Decreasing JPEG quality sometimes works

Question

I've done about a hundred flash files with imported jpegs and
have had no trouble getting the global jpeg quality setting to
affect them. Now I've got a similar project of ten fla files that a
contractor did and I can't get the Jpeg quality setting to affect
the published files.
The point is that I know this setting can have an effect on
imported jpeg bitmaps because I've done it so many times now.
Now, in my own projects, all the jpeg bitmaps are set to "use
document default quality" and the global setting affects them. The
contractor's projects have the same settings but the global setting
has no effect.
The one thing I'm suspicious of is that her bitmap properties
point to originals that are on her computer, not mine. Could the
fact that her sources are unavailable have anything to do with
this?
Rob Mack

Answer 1

Okay, here's the answer. My previous several thousand jpeg
images had been saved with the progressive switch turned on.
Evidently, Flash compresses bitmaps in non-progressive jpeg
format. If the asset is already in this format then flash defaults
to using the original file as is. However, if the jpeg asset is in
progressive format then Flash has to recompress it.
The effect is that progressive jpeg files can be affected by
the project's global jpeg quality settings but non-porgressive
jpegs can't be affected by the settings.
How this affects a user depends on what you want to do. In my
case I'm doing dozens and dozens of files that are much like slide
shows and it's most convenient to adjust compression in the project
settings to get the file size down. For most other users, they've
gone to a bit of trouble to get a jpeg image "just right" and don't
want the global setting to affect it. In most users cases they'd
want to save non-progressive jpegs, in fact since the project
setting will skip them this is the better way to go when mixing
jpeg and png bitmaps.
For someone in my position where you need to be able to
recompress the bitmaps, you either save jpegs with progressive
switched on or you save the bitmaps in PNG format. Overall, using
PNG will probably cause less overall image damage but it increases
your storage requirements quite a bit.
So, what if you saved the jpegs in the wrong format? Maybe
the best choice would be to go back to photoshop and set up a batch
action to resave everything with Progressive on or off. Then in the
FLA file you can select all the bitmaps and update them in bulk.
Rob Mack