On Thu, Jun 24, 2010 at 6:16 AM, Philip Rhoades <p...@pricom.com.au> wrote:
> Confirmation of what is going on from the gurus would be appreciated!

It was as I guessed - the scans are in grayscale mode but the contents
are essentially a 1-bit image.

Open a scan, do image->mode->bitmap, choose 1-bit palette.  Then
rotate and save - the file size will be comparable to the original.

Your image has that fine dot pattern all over the place - that's what
(I think) is causing the huge size blowup when rotated.  Open a scan,
zoom to 100% or greater and look at the character of those small dots.
  If you rotate the image in grayscale mode, look at the dots again at
100% or greater - they are now quite blurry.  Where each dot was n
pixels of solid black, now each dot is something like n+5 pixels of
shades of gray, which makes the image harder to compress efficiently.

On the file increase when just saving - see here:
$ identify TestScanningDoc.png
TestScanningDoc.png PNG 2552x3523 2552x3523+0+0 PseudoClass 2c 8-bit
230.383kb 0.740u 0:02

$ identify TestScanningDoc_nochange.png
TestScanningDoc_nochange.png PNG 2552x3523 2552x3523+0+0 PseudoClass
256c 8-bit 366.076kb 0.870u 0:02

It appears the original has a 2-color palette, while the unchanged,
saved image has a 256-color palette.

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