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	Thu, 1 May 1997 10:24:14 -0400 (EDT)
To: john@giorgio.hart.bbk.ac.uk
cc: tiff@sgi.com
Subject: Re: tiled-pyramidal-LAB-JPEG-TIFF 
In-reply-to: Your message of Thu, 1 May 1997 11:35:10 +0100 
             <9705011035.AA23061@giorgio.hart.bbk.ac.uk> 
Date: Thu, 01 May 1997 10:24:14 -0400
Message-ID: <21907.862496654@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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john@giorgio.hart.bbk.ac.uk writes:
> - I'm using PHOTOMETRIC_CIELAB, and COMPRESSION_JPEG. Does the JPEG
>   layer know about LAB, ie. does it replace its RGB->YCC conversion with
>   something more appropriate? I'm getting some sightly funny effects ...

tif_jpeg.c just turns off all color conversion and downsampling if
the photometric tag is anything but YCbCr.  I'm pretty sure I tested
that case (using PHOTOMETRIC_RGB) but would not want to swear there
are no bugs in it.  What are your "funny effects"?

With the current tif_jpeg code, LAB will not be compressed as efficiently
as it could be, because all three channels will be compressed at luminance
quality.  A and B ought to be compressed at chroma quality, I believe.
They really ought to be downsampled, too, but that's forbidden by the
current TIFF 6.0 spec (because the subsampling tags are defined with
respect to YCbCr only).  This is an oversight that I hope TIFF 7 will
rectify.

I was planning to revisit the code once TIFF 7 was out (Adobe sometimes
makes noises about a new draft), and make it know about LAB specifically.
But if you want to get in there sooner and hack on it, go for it.

			regards, tom lane
			organizer, Independent JPEG Group
