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2003.11.17 20:04 "TIFF JPEG and Upsampling", by Antonio Scuri

2003.11.18 00:19 "TIFF JPEG and Upsampling", by Ross Finlayson

2003.11.17 20:04 "TIFF JPEG and Upsampling", by Antonio Scuri

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