On 2010-06-25 02:54, Chris Mohler wrote:
> On Thu, Jun 24, 2010 at 6:16 AM, Philip Rhoades<>  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.

I resent my second mail (without the attachment) before reading your 
response - sorry about that . .

Many thanks for the explanations - I appreciate it.


Philip Rhoades

GPO Box 3411
Sydney NSW      2001
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