[GreenKeys] bitsavers at Internet Archive

[Sam Hallas]

COURYHOUSE at aol.com wrote:
> As a general rule  though,  books that are in the main parts of
> Internet archive   are scanned   at a higher  res   than   what is at
> bitsavers it seems... Well . for   line  drawings  etc  no biggie,
> but when  you want a nice photograph of  something to include in a
> display the *Prelinger* stuff seems  cleaner.

I would beg to differ, Ed. I expect scans of text and line drawings to
be at least 300 dpi for clarity. Photographs can be as low as 150 dpi,
depending on the screening frequency, and still be clear.

Books that I've looked at from archive.org are often stored in a way 
that uses two different scan desities. Each page is an amalgam of a B&W 
image at a high resolution and a greyscale or colour image at lower 
resolution. Using the extract images tool in Acrobat produces both 
separately.

I just looked at an example from Google Books found at archive.org. (The 
Telephone 1891 Preece & Maier, 1891) The frontispiece photo consists of 
a B&W image at 600 dpi with the caption and some dark shadows at the 
edges. The main photo is at 300 dpi in greyscale. However another (The 
Practical Teelphone Handbook, Poole, 1892) from archive.org, but not 
Google, is only at 166 dpi but in B&W and colour.

One of my gripes with archived books is that the images are very often
in colour for black and white originals. It makes for mega file sizes
without adding any information.  The first book above (in B&W & 
greyscale) 16 MB for 555 pages whereas the second is 27 MB for only  308 
pages in colour but at lower resolution.

An unforgivable error I found in some books is that fold out diagrams
were left folded up, thereby ruining the whole point of the accompanying
text.

Rant over...

Cheers,
Sam