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RAW Image Files - please explain

 
 
PeterH
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      08-11-2004
I am correct in assuming the following about RAW image files.

RAW image files must be converted to say a jpeg or tif in order to be
processed in an image software package (eg Photoshop) for printing.

Or can RAW files be processed/manipulated as RAW files and then converted to
jpeg.

I keep hearing about RAW convertors but I don't understand when the
convertor is used and why.

Is it used before you can do any processing of the image or can the RAW
image be processed without any conversion?

Does conversion mean saving as a jpeg or tif or bmp?

regards

PeterH


 
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PeterH
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      08-11-2004
The earlier post should read "Am I correct in assuming the following about
RAW image files?"

PeterH


"PeterH" <reply to http://www.velocityreviews.com/forums/(E-Mail Removed)> wrote in message
news:HYlSc.50153$(E-Mail Removed)...
> I am correct in assuming the following about RAW image files.
>



 
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E. Le Phant
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      08-11-2004

"Gareth Tuckwell" <ContactGT@NoSpam_hotmail.com> wrote in message
newsgnSc.71$(E-Mail Removed)...
>
> JPG is an acronym (anyone?). It applies an algorithm to the raw bitmap


Joint Photographic Experts Group, thus, more correctly, JPEG.


 
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Gareth Tuckwell
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      08-11-2004
Raw image format is pure bitmap - no compression. I don't think RAW is an
acronym (I might be wrong), but I think it simply means the original and
uncompressed 'raw' image.

JPG is an acronym (anyone?). It applies an algorithm to the raw bitmap
image, discarding information that the human eye does not need to see in
order to understand the image, therefore the end result is an image with
less information, that looks almost identical to the original, but takes up
less file space.

You should work with (crop, colour adjust etc.) the original high quality
bitmap (raw) image and only as a last stage (if at all) should
you save using a compression algorithm such as JPG or GIF.

Photoshop, Paint Shop Pro and all photo manipulation software should be able
to work with raw images - I don't have a digital camera, only film + a
scanner, so I don't know what the files are called when you upload them to
your PC, but raw files should open as 'bitmaps'.

You can try this - take a copy of a bitmap file and do some work on it -
resize it a few times, then perhaps a crop, saving it after each operation
(as bitmap). Take a copy of the same original file and immediately save it
as a JPG, then repeat the resize, crop etc operations, again saving them
each time (as JPG). Once you have done a few operations on the JPG files you
will start to see the compression having an effect on the image quality - it
starts to look blotchy and smudged, whereas the bitmap version should still
be perfectly sharp and clear. This is why you should work with bitmap (raw)
images and save them with compression only at the end.

If you look in the help system for your photo software, or even a quick
search online, you will be able to read about the various compressions -
BMP, GIF, JPG, TIFF etc.



"PeterH" <reply to (E-Mail Removed)> wrote in message
news:HYlSc.50153$(E-Mail Removed)...
> I am correct in assuming the following about RAW image files.
>
> RAW image files must be converted to say a jpeg or tif in order to be
> processed in an image software package (eg Photoshop) for printing.
>
> Or can RAW files be processed/manipulated as RAW files and then converted

to
> jpeg.
>
> I keep hearing about RAW convertors but I don't understand when the
> convertor is used and why.
>
> Is it used before you can do any processing of the image or can the RAW
> image be processed without any conversion?
>
> Does conversion mean saving as a jpeg or tif or bmp?
>
> regards
>
> PeterH
>
>



 
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Justin Thyme
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      08-11-2004
To understand RAW files, it is best to first look at the steps the camera
takes in creating the image. Firstly the sensor - each sensel (sensor
element) in the camera records either red, green or blue. The normal pattern
has red, green and blue in the ratio of 1:2:1. When you take a picture, the
CCD returns an analogue voltage for each of the sensels. This voltage gets
converted to a digital value (anywhere from 8 to 16 bits resolution) which
is associated with each sensel. This information is what goes into the raw
file. At this stage there has been no digital manipulation at all.
Practically all cameras will then take this raw data, and start processing
it. The camera will look at each sensel's neighbours, and interpolate a full
colour value for the resulting pixel in the output image. Depending on the
camera it may at this point enhance colour to give better saturation. At
this point the image could be downsampled to 24bit and saved as a TIFF
image. Most cameras then convert to JPEG prior to saving on the card.
Comments about your post are now sprinkled through....

"PeterH" <reply to (E-Mail Removed)> wrote in message
news:HYlSc.50153$(E-Mail Removed)...
> I am correct in assuming the following about RAW image files.
>
> RAW image files must be converted to say a jpeg or tif in order to be
> processed in an image software package (eg Photoshop) for printing.

