VueScan and Sub-Sampling

  • Thread starter Thread starter Andrew Borland
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A

Andrew Borland

Do any of the VueScan gurus hereabouts happen to know what sub-sampling
options are used by VueScan when saving JPEG files?

I've been running into problems when rotating JPEG images in other
applications and sub-sampling seems a likely cause.

Thanks in advance....

Regards, Andrew Borland (UK)
 
Andrew Borland said:
Do any of the VueScan gurus hereabouts happen to know what sub-sampling
options are used by VueScan when saving JPEG files?
Click to expand...

I don't know which JPEG library was used.
I've been running into problems when rotating JPEG images in other
applications and sub-sampling seems a likely cause.
Click to expand...

The sub-sampling is important when coding the JPEG, it is not going to
'cause' problems when rotating. The application used to rotate (I
presume in 90 degree increments) should be clever enough to do that
lossless, in which case the image is not recoded and the image data
doesn't change.

Bart
 
The sub-sampling is important when coding the JPEG, it is not going to
'cause' problems when rotating. The application used to rotate (I
presume in 90 degree increments) should be clever enough to do that
lossless, in which case the image is not recoded and the image data
doesn't change.
Click to expand...

I agree there is no recoding, but it appears that it is only possible
perform lossless rotation on files of certain dimensions i.e. Pixel
dimensions divisible by 8 or 16 or 32 depending on the sub-sampling. So
the rotation algorithm has to throw away those rows of pixels which it
cannot process. VueScan appears to be in the 16 or 32 camp which limits
one's choices in terms of crop sizes.

Regards, Andrew Borland (UK)
 
SNIP
I agree there is no recoding, but it appears that it is only possible
perform lossless rotation on files of certain dimensions i.e. Pixel
dimensions divisible by 8 or 16 or 32 depending on the sub-sampling. So
the rotation algorithm has to throw away those rows of pixels which it
cannot process. VueScan appears to be in the 16 or 32 camp which limits
one's choices in terms of crop sizes.
Click to expand...

Okay, that part was not clear to me from the original message, I
thought chroma subsampling was meant. Maybe Ed Hamrick can comment
whether 8x8 pixel blocks are possible with the JPEG library he uses,
or whether it is a deliberate choice to use a larger block, if that is
the case (I rarely output JPEG from VS, so I don't have that personal
experience).

I'd assume that the 'loss' of a few rows/columns of pixels only occurs
at first coding, and after that rotations deal with multiples of 8
anyway, so maybe the issue is with the software you use to rotate. You
could try JPEGCROP (http://www.jpegclub.org) to do the lossless
rotation, cropping, or fast scaling.

Bart
 
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