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2006.09.27 04:21 "YCbCr data in tiffcrop", by Richard Nolde
Joris / Bob/list,
The routines that I am using for reading and writing the data are
exactly those used in tiffcp now and they handle strips and tiles in
contiguous and separate planar configurations. I found no indication
that they made any distinctions based on the color model or compression
since they call low level libtiff routines to handle the codecs. They
are documented as delivering the uncompressed data to the specified
buffer in scanlines or tiles as requested. I deduced by experiment, that
this buffer is always arranged as plannar config contiguous for all the
sample formats that I have been able to handle correctly. I surmise
that YCbCr data comes in differently because the resulting image is
shifted half an image width and badly color shifted but vaguely
recognizable when I try to mirror it horizontally. When mirrored
vertically, it gets chopped up into 4 quadrants with a stripe in the
center. Based on the explanation of YCbCr, which is admittedly over my
head, my simple copying and/or chopping of portions of the image based
on bits per sample and samples per pixel is probably not going to work
here unless I can reorder the data in the read buffer to resemble a
multi-byte per pixel sample with a known number of bytes to grab for
each pixel. RGBA works fine, but the sequence described by Joris
doesn't look too promising for pointer offset slicing and dicing. I
don't need YCbCr for my project but I wanted to try to support all the
options that tiffcp does since tiffcrop is a superset of tiffcp. Given
an infinite amount of time, I would like to add more functions like
multi-page splitting capabilities found in tiff2ps so that one image can
be spread over multiple output images. Tiff2ps relies on the Postscript
mask operator to do this but it is very inefficient storage wise as it
keeps a copy of the entire image for each page and just masks off
different regions each time. My code from tiffcrop could be ported to
tiff2ps to store only the cropped portion in each image.
Richard Nolde
Joris wrote:
> Joris wrote:
This applies to all forms of
subsampled YCbCr, uncompressed, LZW compressed, flate compressed,
whatever. Unfortunately, the LibJpeg library returns another
organisation, and a convertion needs to be made...
I meant, a convertion needs to be made inside tif_jpeg.c, that may not
have been clear. The purpose of this conversion is to take data
organisation as returned by LibJpeg, and reformat it into the
compression-independent organisation that is assumed by the rest of the
library, thus making all subsampled YCbCr organisation uniform.
I think there's currently a bug left in exactly this part of tif_jpeg.c,
presumably, seeing the output of some image, but I've not yet had the
time to investigate properly. This may additionally make it hard for
Richard to add subsampled YCbCr support inside tiffcrop, unless he
chooses to work from uncompressed subsampled YCbCr testimages and drops
the use of JPEGCOLORMODE_RGB if he's using it right now, which I'd
recommend anyway since that is the single shortest path to full
compression-independent support for subsampled YCbCr.
Best regards,
Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
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