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February 2009

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2009.02.12 10:45 "Crosstesting request 12 bits per sample JPEG compression in TIFF", by Joris Van Damme
2009.02.12 16:21 "Re: Crosstesting request 12 bits per sample JPEG compression in TIFF", by Bob Friesenhahn
2009.02.13 11:21 "Re: Crosstesting request 12 bits per sample JPEG compression in TIFF", by Joris Van Damme
2009.02.13 16:58 "Re: Crosstesting request 12 bits per sample JPEG compressionin TIFF", by Edward Grissom
2009.02.15 10:53 "Re: Crosstesting request 12 bits per sample JPEG compressionin TIFF", by Joris Van Damme

2009.02.12 10:45 "Crosstesting request 12 bits per sample JPEG compression in TIFF", by Joris Van Damme

Folks,

I've replaced my JPEG subcodec in my proprietary TIFF codec. The old
subcodec used to depend on LibJpeg, the new one depends on my new
proprietary JPEG codec that supports 8 and 12 bits per sample both. Thus,
I've extended my TIFF codec to support 12 bits per sample JPEG compression,
also.

The subject of subsampling was particularly tricky. JPEG compression without
subsampling is silly. Yet, the TIFF spec does not anticipate this need. In
the end, I've decided to logically and unambigiously 'interpolate' the TIFF 
spec, as we've all done before on numerous occasions.

I've assembled a series of testfiles. They're only checked in the sense that
encoder input matches decoder output. I've not been able to find a single 
software that can read back 12 bits per sample JPEG in TIFF, though, so 
there's been no crosstesting yet, and that's the reason for this post. I 
suspect some people in the GEO scene and possibly some others might have 
LibTiff extended with some special or dual LibJpeg build so as to support 
this. I'd be interested to see if these people can read back (some of) my 
testfiles. (Likely not all of them, as I go to some length supporting also 
combination of JPEG compression with planar storage, fillorder bit order 
reversal, CMYK and ITULAB color spaces, etc...) Also, the other way around, 
if anyone has some other 12 bits per sample JPEG in TIFF files to test with 
and/or can produce some, I'd be happy to test and see if my own decoder 
reads them back flawlessly.

The zipped testfile series is at
http://www.awaresystems.be/imaging/tiff/12bpsjpegintiff.zip. It's a pretty
large download, just over 13 megabyte.

All the testfiles in this series should yield the same rendering, being a
nice lady with white skin, green eyes and red lips, before a blackened
window and redish wall in the background. Except the files named
'Grayscale', that are a grayscale equivalent instead. All should be regarded
legit, at least in my opinion, except that the '4x4' file in the
'Subsampling' folder leaves room for discussion (this particular subsampling
is not correct in the JPEG file format, but it is in the TIFF file format,
and in my mind the latter is the encapsulator whose spec has the final say
in this matter), and the ones in the 'Incorrect subsampling values' folder
that willingly have incorrect values for the TIFFTAG_YCBCRSUBSAMPLING tag so
as to test auto-correction.

Let's hope we can ensure good interchange! If/when I find the time, I
certainly hope to seriously extend my site thoroughly documenting all this
stuff.


Best regards,

Joris