https://bugs.documentfoundation.org/show_bug.cgi?id=173679
Bug ID: 173679
Summary: DOCX: transparent PNG background renders horizontal
boundary lines in Writer
Product: LibreOffice
Version: 26.2.5.2 release
Hardware: All
OS: Windows (All)
Status: UNCONFIRMED
Severity: normal
Priority: medium
Component: Writer
Assignee: [email protected]
Reporter: [email protected]
Description:
When opening the attached DOCX in LibreOffice Writer, thin horizontal lines are
rendered at the boundaries of a transparent background image.
The lines are already visible directly in Writer, before exporting the document
to PDF. Exporting to PDF preserves the same artefacts.
The same DOCX opened in Microsoft Word does not display these lines, and a PDF
exported from Microsoft Word does not contain them.
The issue was reproduced with:
LibreOffice 25.2.4.3
LibreOffice 26.2.5.2 (X86_64)
Windows 11
Font availability was tested separately. The document uses Poppins and Calibri.
Installing the missing Poppins font corrected a separate pagination difference
(LibreOffice changed from 3 pages to 4 pages, matching Microsoft Word), but the
graphical artefact remained. Therefore, the horizontal lines do not appear to
be caused by font substitution.
Steps to Reproduce:
Open the attached DOCX in LibreOffice Writer.
Inspect the pale transparent background graphic on the right side of page 1.
Observe the thin horizontal line at the boundary of the background graphic.
Export the document using File → Export As → Export as PDF.
Open the generated PDF.
Observe that the same horizontal artefact is present.
Open the same DOCX in Microsoft Word for comparison.
Actual Results:
LibreOffice Writer renders thin horizontal lines at the boundaries of the
transparent background image. The same lines are retained in PDF export.
Expected Results:
The transparent background image should be rendered without visible boundary
lines, as it is in Microsoft Word.
Reproducible: Always
User Profile Reset: Yes
Additional Info:
Investigation of the DOCX shows that the affected graphic is an embedded PNG
with fully transparent pixels (alpha = 0) on its outermost rows.
The RGB value underneath those transparent pixels is black.
As a test, only the outermost pixel rows of the embedded PNG were changed.
After this modification, the horizontal artefacts disappeared when the DOCX was
rendered by LibreOffice.
Adding an explicit no-outline setting to the picture did not remove the
artefact.
This suggests that the issue is related to LibreOffice's rendering/compositing
of the transparent image rather than a picture border or PDF export setting.
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