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Compression in JPEG files in digital cameras



 
 
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  #11  
Old August 19th 07, 09:40 AM posted to rec.photo.digital,uk.rec.photo.misc
Ron Hunter
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Posts: 4,064
Default Compression in JPEG files in digital cameras

ray wrote:
On Sat, 18 Aug 2007 11:55:45 -0700, aniramca wrote:

I need some help in explaining the JPEG compression feature in digital
cameras. My camera (which likely is similar to most others) has the
feature to compress the photo JPEG files in the storage card. It also
has the choice to have different pixel sizes (example: 3000x2250,
2000x1500, 1024x768, etc). What is the difference of the above two
features? If you store a 3000x2250 pixel data in compressed mode,
does it loose its quality? Can it be re-instated to full uncompressed
size without loosing photo quality?. When I compressed the data, it
will fit more pictures in a single storage card. But, is it the same
if I choose 2000x1500 pixel and no compression instead?
Thanks for info.


It will uncompress to the full resolution - however it will not be an
exact replica of the original image. JPEG is a 'lossy' compression
algorithm - meaning that some detail is lost in the act of compressing the
data. There are 'lossless' compression schemes as well, but the level of
compression with them is generally considerably less. From a practical
standpoint, you can do a lot of compression on a JPEG image before you see
noticeable loss of information. Before I got a camera capable of saving
RAW images, I always chose the highest resolution and the smallest amount
of compressin on JPEG images - now I simply save nearly everything in RAW
- even though it does take a lot of space. Memory cards are very cheap now
- IMHO it is better to get some extra cards and save everything using the
best method possible. I currently have 2-1gb cards and 1-2gb card for my
5mp camera - they run around $15 and $30 respectively.


Currently several places in my local area sell 2GB SD cards for under
$20. 4GB cards go for about $40. Very little excuse to opt for more
compression, or lower resolution, just to save space. Note that if you
have a need for fast shooting, writing very large files (RAW/TIFF), may
slow down your shot rate, considerably.
  #12  
Old August 19th 07, 05:06 PM posted to rec.photo.digital,uk.rec.photo.misc
Don Stauffer in Minnesota
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Posts: 464
Default Compression in JPEG files in digital cameras

On Aug 18, 1:55 pm, wrote:
I need some help in explaining the JPEG compression feature in digital
cameras. My camera (which likely is similar to most others) has the
feature to compress the photo JPEG files in the storage card. It also
has the choice to have different pixel sizes (example: 3000x2250,
2000x1500, 1024x768, etc). What is the difference of the above two
features? If you store a 3000x2250 pixel data in compressed mode,
does it loose its quality? Can it be re-instated to full uncompressed
size without loosing photo quality?. When I compressed the data, it
will fit more pictures in a single storage card. But, is it the same
if I choose 2000x1500 pixel and no compression instead?
Thanks for info.


This is a complicated question, and not suited to quick answers- I'd
get a decent book on digital photography. Actually, the Dummies Guide
series has a reasonable book on the subject.

Briefly, there are two different ways of reducing the size of the file
(memory size) of an image. One method is to "downsample", or average
pixels to create a new array of numbers with fewer pixels. All the
pixels in the image chip are used, but the downsampling or averaging
math reduces the "resolution" or sharpness of the image.

Data compression schemes such as JPEG do not downsample in the same
way. Depending on the subject a jpeg will retain all or most of the
resolution. However, subtleties of color disappear, gradients of
color or brightness get flattened, and little regions of the wrong
color begin cropping up. Jpeg is a lossy compression scheme, meaning
this loss of image quality cannot be reversed after the image file is
compressed. There ARE other compression schemes that ARE reversible.

With the cheapness of very large memory cards these days, and also
large memory banks for computers, there is little reason to downsample
in the camera, or to use excessive values of compression (many cameras
allow you to select how much compression to use). The most common
advice these days is to shoot full resolution (the 3000 x 2250
mentioned), and select the least amount of jpeg compression. This is
frequently called picture quality, as in super high quality, high
quality, medium, or some such combination of words.


  #13  
Old August 19th 07, 07:55 PM posted to rec.photo.digital,uk.rec.photo.misc
Bill Tuthill
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Posts: 361
Default Compression in JPEG files in digital cameras

In rec.photo.digital harrogate3 wrote:

JPEG is a lossy form of saving the picture -


The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument that
JPEG imposes no further loss.

Google on 'explanation of jpg' and it will give you the detail


or read
http://photo.net/jpeg/learn/
  #14  
Old August 20th 07, 12:32 AM posted to rec.photo.digital,uk.rec.photo.misc
Floyd L. Davidson
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Default Compression in JPEG files in digital cameras

Bill Tuthill wrote:
In rec.photo.digital harrogate3 wrote:

JPEG is a lossy form of saving the picture -


The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument that
JPEG imposes no further loss.


You could argue that until you are blue in the face, and it
still wouldn't be anything near correct though! :-)

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)
  #15  
Old August 20th 07, 11:48 AM posted to rec.photo.digital
David J Taylor[_4_]
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Posts: 1,151
Default Compression in JPEG files in digital cameras

Bill Tuthill wrote:
In rec.photo.digital harrogate3 wrote:

JPEG is a lossy form of saving the picture -


The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument that
JPEG imposes no further loss.


However, JPEG imposes an additional loss, because of the approximations it
makes. You can control the degree of approximation by the quality setting
on cameras, or by the compression or quality settings in your software.

