LibTiff Mailing List

On Monday, November 17, 2003, at 12:04 PM, Antonio Scuri wrote:

>
> 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

I think it would require changes in several places.  For example, the 
TIFFStripSize function would have to ignore YCbCrCoefficients.  After 
the JPEG codec converts the data from compressed entropy coded data to 
YCbCr samples (eg Y Y Y Y Cb Cr or Y Y Cb Cr ) then there would have to 
be a function to upsample the data.  I think you might be better off 
getting the downsampled data and then upsampling it in a separate 
function.

The YCbCr photometric interpretation is the only one of those supported 
by libtiff that allows up- and down-sampling, where for each component 
of the image various amounts of samples represent each pixel.  I have 
read that early TIFF versions allowed different bits per sample for each 
component, that is no longer the case, each component has the same 
precision.

It might be a good idea to add the up and down sampling logic to the 
encoder/decoder, execept that other compressions besides JPEG can use 
YCbCr.  TIFFReadRGBAImage will do upsampling of YCbCr image data.  It 
does this separately from the compressor/codec.

What I want to do is know how to pass downsampled data to the JPEG 
compressor.  I try to pass data that is already downsampled to the JPEG 
compressor, for example from the file ycbcrcat-tif of the sample images, 
and don't yet understand how to have the JPEG compressor handle that.  I 
think I might have to read the data from YCbCr image, upsample it to 
constant sampling factors, and then pass the requested sampling factors 
back to the output with setting the coefficients tag in the output.

Ross F.


AWARE [SYSTEMS]		Imaging expertise for the Delphi developer
		TIFF and LibTiff Mailing List Archive
	LibTiff Mailing List TIFF and LibTiff Mailing List Archive November 2003 Previous Thread Next Thread Previous by Thread Next by Thread Previous by Date Next by Date Contact The TIFF Mailing List Homepage This list is run by Frank Warmerdam Archive maintained by AWare Systems	Thread 2003.11.17 20:04 "TIFF JPEG and Upsampling", by Antonio Scuri 2003.11.18 00:19 "TIFF JPEG and Upsampling", by Ross Finlayson 2003.11.18 00:19 "TIFF JPEG and Upsampling", by Ross Finlayson On Monday, November 17, 2003, at 12:04 PM, Antonio Scuri wrote: > > 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 I think it would require changes in several places. For example, the TIFFStripSize function would have to ignore YCbCrCoefficients. After the JPEG codec converts the data from compressed entropy coded data to YCbCr samples (eg Y Y Y Y Cb Cr or Y Y Cb Cr ) then there would have to be a function to upsample the data. I think you might be better off getting the downsampled data and then upsampling it in a separate function. The YCbCr photometric interpretation is the only one of those supported by libtiff that allows up- and down-sampling, where for each component of the image various amounts of samples represent each pixel. I have read that early TIFF versions allowed different bits per sample for each component, that is no longer the case, each component has the same precision. It might be a good idea to add the up and down sampling logic to the encoder/decoder, execept that other compressions besides JPEG can use YCbCr. TIFFReadRGBAImage will do upsampling of YCbCr image data. It does this separately from the compressor/codec. What I want to do is know how to pass downsampled data to the JPEG compressor. I try to pass data that is already downsampled to the JPEG compressor, for example from the file ycbcrcat-tif of the sample images, and don't yet understand how to have the JPEG compressor handle that. I think I might have to read the data from YCbCr image, upsample it to constant sampling factors, and then pass the requested sampling factors back to the output with setting the coefficients tag in the output. Ross F.

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