Il giorno 28-05-2011 23:19, glen ha scritto:
> So I ran some tests for the Geke minded.
> [...]
> Certainly many different file sizes. And I don't have a clue why.
I'll make some guess... and some explanation.
(I could be wrong, though)
BTW, it's all about the format (and options) used when saving; it has
nothing to do with the original scan application: once you scan the data,
that data is THAT data, and file size depends only by the format you use
when saving.
> The test file was 76 KB in its original configuration.
> This is the size is from" Get Info" in Preview as the default viewer
> in 10.4.11. > When saved as uncompressed it grew to 8.1 MB.
This should be the actual data size... but I'm a bit skeptical about Preview
estimate. :-?
> When saved again with LZW compression it was 220 KB.
LZW compression is using - AFAIK - a RLE (Run Lenght Encoded) method; i.e.
it records how many identical contiguous pixels are in a line.
There are more efficient methods (like ZIP), and that's why you can have
smaller sizes when using them.
> When I changed the default application to PhotoShop the original file grew to
> 144 KB.
I believe this has nothing to do with the file content itself, it's just OSX
added some kind of "icon" (or something) to the file. Hence the size growth.
> When saved as uncompressed it grew to only 1.1 MB.
So, Preview said "8.1 MB" but it came out as 1.1 MB when saved?
Mhhh.... something is wrong. Or that file wasn't really uncompressed.
The ACTUAL data size is usually showed by Photoshop in the lower left corner
of the image window (option: file size - I'm using PS here, but it should
true in every image app).
E.g., if you create a new document 1000 x 1000 pixel in RGB, that means 1
million pixels with 3 bytes (R+G+B) each. It makes 3 million bytes, and PS
show (correctly) 2.86MB.
("Wait! Why 2.86 and not 3?" - Because a MB is actually 1024 x 1024 bytes,
not 1000 x 1000)
BTW, my Preview app (OSX 10.4.11), when using Tools/Get Info, doesn't say
the actual data size, just the file size.
> When saved with LZW compression in PS it dropped to 236 KB
That's probably the same as 220KB LZW above, plus some kind of icon or
preview PS inserted.
When PS (or else) adds previews, or color profiles, file size can grow
significantly.
> and when saved with ZIP compression it was 212 KB.
That because ZIP method is more efficient (as I said above) than LZW.
> I wonder what compression was used originally to get the file to 76 KB?
I'm wondering as well.
Maybe it was JPG?
PS offers this method when saving TIFF (I'm using PS CS3).
JPG is surely more efficient than LZW or ZIP, but it's LOSSY (you have a
loss in quality), while LZW and ZIP are LOSSLESS (original data is mantained
identical).
> The file is a 1 bit/channel B/W (line art) not 8 bit greyscale.
You forgot to say the resolution (actual pixel width/height). Knowing that,
you can easily calculate the actual (uncompressed) data.
An A4 paper sheet (21 x 29.7 cm) at 300 DPI, gives 1.04 MB when bitmap (1
bit x pixel), and 8.30 MB when Grayscale (1 byte per pixel).
> I would
> love to get my letter size 1 bit .tiff scans down to 70-100 KB compressed size
> but then I only using inexpensive flatbed scanners.
Again, I'm quite sure the compact file size is about saving format, not some
sort of "magic" by the scanning app. :-)
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