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Jpeg compression info

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Anonymous
May 26, 2005 2:08:43 PM

Archived from groups: rec.photo.digital (More info?)

Is there any way of determining what the compression ratio / quality level
of a Jpeg photo was set at to create the Jpeg?

I had a few rolls of film processed and put onto a CD. I believe the Photo
Lab process produces a higher quality Jpeg compared to the Kodak CD. The
file sizes are quite different. About 1.5 Mb for the Photo Lab compared to
around 600K for the Kodak - 35mm film.

Allan

More about : jpeg compression info

May 26, 2005 2:08:44 PM

Archived from groups: rec.photo.digital (More info?)

On Thu, 26 May 2005 10:08:43 -0400, Allan wrote:

> Is there any way of determining what the compression ratio / quality level
> of a Jpeg photo was set at to create the Jpeg?

I'm sure there are others on here who know more about this than I do, but I
believe the compression ratio/quality level is somewhat arbitrary, because
of differences in compressor implementations.

In other words, 95% quality in one app is not necessarily the same as 95%
compression in another app.

If this is correct, there would be no point in storing the number in the
file.

Pete
Anonymous
May 26, 2005 2:38:07 PM

Archived from groups: rec.photo.digital (More info?)

Allan wrote:

> Is there any way of determining what the compression ratio / quality level
> of a Jpeg photo was set at to create the Jpeg?

No.

> I had a few rolls of film processed and put onto a CD. I believe the Photo
> Lab process produces a higher quality Jpeg compared to the Kodak CD. The
> file sizes are quite different. About 1.5 Mb for the Photo Lab compared to
> around 600K for the Kodak - 35mm film.

The standard JPEG implementation

http://www.ijg.org/

tends to produce larger JPEG's than most other implementations
(PhotoSlop et al) at comparable image appearances. Is this a "higher
quality image" or a "less efficient encoder"?
Related resources
Anonymous
May 26, 2005 8:01:18 PM

Archived from groups: rec.photo.digital (More info?)

On Thu, 26 May 2005 10:08:43 -0400, Allan wrote:

> Is there any way of determining what the compression ratio / quality level
> of a Jpeg photo was set at to create the Jpeg?

If you have IrfanView (it's free, if you don't) you can view Image
Information that shows both the size of the file (you already know
this) as well as the size in memory after it has been expanded.
There's a "quality" setting in the Batch Conversion menu but
IrfanView doesn't indicate any quality value for files that it
loads. I'm not sure that this information could be reliably
determined. Starting with a 92kb JPG file (already compressed by
90%), it was processed and saved using Quality=50%. That file was
reduced to 62kb. Although detail was definitely lost, at first
glance it appeared to be a bit sharper than the original when the
two were viewed side by side. This JPG was then processed again
and saved with the same Quality=50% setting. The file size actually
grew a bit, to 69kb. But it looked pretty awful, with artifacts so
pronounced that it wouldn't be difficult convincing someone that
they were intentionally produced by an "arty" plugin.
Anonymous
May 26, 2005 8:38:59 PM

Archived from groups: rec.photo.digital (More info?)

Pete wrote:
> On Thu, 26 May 2005 10:08:43 -0400, Allan wrote:
>
>
>>Is there any way of determining what the compression ratio / quality level
>>of a Jpeg photo was set at to create the Jpeg?
>
>
> I'm sure there are others on here who know more about this than I do, but I
> believe the compression ratio/quality level is somewhat arbitrary, because
> of differences in compressor implementations.
>
> In other words, 95% quality in one app is not necessarily the same as 95%
> compression in another app.
>
> If this is correct, there would be no point in storing the number in the
> file.
>
> Pete

Hi...

Here's a site that offers a free jpeg quality estimator,
along with some helpful information.

http://www.mediachance.com/digicam/jpgq.htm

Take care.

Ken
Anonymous
May 26, 2005 9:03:56 PM

Archived from groups: rec.photo.digital (More info?)

Allan, then eawckye, wrote:
>> Is there any way of determining what the compression ratio / quality level
>> of a Jpeg photo was set at to create the Jpeg?
>
>No.

