Chuft-Captain wrote:I for one cannot see any significant reduction in
quality in the 80K version ... (after all I didn't explicitly apply any
additional compression, but simply opened it and saved it).
You are right in that in your example the difference isn??t too great.
My copy is a tad sharper than yours, but not by very much.
Please note, however, that you can??t expect this to be the case in
general; all images are different (amount of detail, variation between
neighboring pixels, color ranges etc)..
I do know a little bit about image manipulation after working in the field
professionally for more than twenty years (admittedly in a real darkroom
before the mac arrived) , and can assure you that if this was not so, we
would use high jpg compression for everything. But we don??t. Rather,
we tend to avoid it.
An example:
[1] Original screenshot, no JPG compression:
[2] My usual compression rate applied, bringing filesize down from 284 to 52 kb
[3] Bringing the copy [2] down to a comparable filesize (from your
example) by re-saving my jpg like you did - here fom 52 to 16 kb, i.e.
less than 80%:
As you can see, the difference is considerable between #2 and #3. Actually,
it is far greater than between #1 and #2 even though the variation in
pure filesize should indicate the opposite.
PS: I don??t have the original PNG doc from your example anymore, or i
would have performed the test on that one. But take a set of random
pictures through this, and more often than not, the end result will be bad.
Chuft-Captain wrote:Perhaps you should see what happens if you also
open and save for a second time
What happens is that you destroy more information. By re-saving a jpg
file you introduce more artifacts, in effect garbling the image.
Chuft-Captain wrote:Maybe the conversion to JPG from PNG isn't
happening properly the first time)
It happens properly indeed. My original tends to be around 1.3 to 1.4 MB,
while the copy ends up - usually - between 100 and 200 KB.
- rthorvald