Yes.
>
> Or can RAW files be processed/manipulated as RAW files and then converted

to
> jpeg.

Some manipulation could possibly be done as RAW but it wouldn't be normal.
RAW doesn't actually contain a usable image, it just consists of a bunch of
values representing either Red, Green or Blue depending on which colour the
particular sensel is.
>
> I keep hearing about RAW convertors but I don't understand when the
> convertor is used and why.

As one of the first steps - to convert the image into a usable image that
has RGB values for each pixel.
>
> Is it used before you can do any processing of the image or can the RAW
> image be processed without any conversion?

In theory some processing could be done on the RAW image, as long as the
program doing the processing understands that particular cameras image.
>
> Does conversion mean saving as a jpeg or tif or bmp?

Conversion means converting to an image with RGB values at each pixel - once
that has been done it could be saved in any image format.
>
> regards
>
> PeterH
>
>



 
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PeterH
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      08-11-2004
Thanks Justin - great reply.

Makes more sense now.

regards

PeterH

PS - can you also help me with my second post to this NG re "Canon RAW with
JPG Embedded"?


"Justin Thyme" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> To understand RAW files, it is best to first look at the steps the camera
> takes in creating the image. Firstly the sensor - each sensel (sensor
> element) in the camera records either red, green or blue. The normal

pattern
> has red, green and blue in the ratio of 1:2:1. When you take a picture,

the
> CCD returns an analogue voltage for each of the sensels. This voltage gets
> converted to a digital value (anywhere from 8 to 16 bits resolution) which
> is associated with each sensel. This information is what goes into the raw
> file. At this stage there has been no digital manipulation at all.
> Practically all cameras will then take this raw data, and start processing
> it. The camera will look at each sensel's neighbours, and interpolate a

full
> colour value for the resulting pixel in the output image. Depending on the
> camera it may at this point enhance colour to give better saturation. At
> this point the image could be downsampled to 24bit and saved as a TIFF
> image. Most cameras then convert to JPEG prior to saving on the card.
> Comments about your post are now sprinkled through....
>
> "PeterH" <reply to (E-Mail Removed)> wrote in message
> news:HYlSc.50153$(E-Mail Removed)...
> > I am correct in assuming the following about RAW image files.
> >
> > RAW image files must be converted to say a jpeg or tif in order to be
> > processed in an image software package (eg Photoshop) for printing.

> Yes.
> >
> > Or can RAW files be processed/manipulated as RAW files and then

converted
> to
> > jpeg.

> Some manipulation could possibly be done as RAW but it wouldn't be normal.
> RAW doesn't actually contain a usable image, it just consists of a bunch

of
> values representing either Red, Green or Blue depending on which colour

the
> particular sensel is.
> >
> > I keep hearing about RAW convertors but I don't understand when the
> > convertor is used and why.

> As one of the first steps - to convert the image into a usable image that
> has RGB values for each pixel.
> >
> > Is it used before you can do any processing of the image or can the RAW
> > image be processed without any conversion?

> In theory some processing could be done on the RAW image, as long as the
> program doing the processing understands that particular cameras image.
> >
> > Does conversion mean saving as a jpeg or tif or bmp?

> Conversion means converting to an image with RGB values at each pixel -

once
> that has been done it could be saved in any image format.
> >
> > regards
> >
> > PeterH
> >
> >

>
>



 
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Helge Olsen
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      08-11-2004

"Gareth Tuckwell" <ContactGT@NoSpam_hotmail.com> wrote in message
newsgnSc.71$(E-Mail Removed)...
> Raw image format is pure bitmap - no compression. I don't think RAW is an
> acronym (I might be wrong), but I think it simply means the original and
> uncompressed 'raw' image.
>

The image may very well be (and mostly is) compressed, but with a lossless
compression algorithm. In the camera world, RAW means unprocessed CCD output
data. These files are mostly if not always of proprietary formats and
require vendor specific converters. Most converters have plenty settings for
post processing during conversioin (white balance, sharpening, saturation
and so on).

The main reason I use RAW is that the format has more bits per channel and
opens for more precise color correction and better dynamic range than 8
bits/channel. My camera has 10 bits/channel in RAW. Better cameras typically
have 12. I convert my images to 48 bit TIFF and process those in Photoshop
CS. At the very end I change to 24bit JPEG or TIFF depending on use.

To me, RAW is power. To others it implies a workflow that is too complex for
their needs/ability/interest.

/Helge


 
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Gisle Hannemyr
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      08-11-2004
"PeterH" <reply to (E-Mail Removed)> writes:

> Am I correct in assuming the following about RAW image files.
>
> RAW image files must be converted to say a jpeg or tif in order to
> be processed in an image software package (eg Photoshop) for
> printing.


