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Expires: 21 Jan 2004 00:00:00 GMT
Subject: Medical Image Format FAQ, Part 1/8
From:
[email protected] (David A. Clunie)
Followup-To: alt.image.medical
Reply-To:
[email protected] (David A. Clunie)
Approved:
[email protected]
Summary: This posting contains answers to the most Frequently Asked
Question on alt.image.medical - how do I convert from image
format X from vendor Y to something I can use ? In addition
it contains information about various standard formats.
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Date: Sun, 21 Dec 2003 14:16:25 GMT
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Contents:
part1 - contains index, general information & standard formats
part2 - contains standard formats (continued)
part3 - contains information about proprietary CT formats
part4 - contains information about proprietary MR formats
part5 - contains information about proprietary other formats
part6 - contains information about hosts & compression
part7 - contains general information sources
part8 - contains DICOM information sources
Tools that describe and convert many of the formats described in this document are available in the dicom3tools package from
"
http://www.dclunie.com/dicom3tools.html".
A web browsable version of this FAQ is available at:
"
http://www.dclunie.com/medical-image-faq/html/"
or at the mirror sites:
"
http://www.focus-fr.com/links/faq/medical/"
"
http://www.focus-fr.com/links/"
Html and text forms of the FAQ are available at (postscript and pdf no longer provided):
"
http://www.dclunie.com/medical-image-faq/".
Many FAQs, including this Listing, are available on the archive sites:
"
ftp://rtfm.mit.edu/pub/usenet/news.answers/medical-image-faq/"
"
http://www.faqs.org/faqs/medical-image-faq/part1/"
"
http://www.cs.uu.nl/wais/html/na-dir/medical-image-faq/part1.html"
"
ftp://ftp.univ-lille1.fr/pub/faq/medical-image-faq/part1"
"
http://www.pasteur.fr/infosci/FAQ/medical-image-faq/part1"
"
http://www.panther.net/FAQ/medical-image-faq/part1"
"
http://faqs.jmas.co.jp/FAQs/medical-image-faq/part1"
"
http://www.han.de/usenet/medical-image-faq/part1.gz"
"
http://www.landfield.com/faqs/medical-image-faq/part1/"
The name under which a FAQ is archived appears in the Archive-name line
at the top of the article.
There's a mail server on the FAQ archives. You send a e-mail message to
[email protected] containing the keyword "help" (without quotes!)
in the message body. To fetch this particular FAQ send a message with the following body:
send usenet/news.answers/medical-image-faq/part1
...
send usenet/news.answers/medical-image-faq/part8
Please direct comments or questions and especially contributions to
"mailto:
[email protected]"
or reply to this article. All unknown formats and test images gratefully
accepted.
Changes this issue
Add Chinese and Korean Visible Human sites Add IntuitiveImaging conformance
statement site Add PACSGear document scanning conformance statement site
Remove email addresses to minimize spam Update Tiani conformance statement
link Add UniPACS site Extensively revise conformance statement links Remove
xray.psu dead links Reorganize toolkit summary, and remove lots of dead
toolkit links Add Trevor Morgan's dicomlib toolkit Add JDCM Java DICOM
Toolkit Add PixelMed Java DICOM Toolkit Tidy up Medigration info Tidy up
OFFIS urls Add CDMEDICSPACSWEB site Add Conquest DICOM site Add JiveX site
Update UCDMC sites to http from ftp Update DICOM image sample sites Update
XMedCon PET format convertor site Update ITU T.81 text site Add transfer
syntax determination explanatuon and code Add Madena viewer site Add
idoimaging index site Add iRad Mac viewer site Add IBM conformance statement
site Comment that R Hindel and book no longer contactable Add free PACS web
site Add Apteryx Java Image I/O Plugin site Update Escape QT site Add
YourDICOM site Add MRIConvert site Add explanation of DICOM image
orientation Update JPEG 2000 resources Clean up a lot of conformance
statement links Update UID registration, and elaborate on description Revise
structured reporting resources, and add new web site Add dicomworks site Add
Almacom's J2K codec site Update Mark Nelson's data compression library site
Update TIFF spec site at Adobe Update ALI conformance site Add Sanders
ViewPlus site Update MacAngioView site Add display performance section and
AAPM TG18 reference Tidy up part 7 indentation of compression/jpeg links
Update VMS tools site Update Kodak conformance statement site Add
CardioVista viewer Add sites about reading DICOM in MATLAB Fix IHE MESA
tools site reference Add Analog Devices J2K chip
Changes last issue
Add Tiani Java Image Archive Application with source code Tidy up MultiTech
site Add Sante Viewer and Anonymizer site Update MedX site Update bicubic
spline interpolation site Update Analyze format sites Add SPM site Update
Tiani conformance statement site Add J2K book reference Add Kakadu J2K codec
site Update MultiTech Solutions web site Cull out dead links to image sites
Update ANSI UID registration address Add InviWeb DicomWorks site Update
mirror sites list Add Xinapse software and consulting site Update unuid
pathology image site Update Interfile resources Add IHE and MESA tools sites
Add DICOM SR sample sites Add reViewMD PocketPC DICOM viewer site Add RMIT
Digital Radiography page Update TrueVIEW URL Add NIH etdips 3D software site
Add Kitware vtk site Add Konica images site Add ECRI DICOM Compatability
Analysis Form site Add Marianne's DicomEdit site Add MedicView site
The next part is table of contents.
