[luatex] Prevent stop when tiff file loads

[Hans Hagen]

Hi Patrick,

>> In practice, identifying all image files formats (tiff, png, pdf...) *should* work with the first 4 bytes of the file (they were designed this way).
>
> I think I'll go that way, but I still wish (feature request) that LuaTeX does not fail with a fatal error if an unknown file format is encountered, instead it would be nice to have something like "return nil, errormessage".

In addition to L's remark ...

Concerning img in luatex: when an image is needed (can be a delayed 
action) the backend will create objects that keep some states (e.g. 
requirements and later identified properties). Then at some point there 
is some checking of filename and preamble, then when it's a known format 
the datastructures are checked .. at any point luatex can quit on error 
(recovering is no option due to the many dependencies or maybe 
overflows).  For instance changing the error in a warning (i tested it 
to be sure) aborts luatex.

Now, we could reshuffle code of course (and for instance check first) 
but then we have a chicken-egg issue: we have callbacks for locating 
files and these can decide to choose for instance to report back a jpg 
instead of a gif when present. Think of:

- scan requested
   - image objects made (anchored in other backend mechanisms)
   - image scan happens
     - file is looked up based on info stored in object (1)
     - when known type detailed scan happens (2)
     - extended object is available
    - a valid object is required (contains partially parsed image data + 
properties now)

moving 1 up in the chain could probably work but then we still can't be 
too sure about success later on anyway where we still would need to 
abort the run.

Anyway, I made a catch for an unknown type but beware: you need to check 
the dimensions (zero) in order to figure out that was ignored. Basically 
an image object is still present but with zero dimensions (after all it 
was not scanned).

 From the upcoming 0.89 manual:

You can use \type {pdf.setignoreunknownimages(1)} (or at the \TEX\ end
the \type {\pdfvariable} \type {ignoreunknownimages}) to get around a
quit when no known image type is found (based on name or preamble).
Beware: this will not catch invalid images and we cannot guarantee side
effects. A zero dimension image is still included when requested. No
special flags are set. A proper workflow will not rely in such a catch
but make sure that images are valid.

(Fwiw: personally i'd not rely on such a catch in a workflow.)

Hans

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