Gábor Stefanik created PDFBOX-4832:
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Summary: Binary CCITT image with nearest neighbor interpolation
instead gets upscaled with bicubic, PDFBox thinks it's downscaling
Key: PDFBOX-4832
URL: https://issues.apache.org/jira/browse/PDFBOX-4832
Project: PDFBox
Issue Type: Bug
Components: Rendering
Affects Versions: 2.0.19
Reporter: Gábor Stefanik
Attachments: pdf_scanned-eng-bw.pdf
The attached PDF contains a CCITT-encoded black-and-white image of a roughly US
Letter-sized page (611x783 PDF points page size), scanned at 200DPI. The image
is slightly undersized, at 1696 pixels wide, instead of the expected 1697. The
PDF calls for this image to be upscaled using nearest-neighbor interpolation
(downscaling is always smoothly interpolated, to avoid artifacts - see
PDFBOX-2364).
When I try to render this page to an RGB image in either 200 or 300 DPI, the
image unexpectedly ends up containing shades of gray. Upon closer inspection,
it is clear that the upscaling was performed using bicubic interpolation
instead of nearest-neighbor. Stepping through the PDF rendering code reveals
that, in both cases, PDFBox somehow thinks the image is being _down_scaled, and
applies the behavior from PDFBOX-2364. Additionally, PDFBOX-4831 may also be at
play here, as the computed target image dimensions aren't round integers, but
very slightly undersized (e.g. 2174.999858106px instead of 2175px).
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