[Leica] Re:JPEG compression

[ericm at pobox.com (Eric)]

Larry:

>If I open a JPEG file in Graphic Converter (for the Mac), make a minor al=
>teration, and save in Best quality, the file size more than doubles. As a=
> test I down loaded a JPEG file from my wife's P&S camera (a Leica Digilu=
>x Zoom). The file was 197.3 KB. I then saved the file without alteration =
>at the programs's Best quality. The file size was now 412.8 KB.=20
>
>What's going on? What was added to increase the file size?

I'm giong to open a 1000x1500 picture.  That's 1.5 million pixels.  If it's
24-bit color, that's 3 bytes per pixel, or 4.5M bytes.  Save it in medium
compression, and you'll probably end up with about a 100k file.

4,500,000 --> 100,000

That's pretty good compression.  The jpeg algorthm is tossing out some bits
that are so similar to the surrounding bits that we won't really notice the
loss.

Then, when you load that 100k image, it becames a 4.5M picture because the
program reconstructs all the tossed out bits.

Let's say you now save at a better quality/lower compression jpeg file.  You
end up with a 150k jpeg.  Where did the extra stuff come from?  There is no
extra size.  It's always a 4.5M picture.  That hasn't changed.  What has
changed is that instead of getting a 45:1 compression ratio, we're now
getting a 30:1 ratio.  The lower the ratio, the better the jpeg quality.
There is little value in re-saving with lower compression, because the "made
up" pixels have already been made up from the first round of
compressing/uncompressing.

Does this make sense?



--
Eric
http://canid.com/