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-   -   Does Downsampling Reduce Image Quality? (http://www.photobanter.com/showthread.php?t=53764)

Dave Martindale December 22nd 05 09:00 AM

Does Downsampling Reduce Image Quality?
 
Ken Hall writes:

I understand that upsampling can result in a poor quality image, but I
have always been under the impression that downsampling does not
reduce the image quality.


You have to be more precise about what "image quality" means to,
particularly the size of the image when you are looking at it.

Upsampling results in *the same* quality image when viewed with the
images all the same size, or it can be better if it reduces staircasing
along edges. Downsampling reduces the image quality at the same size,
because there are fewer pixels and less fine detail.

On the other hand, if you're talking about looking at a small area of
the image at 100% scale, then the comments above make more sense.
Upsampling spreads the same information over more pixels, so the image
looks less sharp at 100%. Downsampling usually gives an image that
looks sharper at 100%, because features are smaller - but there is less
information.

The view at 100% is not what matters - it's the view of the full image
that's important.

Dave

Bill Tuthill December 22nd 05 07:14 PM

Does Downsampling Reduce Image Quality?
 
Dave Martindale wrote:

Ken Hall writes:
I understand that upsampling can result in a poor quality image,
but I have always been under the impression that downsampling
does not reduce the image quality.


You have to be more precise about what "image quality" means to you,
particularly the size of the image when you are looking at it.

Upsampling results in *the same* quality image when viewed with the
images all the same size, or it can be better if it reduces staircasing
along edges. Downsampling reduces the image quality at the same size,
because there are fewer pixels and less fine detail.


I have read answers to this point, but didn't see any practical advice.

Some downsampling algorithms work better than others. In particular,
the Lanczos algorithm works better than Bicubic for downsampling.
ImageMagick "convert" produces excellent results for me, better than
"bicubic sharper" in Photoshop CS2. Examples/proof he

http://www.photo.net/bboard/q-and-a-...?msg_id=00E7py

Many algorithms produce identical results at 50% (modulo sharpening)
so perhaps Ken should not have asked such a specific question.

Furthermore, you will find that the finer downsampling algorithms
(such as Lanczos) produce much better results than Epson software,
and probably most digital minilabs as well, so you are best advised
to do your own downsampling.



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