From: scuri@tecgraf.puc-rio.br (Antonio Scuri)
Date: Mon, 17 Nov 2003 18:04:56 -0200
Subject: [Tiff] TIFF JPEG and Upsampling
Message-ID: <6.0.0.22.0.20031117174233.022c9a38@pop.via-rs.com.br>

   Hi,

   I´m using LibTIFF 3.6.0 - tif_jpeg.c,v 1.15, in Visual C++ 7.0 (.NET). 
If I set JPEGCOLORMODE_RGB it works fine.

   Since I do not want to auto convert to RGB, and I want an upsampled 
YCbCr data, I look at the function JPEGPreDecode in line 745 of 
"tif_jpeg.c" and noticed that what I want is not possible if the original 
data is downsampled with "sp->xxx_sampling != 1", it will assume that I 
want a downsampled output.

   So I just comment lines 743-745 to avoid the "downsampled_output = 
TRUE;", and the decoding works fine because it changed the initialization 
of the decoder functions and libJPEG is upsampling the data for JPEGDecode.

   In the today logic it seems that I can not have upsampled output for a 
downsampled data if not using JPEGCOLORMODE_RGB. Is that true? Did I miss 
something?

   I noticed some "XXX" comments about upsampling in the code. Can a pseudo 
tag control upsampling? It could make the upsampling/downsampling logic easier.

Thanks,
Antonio Scuri


