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March 2005
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2005.03.29 11:24 "Re: tif_jpeg.c and 12bit JPEG Files", by Jean-yves Le Ridant
> > > The only way that makes sense is for 12 bit samples to be > > > returned the same as for an uncompressed 12 bit TIFF file. > > > Likewise, 12 bit samples should be consumed so that the only > > > thing special the library user needs to do is request JPEG > > > compression, and possibly JPEG RGB mode. > > > > Well... the "issue" is that the jpeg library treats 12 bit samples as > > 12 bit stored in 16 bit integers. JPEG's transformation uses 16 bit > > ints for the math anyways ... so someone has to do the upsampling. > > Both the classic and the MK1 jpeg expect the user to do this. > > This is not really compatible with the way the libtiff interfaces > work. Libtiff normally provides the data as a scanline, strip, or > tile where the bits-per-sample of the data is the same as reported by > libtiff, and the values occupy the full range of the sample. As part > of the "decompress" step, the data should be converted to packed > 12-bit form. This can easily be done "in-place" if necessary while > reading since 12-bits is smaller than 16-bits. A secondary buffer > would be needed while writing. > > In order to make 12-bit JPEG in TIFF available to a broader variety of > applications, an option could be provided via a virtual tag (similar > to that used by the LogLuv support) to set libtiff in a mode where > 12-bit JPEG in TIFF pretends to be a 16-bit format. 12-bit JPEG in > TIFF files would be reported as 16-bits/sample. In this case the > samples should be scaled to 16-bits. The only change necessary for > applications already supporting 16-bit TIFF is to set the option > requesting that 12-bit TIFF be supported like 16-bit TIFF. Yes, but the problem of a "clean" release with jpeglib for 8 and 12 bits samples ? As said by other contributors, I agree that's much a burden to carry. JSAMPLE, JSAMPROW, GET_JOCTET .... that parse the code. And 12 bits compression by jpeglib *must* be two pass mode ... I use Jpeglib v6 in the way of Joris, 12 bits CIELAB, with JCS_UNKNOWN ... , and it work very fine. The only matter, for a 'clean' release, is having 2 build of the Jpeglib, includes/typedef, .... and it's a matter. The less changes, would be having a 12 bits jpeglib build, and feed 8 or 12 bits data in a 16 bits JSAMPLE ... ? -- Jean-Yves ( sorry for his poor english ).