I got real curious last night, ran a bunch of simple tests using the
same raw 3mb jpg. In the first test all I did was to add -strip and
rename the file. All the other including resizing it to 600 px wide. I
used combinations -quality, -strip and -depth. Some of the -quality
settings were almost pointless but I did them just because it's so
easy. Adding -strip seemed to be the biggest gain.
If you're using the image on the internet, somewhere between 70-80%
quality is typically 'good enough' in my opinion. Most people are not
very discerning in quality and the differences with higher quality are
so subtle that you may need to sit there with images side by side to
tell them apart. Image content is a big factor. On rare occasions
you'll see some that can live with 50%. I say this speaking as an
x-photographer who used to shoot with both 2 1/4 and 4x5 (film size)
formats.
ls -l | awk -F" " '{print $5, $8}'
216774 jensen.jpg
165523 jensen-strip.jpg
75167 jensen-s600.jpg
35269 jensen-s600-strip.jpg
3005 jensen-s600-strip-q1.jpg
6124 jensen-s600-strip-q10.jpg
9927 jensen-s600-strip-q20.jpg
13280 jensen-s600-strip-q30.jpg
15998 jensen-s600-strip-q40.jpg
18751 jensen-s600-strip-q50.jpg
21787 jensen-s600-strip-q60.jpg
26459 jensen-s600-strip-q70.jpg
29416 jensen-s600-strip-q75.jpg
33943 jensen-s600-strip-q80.jpg
40697 jensen-s600-strip-q85-d24.jpg
40665 jensen-s600-strip-q85-d8.jpg
40697 jensen-s600-strip-q85.jpg
52159 jensen-s600-strip-q90.jpg
77718 jensen-s600-strip-q95.jpg
183633 jensen-s600-strip-q100.jpg
Not knowing how the compression works, what I really find curious is
that when using -strip and a quality around 80% or above, the size is
about the same size or larger than -strip alone. To me it would make
sense that the following 2 commands produce the same result:
convert jensen.jpg -resize 600 -strip jensen-s600-strip-q100.jpg (by
ignoring quality isn't 100% assumed?)
convert jensen.jpg -resize 600 -quality 100 -strip
jensen-s600-strip-q100.jpg
-q 1 has the effect of a mind altering experience!
-Bob
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