David


  #16  
Old August 20th 07, 06:48 PM posted to rec.photo.digital,uk.rec.photo.misc
Bill Tuthill
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Posts: 361
Default Compression in JPEG files in digital cameras

In rec.photo.digital Floyd L. Davidson wrote:

JPEG is a lossy form of saving the picture -


The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument that
JPEG imposes no further loss.


You could argue that until you are blue in the face, and it
still wouldn't be anything near correct though! :-)


I'm not going to argue it that long,
but I would say that nobody has quantified the loss that comes from
the Bayer sensor versus the loss that comes from JPEG.

The real problem with JPEG is that it is not edit-safe,
rather than loss of information at creation time.

  #17  
Old August 20th 07, 08:25 PM posted to rec.photo.digital,uk.rec.photo.misc
Thomas T. Veldhouse
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Default Compression in JPEG files in digital cameras

In rec.photo.digital Bill Tuthill wrote:

I'm not going to argue it that long,
but I would say that nobody has quantified the loss that comes from
the Bayer sensor versus the loss that comes from JPEG.

The real problem with JPEG is that it is not edit-safe,
rather than loss of information at creation time.


Is it not obvious to you that the "loss" from using a bayer sensor (which
isn't really a loss in the same sense) and the loss due to JPEG compression
are going to be cummulative in nature. Thus, the JPEG losses will only add to
any losses due to using a bayer sensor.

--
Thomas T. Veldhouse

We have more to fear from the bungling of the incompetent than from the
machinations of the wicked.

  #18  
Old August 20th 07, 09:44 PM posted to rec.photo.digital,uk.rec.photo.misc
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

added these comments in the current discussion du jour ...

I need some help in explaining the JPEG compression feature in
digital cameras. My camera (which likely is similar to most
others) has the feature to compress the photo JPEG files in
the storage card. It also has the choice to have different
pixel sizes (example: 3000x2250, 2000x1500, 1024x768, etc).
What is the difference of the above two features? If you
store a 3000x2250 pixel data in compressed mode, does it loose
its quality? Can it be re-instated to full uncompressed size
without loosing photo quality?. When I compressed the data, it
will fit more pictures in a single storage card. But, is it
the same if I choose 2000x1500 pixel and no compression
instead? Thanks for info.

Resolution is measured by horizontal and vertical pixels for an
image, the area of which constitutes mega pixels. JPEG
compression is how much or little the uncompressed data is made
smaller. Since JPEG is a so-called "lossy" format, it literally
throws away pixels in order to drastically shrink the file size.
The idea of the algorithm is to choose pixels to disgard that
have a mathmatical probability of not being noticed by the human
eye.

However, when compression begins to get even moderately high,
defects begin to appear, commonly called "JPEG artifacts",
artifact meaning "what is left behind". This can easily be seen
by blobs, streaks, blurry areas, minor destruction of fine
detail, and sometimes posterization.

"Better" digital cameras will give you choices as to how much to
compress for a given mega pixel image. Lots of words used but
"basic", "normal", and "fine" are common. I don't think any
cameras tell you the actually number they use nor the Chroma
subsampling they use, but under most circumstances, one can
quickly show themselves that "basic" is pretty awful, "normal"
MAY produce artifacts 5, 10, 15% of the time, and "fine" rarely
does.

However, many - not all - of the less expensive P & S cameras
only give you marketing BS like "good", "better" and "best", but
what they're really doing is maintaining the same JPEG
compression but upping the MP. The reason that so many lower cost
but high MP cameras do that is that they also want to advertise
how many pictures you can fit on even a small memory card.

I'm not sure about the rest of your question to exampand more on
my answer. Some cameras allow TIFF, which is lossless, and even
better cameras - certainly DSLRs - can also save in RAW.

Without starting another religious war, if you can get by with
JPEG and it fulfills what you want and need the camera to do for
you, you'll be just fine. It is universally readable, lots of
free or almost free editing apps as wells as commercial apps, and
you can save money on memory if that is a consideration.

But, since I know of NO camera buyer who isn't interested in the
best possible quality, I would look for cameras that offer a
choice of compression so that you can run some tests for yourself
and make up your own mind.

--
HP, aka Jerry
  #19  
Old August 20th 07, 09:46 PM posted to rec.photo.digital,uk.rec.photo.misc
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

Bill Tuthill added these comments in the current discussion du jour
....

In rec.photo.digital harrogate3 wrote:

JPEG is a lossy form of saving the picture -


The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument that
JPEG imposes no further loss.

Google on 'explanation of jpg' and it will give you the detail


or read
http://photo.net/jpeg/learn/


Bill, I'm not nearly enough of a mathematicion to understand this,
but it is WAY over the OP's head. Might it be better to describe
the issue in more qualitative than theoretical ways he can
understand?

--
HP, aka Jerry
  #20  
Old August 20th 07, 09:48 PM posted to rec.photo.digital
HEMI-Powered[_2_]
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Posts: 447
Default Compression in JPEG files in digital cameras

David J Taylor added these comments in the current discussion du
jour ...

JPEG is a lossy form of saving the picture -


The Bayer sensor is already lossy, having only one of RGGB at
any of four pixel locations, so one could make the argument
that JPEG imposes no further loss.


However, JPEG imposes an additional loss, because of the
approximations it makes. You can control the degree of
approximation by the quality setting on cameras, or by the
compression or quality settings in your software.

Here we go again! David, you and I get along OK. You're level
headed and not any kind of elitist, while at the same time, you
have an excellent foundation in the theory of all of this. I think
the OP is a pretty rank novice and has probably gone unconscious by
now in this thread. You have such a great way to express your ideas
in words that people of all technical levels can understand that I
bet you can do far better than my feeble attempt at helping the OP.

--
HP, aka Jerry
 




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