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If I want to scan a document, 10 pages, each page as PDF 300dpi, greyscale. 10 pages 6MB.. 40 pages 24MB. FINE. Color, each page is 25MB Why is color 25MB?

Is PDF very inefficient? it does the text and images.

But a far bigger question,

And far crazier than that.. I scan in 10 pages into a document. It takes up 1GB in my TEMP folder. ONE GIGABYTE!!!! It processes it then the 1GB is freed up.

But why the heck as much as 1GB during processing!! For a 6MB document!

I think it was here C:\Documents and Settings\user\Local Settings\Temp\MaglevExpressTemp

Software is HP Solution Center.

Does other software take up that much space? is some worse than others? Should I not be using HP solution center, for my HP Scanner?

share|improve this question
A4 is about 8.3x11.7 inches (letter is not too far off this, OK for approximation). At 300dpi that's ~8.7 million pixels per page. With 24 colour (likely the default) that's not too far off 25MB. It is quite possible that the scanner is using a higher resolution and this is being down sampled for final output. 1200dpi (not very high for a scanner) gives about 122MB per page for greyscale, colour will be ~400MB. So you numbers seem quite reasonable. (As a comparison, my two displays combined would need ~12MB, and thats at less than 100dpi). – Richard Sep 29 '10 at 11:01
@Richard - was half way though posting that as an answer myself when I spotted your comment. Stick that in an answer and you can have +1. – DMA57361 Sep 29 '10 at 12:32
be good to see the calculation that goes from dpi to MB. to get 25MB for color or 8MB for greyscale. Gets weirder 'cos the PDF of 10 greyscale pics, is 6.35MB! Bot 80MB. But is separate from the really weird problem and big question, of 1GB! – barlop Sep 29 '10 at 13:59
@DMA57361: OK, coming shortly :-) – Richard Sep 29 '10 at 14:34
up vote 8 down vote accepted

Is PDF very inefficient?

Not particularly, as your comment to the Q shows:

'cos the PDF of 10 greyscale pics, is 6.35MB!

It all depends on the resoution and level of compression.

Fundamentally the answer is: full pages are quite large and typical scanning and printing resolutions are quite high: thus lots of data.

Basic calculation:

A4 is about 8.3x11.7 inches (letter is similar, different is not significant). So one page is 97in2.

At 300dpi, 1in2 is 90,000 pixels. At 24 bits per pixel (enough for 16 million colours) that’s about 260kB.

So 100in2 will require about 26MB. This matches your figures. NB greyscale at 8 bits per pixel (256 levels) requires about a third of this.

The 1GB Of Intermediate Files

But if you want a final output of 300dpi you really need to work a higher resolution until the final output stage (to avoid artefacts: just try opening and saving anew a JPEG with even moderate compression a few times to see how these quickly appear).

My old scanner (>8 years old?) would operate at 2400dpi (IIRC), so something like 600dpi would be in easy range of most scanners, since that is the linear measurement, the per-area—and thus file size—will increase with the square, leading to file sizes four times bigger.

I.e. ∼100MB per image, so 1GB for 10 is to be expected.

But the final output is much smaller

This is almost certainly compressed. Non-loosy compression of images with large areas of constant colour can often be compressed by x10 with no significant loss of perceived information, for scans and photos (which tend to have a lot of small amounts of variation that we only see at extreme zoom levels) some loss is normal (as JPEG uses) and similar compression ratios are achieved.

How Big Could It Be?

Pro-DSLRs use 14bits per colour (a high end scanner should also be able to), i.e. 42bits per pixel. Scanning A4 at 4800dpi would lead to a raw data size of ∼11GB for a single page.

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Good job! Here's that +1 – DMA57361 Sep 29 '10 at 15:19
impressive! but why do you say " At 24 bits per pixel (enough for 24 million colours)" The "obvious" thought would be 2^24, 16 million 'cos knowing powers of two up to 2^20 one can see 2^24 is 16 million. – barlop Sep 29 '10 at 15:37
@barlop well spotted. 24bits / pixel is common (just look at the colour depth of most windows desktops), because this is three bytes to represent each of red, green and blue (normally) as a value from 0 to 255. This is why grey scale is normally a third the size as it only uses one byte to record an intensity of 0 to 255. I imagine 24million colours is an error, and you correctly say 16.7M colours is correct. – DMA57361 Sep 29 '10 at 15:44
@barlop: I call it a typo :-) Now fixed. – Richard Sep 30 '10 at 9:53

if you scan files as a .tiff image it will create a very large image. You should have the option of scanning to .png. I also prefer using the XP Scanner and Camera Wizard to any scanner package (Vista/7 has Scanners and Faxes).

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XP scanner and camera wizard with png, doesn't sound like a good way to scan a multi-page document, which what i'm doing and where i've found the space issue. – barlop Sep 29 '10 at 10:50

I notice that professional scans take about a tenth of the space as mine even though their quality is a bit better. They obviously use a different/better scanner than mine. (Its all pdf so we are talking apples to apples) I mean they can send me a 10 page color document in a reasonable size. I could never do that with my scanner.

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i'm sure that's nothing to do with the quality of your scanner, but probably to do with something like resolution of images. or the software that their scanner uses, which you might be able to get for your scanner and if not then you could convert your pdf to a smaller size. How, would be an Adobe question. Also the really massive amount of space I refer to, is during the scan rather than after. – barlop Mar 13 '13 at 0:26

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