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March 2005
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2005.03.22 20:38 "Re: tif_jpeg.c and 12bit JPEG Files", by Frank Warmerdam
On Tue, 22 Mar 2005 14:25:45 -0600 (CST), Bob Friesenhahn wrote: > 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. Bob, I have modified the JPEGDecode in tif_jpeg.c to take care of repacking the 16bit data into 12bit packed format. BTW, the changes are now all committed in tif_jpeg.c in the CVS head for libtiff for use with the Mk1 libjpeg. > 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. While I agree this would be desirable (auto-upgrade to 16bit), I am not personally keen to try and implement it. It might be fairly challenging to do properly without any adverse effects. Best regards, -- ---------------------------------------+-------------------------------------- I set the clouds in motion - turn up | Frank Warmerdam, warmerdam@pobox.com light and sound - activate the windows | http://pobox.com/~warmerdam and watch the world go round - Rush | Geospatial Programmer for Rent