvoked resource entries are not scanned.
 Top-level content is keyed per page; a content stream shared between pages is
@@ -178,6 +183,7 @@ python -m unittest discover -s tests -v
 Synthetic PDF fixtures test parser-backed payload measurements, filter arrays,
 long-form keys, false-positive text strings, bit-packed rows, image masks, repeated
 Forms, recursion, indirect named color spaces, Form resource precedence,
+null-versus-empty Form resources,
 context-dependent reuse, cyclic aliases, unknown color spaces, corruption,
 encryption, scan limits,
 timeouts and the Windows-compatible spawn-based CLI.

pdf_inline_audit.py

@@ -15,9 +15,9 @@
 
 import pypdf
 from pypdf import PdfReader
-from pypdf.generic import ContentStream, DictionaryObject
+from pypdf.generic import ContentStream, DictionaryObject, NullObject
 
-VERSION = "0.2.0"
+VERSION = "0.2.1"
 
 
 @dataclass(frozen=True)
@@ -168,6 +168,8 @@ def walk(stream: Any, resources: Any, label: str, page: int,
                 nested_resources = form.get("/Resources", resources)
                 if hasattr(nested_resources, "get_object"):
                     nested_resources = nested_resources.get_object()
+                if isinstance(nested_resources, NullObject):
+                    nested_resources = resources
                 walk(form, nested_resources, form_label(form), page, depth + 1, active)
 
     try:

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pdf-inline-audit"
-version = "0.2.0"
+version = "0.2.1"
 description = "Reproducible corpus statistics for PDF inline image payloads"
 readme = "README.md"
 requires-python = ">=3.10"

tests/test_audit.py

@@ -1,7 +1,7 @@
 import json
 from pathlib import Path
 from pypdf import PdfWriter
-from pypdf.generic import ArrayObject, BooleanObject, ByteStringObject, DecodedStreamObject, DictionaryObject, NameObject, NumberObject
+from pypdf.generic import ArrayObject, BooleanObject, ByteStringObject, DecodedStreamObject, DictionaryObject, NameObject, NullObject, NumberObject
 from pdf_inline_audit import Limits, distribution, isolated_scan, main, sample_estimate, scan_pdf
 
 def make_pdf(path, streams, form_data=None, recursive=False, color_spaces=None, form_color_spaces=None):
@@ -171,6 +171,36 @@ def test_form_colorspace_overrides_page_resource_name(self):
         assert result['status'] == 'ok'
         assert result['images'][0]['estimated_packed_sample_bytes'] == 12
 
+    def test_null_form_resources_match_absence_but_empty_dictionary_does_not(self):
+        source = make_pdf(self.tmp_path / 'source.pdf', [b'/Fm Do /Fm Do'],
+                          inline(b'00>', b'/W 1 /H 1 /BPC 8 /CS /Custom /F /AHx'),
+                          color_spaces=[{'/Custom': NameObject('/DeviceGray')}])
+        cases = [
+            ('absent', None, False, 1),
+            ('direct-null', NullObject(), False, 1),
+            ('indirect-null', NullObject(), True, 1),
+            ('empty-dictionary', DictionaryObject(), False, None),
+        ]
+        for name, resources, indirect, expected in cases:
+            with self.subTest(resources=name):
+                path = self.tmp_path / f'{name}.pdf'
+                writer = PdfWriter(clone_from=str(source))
+                form = writer.pages[0]['/Resources']['/XObject']['/Fm'].get_object()
+                if resources is not None:
+                    form[NameObject('/Resources')] = (
+                        writer._add_object(resources) if indirect else resources)
+                writer.write(path)
+                writer.close()
+                result = scan_pdf(path)
+                assert result['status'] == 'ok'
+                assert result['errors'] == []
+                assert len(result['images']) == 1
+                image = result['images'][0]
+                assert image['invocations'] == 2
+                assert image['estimated_packed_sample_bytes'] == expected
+                reason = 'unresolved_or_unsupported_color_space' if expected is None else None
+                assert image['estimate_unknown_reason'] == reason
+
     def test_reused_form_with_conflicting_inherited_colorspaces_is_unknown(self):
         path = make_pdf(self.tmp_path / 'context-dependent.pdf', [b'/Fm Do', b'/Fm Do'],
                         inline(b'000102>', b'/W 1 /H 1 /BPC 8 /CS /Custom /F /AHx'),