On 1/14/2017 12:03 AM, David Carlisle wrote:
latex (far more than context or plain) has an expectation for working
over multiple
engines. Documented (and documentable) differences are understandable,
luatex
isn't expected to be fully compatible with pdftex. But this is a
difference in internal
data in a binary file where luatex is taking a different choice to
pdftex (and image
magic at least as far as I can see) It is going to be very hard to
document to end users
when this might occur or what to do if it does.
fwiw ... Traditionally graphics are unrelated to engines and when we had
reasons to support different backend (dvi+special flavours as well as
pdf) context always had a clear separation between front- and backend
via some intermediate common interface. (In fact this was quite a
handicap for graphics as one had to deal with scale based vs size based
vs .. backend support.) As in dvi mode we had no means to know
properties of graphics we always used either parsing preambles or
external programs to identify these properties.
Anyway, graphics and especially ones with errors in them always will be
a problem and no engine can handle them all. Recently we added
orientation support in luatex which is also tricky. There has been
issues with 1 pixel resolutions (eventually we decided that messing up
internals to catch one bad graphic is a no-go). Color in images
(included in pdf) can be an issue too (but we deal with that in
different ways). Messed up external graphics and fonts will always be an
issue I fear. And one can always code around it (e.g. using some flag)
but another one (or the reverse case) will pop up. The nice thing about
tex is that one can always say: rubish in - rubish out.
We can cook up some flag (0=current, 1=exif jfif swapped, 2=use
specified resolution) but in the end the user still has to make a choice
in case of troubles (of course the macro package can decide for some
default).
Hans
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