Sounds like the spec guarantees that that's the number of the first
page.
(In practice, all but one of all jbig2 files I've found contain just
page 1. PDFs almost always contain just page 1, and very rarely a
page 0 for globally shared parameters.)
This allows `file` to correctly print the dimensions of a .jbig2 file,
and it allows us to write a test that covers much of all the code
written so far.
Several ramifications:
* /JBIG2Globals is an indirect reference, which means we now need
a Document for unfiltering. (Technically, other decode parameters
can also be indirect objects and we should use the Document to
resolve() those too, but in practice it only seems to be needed
for /JBIG2Globals.)
* Since /JBIG2Globals are so rare, we just parse once for each
image that use them, and decode_embedded() now receives a
Vector<ReadonlyBytes> with all sections of sequences of
segments.
* Internally, decode_segment_headers() is now called several times
for embedded JBIG2s with multiple such sections (e.g. PDFs with
/JBIG2Globals).
* That means `data` is now no longer part of JBIG2LoadingContext
and things get slightly reshuffled due to this.
This completes the LibPDF part of JBIG2 support. Once LibGfx
implements actual decoding of JBIG2s, things should start to
Just Work in PDFs.
They're in different places for Sequential/Embedded (right after
the header) and RandomAccess (which has all headers first, followed
by all data bits next).
We don't do anything with the data yet, but now everything's in
place to actually process segment data.
Except for /JBIG2Globals, which we bail out on for now. In my 1000
files, 13 use JBIG2, and of those, 2 use JBIG2Globals (0000372.pdf e.g.
page 11 and 0000857.pdf e.g. page 1), and only one (the latter) of the
two uses the same JBIG2Globals stream for more than a single image.
JBIG2ImageDecoderPlugin cannot decode the data yet, so no behavior
change, but with `#define JBIG2_DEBUG 1` at the top of that file,
it now prints segment header info for PDFs containing JBIG2 data :^)
With `#define JBIG2_DEBUG 1` at the top of the file:
% Build/lagom/bin/image --no-output \
.../JBIG2_ConformanceData-A20180829/F01_200_TT10.jb2
JBIG2LoadingContext: Organization: 0 (Sequential)
JBIG2LoadingContext: Number of pages: 1
Segment number: 0
Segment type: 48
Referred to segment count: 0
Segment page association: 1
Segment data length: 19
Segment number: 1
Segment type: 39
Referred to segment count: 0
Segment page association: 1
Segment data length: 12666
Segment number: 2
Segment type: 49
Referred to segment count: 0
Segment page association: 1
Segment data length: 0
Runtime error: JBIG2ImageDecoderPlugin: Draw the rest of the owl
With `#define JBIG2_DEBUG 1` at the top of the file:
% Build/lagom/bin/image --no-output \
.../JBIG2_ConformanceData-A20180829/F01_200_TT10.jb2
JBIG2LoadingContext: Organization: 0 (Sequential)
JBIG2LoadingContext: Number of pages: 1
Segment number: 0
Segment type: 48
Referred to segment count: 0
Segment page association: 1
Segment data length: 19
Runtime error: JBIG2ImageDecoderPlugin: Draw the rest of the owl
