The splash backend uses bilinear interpolation for rendering images,
even when the image lacks the Interpolate flag. This seems to be
contrary to what PDF Reference 1.7 says:

I've attached a test PDF and two pdftoppm outputs that illustrate the
problem:
- with poppler 0.18.4, you get black&white image, as expected;
- with poppler 0.26.5, you get gray goo instead.


See also bug #760396 for some background information.
Suggestions as to how I could work around this bug in pdf2djvu are
welcome.

Hi,

 From a quick look, it seems like
https://bugs.freedesktop.org/show_bug.cgi?id=68360
could be your case.

If not, can you please report the findings in #760396 (with links, etc)
to a new bug in the upstream bug tracker?

I'm not sure you can do anything without changes in Poppler.

Thanks for your report,

* Pino Toscano <pino@debian.org>, 2014-10-08, 13:21:

Ah yes, that would be it. Although I would argue that interpolating
images that lack the Interpolate flag is always wrong, not only for
"native resolution".

Reading the upstream bug gave me idea for a work-round for the cases
when scale factor is almost-but-not-quite an integer. I think I could
slightly modify the CTM, so that the scale is exactly integer. (Yes,
it's crazy, but it could work.)