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Fix OCR text displacement on PDFs with non-zero MediaBox origins

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_build_text_layer_ctm() returned None when text_rotation was 0,
skipping the origin translation needed for pages with non-zero
MediaBox origins (e.g. JSTOR PDFs with [0, 100, 595, 982]). This
caused the text layer to be offset by the page origin amount.

Always compute the full CTM and only return None when the result
is the identity matrix.

Fixes #1630
This commit is contained in:
James R. Barlow
2026-02-17 23:34:33 -08:00
parent 5890d1855e
commit 10b71937c4
2 changed files with 25 additions and 25 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tests/test_graft.py
+12 -20
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@@ -46,24 +46,19 @@ def test_redo_ocr_with_offset_mediabox(resources, outdir):
"""Test that --redo-ocr handles non-zero mediabox origins correctly.
Regression test for issue #1630 where PDFs with mediabox origins like
[0, 100, width, height+100] (common in cropped PDFs)
would have OCR text shifted vertically because the Form XObject BBox
used the text layer's mediabox [0, 0, w, h] instead of the base page's
mediabox [0, 100, w, h+100].
Before the fix, the BBox would be [0, 0, w, h] but the transformation
matrix would expect [0, 100, w, h+100], causing a 100pt vertical shift.
[0, 100, width, height+100] (common in cropped/JSTOR-style PDFs)
would have OCR text shifted vertically because the text layer CTM
did not account for the page origin offset.
"""
# Create a PDF with a non-zero mediabox origin
input_pdf = outdir / 'offset_mediabox_input.pdf'
y_offset = 100
with pikepdf.open(resources / 'graph_ocred.pdf') as pdf:
page = pdf.pages[0]
original_mb = list(page.MediaBox)
# Shift mediabox Y origin to simulate cropped/JSTOR-style PDFs
# This is the scenario that triggers the bug
y_offset = 100
page.MediaBox = [
original_mb[0],
original_mb[1] + y_offset,
@@ -84,18 +79,15 @@ def test_redo_ocr_with_offset_mediabox(resources, outdir):
# MediaBox origin should be preserved
assert (
float(mediabox[1]) == 100.0
), f"MediaBox Y origin should be preserved at 100, got {mediabox[1]}"
float(mediabox[1]) == y_offset
), f"MediaBox Y origin should be preserved at {y_offset}, got {mediabox[1]}"
# MediaBox should have valid dimensions
width = float(mediabox[2]) - float(mediabox[0])
height = float(mediabox[3]) - float(mediabox[1])
assert width > 0 and height > 0, "MediaBox should have positive dimensions"
# Page content should be present (may include XObject references
# for the OCR text layer and/or inline image operators)
text_content = page.Contents.read_bytes()
assert len(text_content) > 0, "Page should have content"
# The content stream should include a CTM with the Y origin translation.
# Without the fix, the CTM was omitted for rotation==0, causing a shift.
content = page.Contents.read_bytes()
assert b'cm' in content, (
"Content stream should include a CTM to translate by the page origin"
)
def test_strip_invisble_text():