Actually you *can* extract this data reasonably reliably from the
quantisation info, and there are programs that do this.. but there is
a problem - if it has been resaved, the information is not going to
tell you about the previous compression, and so on.. So the
information is not as useful as you might think...
Anonymous
May 26, 2005 11:52:57 PM

Archived from groups: rec.photo.digital (More info?)

"Allan" <pabroon@sympatico.ca> schrieb
> Is there any way of determining what the compression ratio / quality level
> of a Jpeg photo was set at to create the Jpeg?
You can try cPicture (Quality level tab). Its a free feature and it does it
without file size dependencies (e.g. if you have large EXIF info in a rather
small image).
Anonymous
May 27, 2005 1:39:47 AM

Archived from groups: rec.photo.digital (More info?)

On Thu, 26 May 2005 10:08:43 -0400, "Allan" <pabroon@sympatico.ca>
wrote:

>Is there any way of determining what the compression ratio / quality level
>of a Jpeg photo was set at to create the Jpeg?
>
>I had a few rolls of film processed and put onto a CD. I believe the Photo
>Lab process produces a higher quality Jpeg compared to the Kodak CD. The
>file sizes are quite different. About 1.5 Mb for the Photo Lab compared to
>around 600K for the Kodak - 35mm film.


Easy. What are the pixel dimensions of the
image in question?

Multiply the height in pixels by the width in
pixels, times three. (H x W x 3).

That's how many bytes the file would be in
a lossless RGB format (not including header,
footer, etc.)

Now look at the JPG file using Explorer or
whatever, and get the file size in bytes.

Best quality JPG quite typically gets about
2.5:1 compression. Low quality JPG can be
25:1 or higher.

JPG "compression" is varied by using more
or fewer bits to represent the DCT
coefficients of each 8x8 pixel block.


rafe b.
http://www.terrapinphoto.com
Anonymous
May 27, 2005 1:58:24 AM

Archived from groups: rec.photo.digital (More info?)

> Is there any way of determining what the compression ratio / quality level
> of a Jpeg photo was set at to create the Jpeg?

Yes. If I load a JPG in Thumbs Plus and resave it, TP denotes and
suggests the "original" JPG compression rate. I can select another
compression rate, if I like.

-Michael
Anonymous
May 27, 2005 12:25:48 PM

Archived from groups: rec.photo.digital (More info?)

Allan wrote:
> Is there any way of determining what the compression ratio / quality level
> of a Jpeg photo was set at to create the Jpeg?

Yes. Although some applications like PhotoShop (and others) use custom
quantisation tables so the answer is not always exact. JPEGDUMP will
make an attempt at guesstimating the previous compression settings. It
assumes that the qtable was a scaled copy of the sample given in the
original JPEG specification (this is not alway a valid assumption).

You can always inspect the quantisation tables directly. That is a very
precise description of how the lossy JPEG encoding step was done -
needed to enable the decoder to reconstruct the image JPEG coefficients.
>
> I had a few rolls of film processed and put onto a CD. I believe the Photo
> Lab process produces a higher quality Jpeg compared to the Kodak CD. The
> file sizes are quite different. About 1.5 Mb for the Photo Lab compared to
> around 600K for the Kodak - 35mm film.

JPEGDUMP should be good enough for this. PictureCD uses poxy resolution
and compression levels - most 2Mpixel digicams will beat it hollow.

Regards,
Martin Brown
Anonymous
May 30, 2005 6:43:16 PM

Archived from groups: rec.photo.digital (More info?)

Whew, this follow-up post is as wrong as it could be! Sorry to top post.

eawckyegcy@yahoo.com wrote:
>> Is there any way of determining what the compression ratio / quality level
>> of a Jpeg photo was set at to create the Jpeg?
> No.

The "jpegdump" program shows chroma subsampling and estimates quality level,
as Martin Brown correctly pointed out.

> The standard JPEG implementation
> http://www.ijg.org/
> tends to produce larger JPEG's than most other implementations
> (PhotoSlop et al) at comparable image appearances. Is this a "higher
> quality image" or a "less efficient encoder"?

Wrong! The IJG implementation produces tighter JPEG (same quality
at smaller file sizes, or higher quality at same size) than Photoshop.
!