Hi Folks,
I have a little query which someone may be able to clarify.
Background: I download photos off my digital camera. These
are 2304x1728 (4:3) JPEGS, 8pits/pixel, around 2MB in size.
As well as storing the originals, I also re-size them to
1024x768 (also 4:3, linear reduction by factor 2.25) because
(a) at that geometry they exactly fit my screen, (b) they then
come out much smaller in byte-size, convenient for emailing
for instance.
I do the re-sizing as follows, in ImageMagick:
1. display photo.jpg
2. View --> Resize --> 1024x768 --> Resize
3. File --> Save --> new filename --> Save
and here's where my query comes. At this point I get a popup
window which asks me the "Choose JPEG quality" with default
option 75%.
I've wondered how much this "quality" matters, so I've done
a bit of experimentation, saving the re-sized file both at
75% (the default) and 100%. Then I put both of them up (on
different "workspace" panels) and flip between them.
Try as I may, I can't visually perceive any difference between
them.
Yet, for example, one 75% file has 171064 bytes, and its 100%
counterpart has 694833 bytes, 4 times larger.
At that difference in byte size, I'm surprised that there's
no perceptible difference!
Any ideas?
With thanks,
Ted.
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E-Mail: (Ted Harding) <ted.harding(a)nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 11-Mar-07 Time: 18:00:49
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