[jira] [Created] (PDFBOX-4831) Rounding errors when rendering non-interleaved binary CCITT image at 1:1 scale cause gray pixels in output

["Gábor Stefanik (Jira)" <ji...@apache.org>]

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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