TST: Add test for correct rendering of watermarks (#2130)

This adds a test for the correct rendering of watermarked files.

Closes #2112

.github/workflows/github-ci.yaml

@@ -30,6 +30,12 @@ jobs:
  - python-version: "3.10"
  use-crypto-lib: ""
  steps:
+ - name: Update APT packages
+ run:
+ sudo apt-get update
+ - name: Install APT dependencies
+ run:
+ sudo apt-get install ghostscript
  - name: Checkout Code
  uses: actions/checkout@v3
  with:

tests/test_writer.py

@@ -1,5 +1,7 @@
 """Test the pypdf._writer module."""
 import re
+import shutil
+import subprocess
 from io import BytesIO
 from pathlib import Path
 
@@ -29,11 +31,13 @@
 )
 
 from . import get_data_from_url, is_sublist
+from .test_images import image_similarity
 
 TESTS_ROOT = Path(__file__).parent.resolve()
 PROJECT_ROOT = TESTS_ROOT.parent
 RESOURCE_ROOT = PROJECT_ROOT / "resources"
 SAMPLE_ROOT = Path(PROJECT_ROOT) / "sample-files"
+GHOSTSCRIPT_BINARY = shutil.which("gs")
 
 
 def test_writer_exception_non_binary(tmp_path, caplog):
@@ -1575,6 +1579,35 @@ def test_watermarking_speed():
  assert pdf_size_in_mib < 20
 
 
+@pytest.mark.enable_socket()
+@pytest.mark.skipif(GHOSTSCRIPT_BINARY is None, reason="Requires Ghostscript")
+def test_watermark_rendering(tmp_path):
+ """Ensure the visual appearance of watermarking stays correct."""
+ url = "https://github.com/py-pdf/pypdf/files/11985889/bg.pdf"
+ name = "bgwatermark.pdf"
+ watermark = PdfReader(BytesIO(get_data_from_url(url, name=name))).pages[0]
+ url = "https://github.com/py-pdf/pypdf/files/11985888/source.pdf"
+ name = "srcwatermark.pdf"
+ page = PdfReader(BytesIO(get_data_from_url(url, name=name))).pages[0]
+ writer = PdfWriter()
+ page.merge_page(watermark, over=False)
+ writer.add_page(page)
+
+ target_png_path = tmp_path / "target.png"
+ url = "https://github.com/py-pdf/pypdf/assets/96178532/d5c72d0e-7047-4504-bbf6-bc591c80d7c0"
+ name = "dstwatermark.png"
+ target_png_path.write_bytes(get_data_from_url(url, name=name))
+
+ pdf_path = tmp_path / "out.pdf"
+ png_path = tmp_path / "out.png"
+ writer.write(pdf_path)
+
+ # False positive: https://github.com/PyCQA/bandit/issues/333
+ subprocess.run([GHOSTSCRIPT_BINARY, "-sDEVICE=pngalpha", "-o", png_path, pdf_path]) # noqa: S603
+ assert png_path.is_file()
+ assert image_similarity(png_path, target_png_path) >= 0.95
+
+
 @pytest.mark.enable_socket()
 def test_da_missing_in_annot():
  url = "https://github.com/py-pdf/pypdf/files/12136285/Building.Division.Permit.Application.pdf"