Gábor Stefanik created PDFBOX-4831:
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Summary: Rounding errors when rendering non-interleaved binary
CCITT image at 1:1 scale cause gray pixels in output
Key: PDFBOX-4831
URL: https://issues.apache.org/jira/browse/PDFBOX-4831
Project: PDFBox
Issue Type: Bug
Components: Rendering
Affects Versions: 2.0.19
Reporter: Gábor Stefanik
I have a 300dpi scanned PDF file with a single CCITT-encoded black-and-white
image in each page, spanning the whole page. The images all have a resolution
of 2480x3504.
When I try to render a page from this PDF into a PNG at 300DPI, the resulting
PNG has some pixels with colors #010101 and #fefefe. The PNG has the same
2480x3504 dimensions as the embedded CCITT images, but stepping through the
PDFBox code reveals it's trying to downscale the image by a tiny fraction of a
pixel (e.g. to 2479.999964573x3503.9999537378) using bicubic interpolation,
introducing these "near-black" and "near-white" pixels due to rounding errors.
Additionally, the actual image conversion code goes to a slow path intended for
"proper" interpolated scaling, rather than hitting the fast path for copying
1:1-scale images.
For now, we worked around this by treating images containing only #000000,
#010101, #fefefe and #ffffff as binary, but the performance hit from the slow
path is still there.
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