This is indeed confusing. In Photoshop, there is even a file format
called "Adobe RAW" - which has nothing whatsoever to do the RAW image
files you probably ask about. Adobe calls the type of files you are
interested in "camera RAW".

"Camera RAW" - or just "RAW" is the original (i.e. un-interpolated
and un-corrected) linear sensor data from the camera's image sensor.
There is no «standard» for RAW, each camera/sensor have its own
proprietary RAW format. In additon to sensor data, the RAW file
contains metadata (i.e. in-camera settings such as white balance,
default sharpening, etc.) RAW data can be compressed or uncompressed,
but if compressed, they should be losslessly compressed.

There are (with the possible exception of Foveon-cameras), no pixels
in a RAW file. To be able to view the images, and do all the normal
things we do in a image editor (cropping, burning, retouching, etc.),
and for printing, the RAW data must be converted to pixels
(i.e. de-mosaiced), and the scale of the bit depth must be converted
from linear RAW data to something that is sharpened, colour balanced,
gamma corrected, and on a log scale.

All the above is what is done by a "RAW converter".

> Or can RAW files be processed/manipulated as RAW files and then
> converted to jpeg.


Not "or" - it is "and".

To convert sensel data to pixels, a lot of image processing takes
place. By working on the RAW data, instead of the converted pixels,
you have a lot more flexibility to change things. The more obvious
things you may want to change are white balance and sharpening, but
the fact that most cameras these days gives you at least 12 bit linear
RAW data gives you about 1.5 stops more dynamic range than 8 bit log
TIFF/JPEG - which you can make use of by fishing out detail from the
shadow and highlights in the linear data, and save (after conversion)
as 16 bit/channel log output.

If you look careful at Adobe Camera Raw (ACR, the RAW converter that
comes with Photoshop CS), you'll see that it doesn't stop there.
ACR will even let you profile your lenses and camera sensor, and
then be able to correct for lens defects such as CA and vignetting,
and sensor defects such as colour faults, as part of the conversion
process. The 4/3 system seems to have provisions for embedding
even more lens profile metadata in the RAW file, making other type
of corrections possible.

> I keep hearing about RAW convertors but I don't understand when the
> convertor is used and why.
>
> Is it used before you can do any processing of the image or can the
> RAW image be processed without any conversion?


All digital images from cameras start as RAW (i.e. as sensel data).
Most cameras (Sigmas being the exception) have a built-in RAW
converter that use defaults or presets to convert from sensel data to
JPEG and/or TIFF. Some cameras let the photographer reset some of the
settings for things like sharpening and the white balance. These
setting change the presets of the built-in RAW converter in the
camera - but in-camera RAW converters run pretty much on autopilot.

TheA "RAW converters" you keep hearing about are pieces of software
that does the same thing out-of-camera - where the result can be
inspected visually on a computer screen and re-done if the effect
isn't as desired - all under control of a human operator.

> Does conversion mean saving as a jpeg or tif or bmp?


The /end result/ of the conversion is saved in a pixel-format.

/Doing/ a conversion involves much more than just saving.
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
================================================== ======================
«To live outside the law, you must be honest.» (Bob Dylan)
 
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Gisle Hannemyr
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      08-11-2004
"PeterH" <reply to (E-Mail Removed)> writes:
> PS - can you also help me with my second post to this NG re "Canon
> RAW with JPG Embedded"?


Most (all?) Canon cameras store a segment with a tiny JPEG thumbnail
inside (embedded in) the RAW file. This thumbnail is quite small and
highly compressed, so it doesn't add much to the file size, but it
adds a lot of convenience. EXIF metadata, for example, is stored in
the embedded JPEG, and if you use the camera's "review" button, it is
the JPEG thumbnail image that's used when you review the "RAW" image
on the LCD on he back of the camera (saves time, since the camera
doesn't have to de-mosaic and colour correct every time you review).
Utilities like Canon Zoombrowser also show the embedded JPEG instead
of converting the RAW data for dispaly - again to make the program
appear to run faster.

Some of Canon cameras also let the user specify that a second, full
size JPEG should be embedded in the RAW file - so that the user have
"both" options (RAW and SF JPEG) available at all times. I don't see
much point in doing this - it add significant to the file size and you
can always reproduce SF JPEG on your PC if you just have the RAW data.
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
================================================== ======================
«To live outside the law, you must be honest.» (Bob Dylan)
 
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eawckyegcy@yahoo.com
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      08-11-2004
"PeterH" <reply to (E-Mail Removed)> wrote:

> I keep hearing about RAW convertors but I don't understand when the
> convertor is used and why.


There is no way to be polite or kind about this:

http://www.google.com/help/index.html

Go ahead, enter the exact subject line of your posting at
www.google.com and click "search". What do you have to lose?
 
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