I think the context normally changes for every bit. But this here
is enough to correctly decode the test bitstream in Annex H.2 in
the spec, which seems like a good checkpoint.
The internals of the decoder use spec naming, to make the code
look virtually identical to what's in the spec. (Even so, I managed
to put in several typos that took a while to track down.)
EXTTEMPLATE=1 was added later and doesn't seem to be used much in
practice -- it doesn't appear in no simple generic regions in any PDF
I tested so far at least. Since the spec contradicts itself on what
to do with these as far as I can tell, error out on them for now and
then add support once we find actual files using this, so that we can
check our implementation actually works.
Deduplicate the data reading for the different cases, and
zero-initialize all adaptive template pixels to zero to make that
possible.
Other than prohibiting EXTTEMPLATE=1, no behavior change.
The memmem() call passes `data.size() - 19 - sizeof(u32)` for big_len,
(18 prefix bytes skipped, the flag byte, and the trailing u32), so the
buffer needs to be at least that large.
Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67332
If the "MMR" bit is set, the generic region decoding procedure just
uses ITU-T T.6 (2D CCITT), which we already have an implementation of.
In practice, this is used almost never in .jbig2 files and in none of
the PDFs I have.
The two files that do use MMR are:
1.) JBIG2_ConformanceData-A20180829/F01_200_TT9.jb2
2) 042_3.jb2 from the ghostpdl tests
The first uses an immediate _lossless_ generic region, which we don't
implement yet (but I think it should just forward to the normal
immediate generic region code? Not in this commit, though). The second
uses a regular immediate generic region, and we can decode it now:
Build/lagom/bin/image -o out.png \
path/to/ghostpdl/tests/jbig2/042_3.jb2
With this, `image` can convert any jbig2 file, as long as it's
black (or white), and LibPDF can draw jbig2 files (again, as long
as they only contain a single color stored in just a
PageInformation segment).
Also make scan_for_page_size() not early return, so that it has the
same behavior as the main decoding look. (Multiple page information
segments for a single page are likely invalid and don't happen in
practice, so this is mostly an academic change.)
Add a BitBuffer class to store the bit image data, and introduce a
Page struct for storing data associated with a page. We currently
only handle a single page, but a) this makes it easier to decode
multiple pages in the future if we want b) it makes the code easier
to understand.
7.4.8.2 Page bitmap height:
"In some cases, this value may not be known at the time that the page
information segment is written. In this case, this field must contain
0xFFFFFFFF, and the actual page height may be communicated later, once
it is known."
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
