The lossless JPEG feature was originally implemented by Ken Murchison as
a patch against libjpeg v6b in 1999. (The patch was never adopted into
libjpeg, probably because Tom Lane was already moving away from the
project at that point.) As implemented, Ken's patch introduced new
fields into the public jpeg_compress_struct/jpeg_decompress_struct
structures that allowed lossless mode to be enabled (compression) and
queried (decompression.) However, that broke backward API/ABI
compatibility, which is a non-starter for libjpeg-turbo. Thus, in the
process of integrating Ken's patch, I heavily restructured it and moved
the new lossless parameters into the private
jpeg_comp_master/jpeg_decomp_master structures. (Refer to
https://github.com/libjpeg-turbo/ijg/commits/6b_lossless to see how this
all played out prior to the feature being merged into libjpeg-turbo.)
The lack of an intuitive way to query lossless mode in the public
decompressor API was an oversight on my part. However, the good news is
that you can still do it. It's just really subtle and requires digging
through wizard.txt to figure it out. In a lossless JPEG image, Ss will
always be non-zero and Se will always be 0. That's because Ss in a
lossless JPEG image is the predictor selection value, which must be
between 1 and 7 (inclusive), and Se is specified to be 0. That
combination of scan parameters is illegal for a lossy JPEG image, in
which Se must be greater than or equal to Ss. Thus, if any scan has Ss
!= 0 and Se == 0, then the JPEG image is lossless. The compressor takes
advantage of that fact to automatically enable lossless mode if a scan
script specifies a scan with Ss != 0 and Se == 0:
https://github.com/libjpeg-turbo/libjpeg-turbo/blob/335ed793f92370edbb425d56fe9c07aab5a93d9a/jcmaster.c#L184-L193
Thus, lossless mode can implicitly be enabled through a scan script
without explicitly calling jpeg_enable_lossless(), just like progressive
mode can implicitly be enabled through a scan script without explicitly
calling jpeg_simple_progression().
To make this more intuitive, I would probably need to introduce the
get/set API that I implemented for mozjpeg, which extends the public
libjpeg API to allow getting and setting the value of arbitrary
parameters (including those that are stored in the opaque master
structures.) That would not break backward API/ABI compatibility, but
it would break forward API/ABI compatibility (meaning that applications
that took advantage of the new functions would not work with prior
versions of libjpeg-turbo.) Thus, I would have to be really careful
when integrating the feature, and it would probably have to land in a
major new release rather than a bug-fix release.
Since the decompressor automatically handles lossless vs. lossy mode, it
isn't functionally critical for applications to have that information.
Thus, since there is already a non-intuitive but still straightforward
and reliable way to obtain that information, my inclination is not to
introduce a major new feature solely for that purpose.
On 1/21/24 6:01 AM, kevle wrote:
Hello everyone,
I have been trying to find out if I can determine whether a jpeg file
is lossless or lossy when decompressing it using the libjpeg API.
From what I can tell, it might be possible that the internal state in
"master->lossless" is setup after a call to "jpeg_read_header".
However, I have not seen a way to access that property by using the
public API.
In TurboJPEG API 3, there is "tj3Get" and "TJPARAM_LOSSLESS" that can
probably be used for the same purpose.
The question is, can I also access the "lossless" property using the
public libjpeg API or is there an indirect way of deriving that
information with public API calls?
Best regards,
Kevin
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