[Poppler-bugs] [Bug 68360] Fuzzy rendering in SplashOutputDev in latest releases

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=68360

--- Comment #2 from Charles Hyder <dr.hyder at gmail.com> ---
(In reply to comment #1)
> It is an image of a resolution being parent with a different resolution. We
> have improved the code to do bilinearFiltering now instead of the crude
> filtering we had before.
> 
> ImageOutputDev does not do the bilinearFiltering since it's just writing the
> raw data to a file, not painting it to a page.
> 
> Sincerely I don't see the problem here.

Perhaps, I haven't clearly stated why this situation constitutes a problem.
Here's why I think it does: if I have, say, a JPEG image at 300dpi and I create
a PDF file by embedding the image in it and then render the page of the PDF
that contains the image with a resolution of exactly 300dpi then I should get,
ideally, the image that's IDENTICAL to the original JPEG image. In fact, this
is how it was in previous versions. But it is no longer the case: some
filtering is unnecessarily applied.

When describing the problem originally, by "rendering" I meant rendering at the
original resolution, 400dpi in that case. So, the rendering resolution is the
same as the original image resolution, not different!

In fact, bilinear filtering (by "filtering" you probably mean "interpolation)
is the way to go (bicubic is even better and is not very hard to implement
either) for rendering at resolutions DIFFERENT from the original one. But for
the same resolution (and resolutions that are integer multiples of the original
one) no filtering should be applied to embedded images. On the other had, even
if filtering is applied, it's based on interpolation, and as such, interpolated
values should coincide EXACTLY with the original pixel intensities if the
scaling factor is an integer.

To sum up, 1)no filtering is necessary when rendering images at certain
resolutions; 2) whatever filtering is used - even it's used at all times - it
is, apparently, not implemented correctly.

As a matter of fact, accurate rendering of embedded images is critical for the
project I'm working on. I'll appreciate very much if this issue is resolved so
that I can update the Poppler code to the latest version.

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