Date   : Mon, 23 Jun 2008 16:25:57 +0100
From   : faz@... (neil f)
Subject: The Micro User

 

> -----Original Message-----
> From: bbc-micro-bounces+faz=nildram.co.uk@... 
> [mailto:bbc-micro-bounces+faz=nildram.co.uk@...
> ] On Behalf Of Dave Moore
> Sent: Monday, June 23, 2008 3:51 PM
> To: Jules Richardson; bbc-micro@...
> Subject: Re: [BBC-Micro] The Micro User
> 
> > Mike Tomlinson wrote:
> > > In article 
> <314mFTNIO6388S10.1213970833@...>, Dave 
> > > Moore <dllm@...> writes
> > >> (I can't help but feel 600dpi is overkill for magazines, 
> as you'd 
> > >> end up requiring terabytes and terabytes of space to store the 
> > >> lossless
> versions).
> > > 
> > > The files should be highly compressible though?
> > 
> > Not in my experience - there tends to be a lot of noise in the data 
> > which reduces the compression algorithm's efficiency. It is 
> possible 
> > to skew the greyscale range (i.e. fold several tones into one, just 
> > not linearly) in
> order 
> > to reduce filesize, but then it requires extra visual 
> checks to make 
> > sure
> that 
> > important data's not being lost.
> 
> With reference to the greyscale pages featured here:
> http://mags.acornpreservation.org/Micro_User/v3n1
> 
> Regarding my OFFLINE copies, I was able to reduce the 
> grayscale images from approx 8Mb per page (lossless 600dpi 
> PNG) to 1Mb-2Mb per page (*still* lossless 600dpi PNG) by 
> simply increasing the contrast and 'blanking' the plain/empty 
> areas of each page i.e. removing the paper texture, so white 
> areas were pure white
> 
> But as Jules points out, you do need to carefully inspect 
> every page to ensure you aren't losing any detail (i.e. pale 
> shades of gray).
> 
> --
> Dave.
> 

Guys, there's very little point in scanning half-toned pages at greater than
300dpi, whether colour, greyscale or b/w.

The images on the page would have been printed at 150dpi max, probably less.
Computer printers can't print finer than 300dpi (higher claimed figures are
interpolated and in any case they're only any use if the original image was
high-res in the first place - such as a neg or transparency scan). And the
finest monitor screen resolutions are still far more coarse than 300dpi, so
600dpi is not even an advantage for screen viewing.

Seriously, 300dpi greyscale or colour, as appropriate, is as high as you
need to go. A good descreening filter is the next most important
requirement, to avoid those annoying moire patterns on pictures.

Save your storage capacity and processing speed for objects that can do
justice to high scanning resolutions - such as continuous tone photographic
prints, negatives or slides. Negs and transparencies benefit in particular
from high res settings because they often need to be enlarged considerably
before printing or viewing. Not so magazine pages.

There you go: that's a lossless 75% reduction in file size and you've still
got compression in reserve ;-) 

-Neil F.