2005.03.22 17:20 "[Tiff] tif_jpeg.c and 12bit JPEG Files", by Frank Warmerdam
I am planning to commit some changes to tif_jpeg.c in CVS to support 12bit jpeg encoded TIFF files using a new working version of the IJG libjpeg (which we have been calling Mk1).
In investigations, I have come to the conclusion that calling TIFFReadEncodedTile() to read YCbCr downsampled JPEG data is completely broken for 8 and 12 bit JPEG encoded imagery currently. Does anyone know any differently? It seem that the only way to get these working is to force libjpeg to convert to RGB on the fly by setting TIFFTAG_JPEGCOLORMODE to JPEGCOLORMODE_RGB.
In tif_jpeg.c direct YCbCr access is done via a set of "raw" access functions. But it is these that seem to be broken. Does anyone feel a compelling need to be able to read YCbCr downsampled raw data from JPEG encoded TIFF files? That is, should I also try and fix the "raw" access while I am in here? (no promises... it looks yucky).
My general intent then is to modify the tif_jpeg.c JPEGDecode() (and likely JPEGEncode()) to support repacking 12bit data from 16 bit JSAMPLEs returned by libjpeg into packed 12bit format in the TIFF buffer. This seems to be the main barrier to correct operation currently. But there are a few other patches needed, and also the ability to autodetect at build time which style of libjpeg is being used. The traditional "6b" libjpeg, or the newer "Mk1".
BTW, the 6b libjpeg has to be built either in 8bit mode or 12bit mode. One build of the library cannot support both forms. The new Mk1 library always returns samples as 16 samples and can read either 8bit or 12bit files dynamically.
Finally, the "Mk1" library is essentially a libjpeg fork. There is discussion on the JPEG mailing list about how to move to a libjpeg7 with similar capabilities. While it is likely that a libjpeg7 would take a somewhat different approach than the Mk1 library, it is hoped that the work being done now will help in that transition.
Anyone interested in using the Mk1 library can contact me or Dave Gilbert (see cc) for details. It is sitting on the same CVS server as libtiff with a module name of "libjpeg". Make sure to check out the mk1 tag.
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