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https://issues.apache.org/jira/browse/PDFBOX-4831?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17295408#comment-17295408
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Tilman Hausherr commented on PDFBOX-4831:
-----------------------------------------
Please change the segment in PageDrawer.java to this:
{code}
Matrix xformMatrix = new Matrix(xform);
boolean isScaledUp = bim.getWidth() <=
Math.round(ctm.getScalingFactorX() * xformMatrix.getScalingFactorX()) ||
bim.getHeight() <=
Math.round(ctm.getScalingFactorY() * xformMatrix.getScalingFactorY());
{code}
This takes two of your suggestions and also considers rotations.
When rendering your file at 300 dpi {{isScaledUp}} is now true, and the number
of unique colors changes from 4 to 2, and the rendering gets faster, so I guess
that's what you wanted.
However the rendering of a lot of files changes, sometimes worse. But it looks
closer to Adobe Reader. A good example is gs-bugzilla688728.pdf, where the new
rendering is now the same as in Adobe. An example for "worse" is
K_UPMEYER_SPRING10.pdf, the image at the lower right of page 1, or
outside-in.pdf the "MARKET NOTE" top right. (But Adobe is also bad there).
(Files mentioned are available by googling, if not, mention them and I'll
upload them)
> 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
> Priority: Major
> Attachments: 13._Korona_szallo_vegzes_13.09.26.eredeti-1.png,
> 13._Korona_szallo_vegzes_13.09.26.eredeti.pdf
>
>
> 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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