[sdiy] GR-500 schematics

[Andre Majorel]

On 2006-07-25 10:21 +0000, sergio koval wrote:

> Due this, I logged an account in photobucket.com, but I found
> that the site has a limit of 1024kb per file, and the biggest
> scan weights 6.6Mb.

To anyone with scans to publish :

the average 300 DPI black and white schematic page can be
compressed to 50 kB (100 kB at 600 DPI).

The secret is to reduce the scan to black and white (as opposed to
greyscale). Once that is done, a scan of a typical text or line
art document becomes very redundant and lossless compression
algorithms can be used. I use TIFF with the Group 4 compression
method. It yields compression ratios on the order of 100.

If you use black and white, you have to use a very high
resolution (>= 300 DPI).

For low resolutions, it's best to stay in greyscale. Either use a
lossy compression (JPEG) or posterise to as few grey levels as you
can get away with (usually 5 to 10) and use a lossless compressed
format such as PNG. Try both and see what bothers you more : low
intensity resolution (posterised PNG) or artefacts (JPEG). I find
that artefacts hurt readability more than posterisation does so I
usually avoid JPEG for this application.

In my experience, a 300 DPI TIFF/G4 file and the same scaled down
to 100 DPI and posterised to 8 levels are roughly equivalent in
size.

If you save as JPEG and find yourself lowering the quality factor
because the file is too big, stop and think about what you're
doing. For highly redundant images like text and line art, JPEG
rarely provides the highest quality/size ratio.

-- 
André Majorel <URL:http://www.teaser.fr/~amajorel/>
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