What does "easy" compression inform me about a jpeg?

[Zak]

Sometimes I compress a jpeg which I have downloaded from the Net.

I always use the same old version of ACDSEE (v3.1) for compression
mainly because it is simple and convenient.

I also have Photoshop and Paint Shop Pro but they are slower, more
cumbersome and more complicated.

For example, I notice that (with the SAME settings for compression in
Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs
will compress right down to a surprisingly small 100 KB.

What does the final size in KB compared to the original size tell me
about the properties of original jpeg?

Or to put it another way, how is the degree of compression (given the
same settings on Acdsee) linked to the original jpeg's properties such
as pixel size, print size, etc?

 

[Don]

Sometimes I compress a jpeg which I have downloaded from the Net.

Not a good idea, in theory. Any JPG has already compromised data and
compressing it twice like that will compromize the data even more.

For example, I notice that (with the SAME settings for compression in
Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs
will compress right down to a surprisingly small 100 KB.

That's because the rate of compression depends on the data in the
image being compressed. An image with lots of detail will compress
badly, an image with large areas of same color with compress much
better. This goes for both lossless and lossy compression equally.

What does the final size in KB compared to the original size tell me
about the properties of original jpeg?

See above. Complicated source results in less compression. Uniform
source results in more compression.

Or to put it another way, how is the degree of compression (given the
same settings on Acdsee) linked to the original jpeg's properties such
as pixel size, print size, etc?

No relation. The only thing that matters is image content.

Also, the degree of JPG compression is not standardized. Each program
picks a number out of thin air an then declares it a certain level of
compression.

Don.

 

[Alan Meyer]

...
I always use the same old version of ACDSEE (v3.1) for compression
mainly because it is simple and convenient.
...

You might also look at Irfanview - fast, free, and capable of batch
processing.

For example, I notice that (with the SAME settings for compression in
Acdsee) some 500 KB jepgs will compress to 300 KB but other 500 KB jpegs
will compress right down to a surprisingly small 100 KB.

What does the final size in KB compared to the original size tell me
about the properties of original jpeg?

Or to put it another way, how is the degree of compression (given the
same settings on Acdsee) linked to the original jpeg's properties such
as pixel size, print size, etc?

As Don said, there are no standards for how to specify JPEG
compression and different programs have their own approaches.

There are several factors that influence compression. Print size
is not one of them. Pixel count plays a role, but it's really the
amount of change between pixels (i.e., the amount of detail in
the image) that determines the degree of compression that can
be achieved for any given level of quality.

A large image (many pixels) will compress to a larger file than a
small iamge that has the same level of detail. But a large image
with little detail may compress more than a smaller image with
more detail. A pure black or pure white image of any size can
be compressed to just a few bytes.

One thing I've noticed is that some programs seem to aim at an
output size when a compression number is specified while others
aim at an output quality.

A program of the first sort (size oriented) will produce similar sized
compressions from similar input sizes. Images with lots of detail
will lose more than images with less detail. The output quality will
vary significantly between images.

A program of the second sort will produce similar quality images
from input sources. Images with little detail will compress more
than images with lots of detail. The output size will vary
significantly
between images.

From your description, it sounds like ACDSee uses the second

approach to compression - which in my view is much better choice
than the first.

Alan

 

[Pete Fraser]

One thing I've noticed is that some programs seem to aim at an
output size when a compression number is specified while others
aim at an output quality.

I haven't seen the former in a program
(though many still cameras seem to do this).

All the programs I've played with give an
image-independent quantizing table.

What programs tweak the table for a fixed size?

 

[Alan Meyer]

I haven't seen the former in a program
(though many still cameras seem to do this).

All the programs I've played with give an
image-independent quantizing table.

What programs tweak the table for a fixed size?

I may be mistaken, but it seems to me that my Pentax Optio 750Z
tends to produce a relatively small range of file sizes regardless
of the amount of detail in the image. My older Canon S30 had a
much wider range.

However, as you say, that's a program running in a camera, not
in a desktop computer. I haven't noticed this in the limited
number of desktop computer programs I've worked with. I'm
less likely to notice it with them because, in the desktop
programs I run, I'm working with all sizes of input images
whereas with the cameras I had them fixed at the maximum
sizes the cameras produced.

Alan

 

[Pete Fraser]

I may be mistaken, but it seems to me that my Pentax Optio 750Z
tends to produce a relatively small range of file sizes regardless
of the amount of detail in the image. My older Canon S30 had a
much wider range.

Correct on both.

However, as you say, that's a program running in a camera, not
in a desktop computer. I haven't noticed this in the limited
number of desktop computer programs I've worked with. I'm
less likely to notice it with them because, in the desktop
programs I run, I'm working with all sizes of input images
whereas with the cameras I had them fixed at the maximum
sizes the cameras produced.

Yes. There may well be an application that aims for a particular
file size, but I haven't stumbled across it yet.