Subject: Contents
1. Introduction
1.1 Objective 1.2 Types of Formats 1.3 In Desperation - Quick & Dirty Tricks
2. Standard Formats
2.1 ACR/NEMA 1.0 and 2.0 2.2 DICOM 3.0
2.2.1 Localizer lines on DICOM images
2.3 Papyrus 2.4 Interfile V3.3 2.5 Qsh 2.6 DEFF
3. Proprietary Formats
3.1 Proprietary Formats - General Information
3.1.1 SPI (Standard Product Interconnect) 3.1.2 Siemens - Features
common to multiple families
3.1.2.1 Siemens Vax/VMS 3.1.2.2 Siemens Sparc SunOS
3.1.2.2.1 Starting up 3.1.2.2.2 Getting a console
3.1.2.2.3 Native images 3.1.2.2.4 Exporting images
3.1.2.2.5 Physical connection 3.1.2.2.6 Archive devices
3.1.2.2.7 Becoming root 3.1.2.2.8 Reset
3.2 CT - Proprietary Formats
3.2.1 General Electric CT
3.2.1.1 GE CT 9800
3.2.1.1.1 GE CT 9800 Image data 3.2.1.1.2 GE CT 9800 Tape
format 3.2.1.1.3 GE CT 9800 Raw data MR
3.2.1.2 GE CT Advantage - Genesis
3.2.1.2.1 GE CT Advantage Image data 3.2.1.2.2 GE CT
Advantage Archive format 3.2.1.2.3 GE CT Advantage Raw
data
3.2.1.3 GE CT Pace 3.2.1.4 GE CT Sytec 3.2.1.5 GE CTI
3.2.2 Siemens CT
3.2.2.1 Siemens Somatom DR 3.2.2.2 Siemens Somatom Plus 3.2.2.3
Siemens Somatom AR
3.2.3 Philips CT 3.2.4 Picker CT 3.2.5 Toshiba CT 3.2.6 Hitachi CT 3.2.7
Shimadzu CT 3.2.8 Elscint CT 3.2.8 Imatron CT
3.3 MR - Proprietary Formats
3.3.1 General Electric MR
3.3.1.1 GE MR Signa 3.x,4.x
3.3.1.1.1 GE MR Signa 3.x,4.x Image data 3.3.1.1.2 GE MR
Signa 3.x,4.x Tape format 3.3.1.1.3 GE MR Signa 3.x,4.x
Raw data
3.3.1.2 GE MR Signa 5.x - Genesis
3.3.1.2.1 GE MR Signa 5.x Image data 3.3.1.2.2 GE MR Signa
5.x Archive format 3.3.1.2.3 GE MR Signa 5.x Raw data
3.3.1.3 GE MR Max 3.3.1.4 GE MR Vectra
3.3.2 Siemens MR
3.3.2.1 Siemens Magnetom GBS/GBS II
3.3.2.1.1 Siemens Magnetom GBS/GBS II Native Format
3.3.2.1.2 Siemens Magnetom GBS/GBS II SPI Format
3.3.2.2 Siemens Magnetom SP
3.3.2.2.1 Siemens Magnetom SP Native Format 3.3.2.2.2
Siemens Magnetom SP SPI Format
3.3.2.3 Siemens Magnetom Impact
3.3.2.3.1 Siemens Magnetom Impact Native Format 3.3.2.3.2
Siemens Magnetom Impact SPI Format
3.3.2.4 Siemens Magnetom Vision
3.3.2.4.1 Siemens Magnetom Vision Native Format 3.3.2.4.2
Siemens Magnetom Vision SPI Format
3.3.3 Philips MR
3.3.3.1 Philips Gyroscan S5 3.3.3.2 Philips Gyroscan ACS 3.3.3.3
Philips Gyroscan T5 3.3.3.4 Philips Gyroscan NT5 & NT15
3.3.4 Picker MR 3.3.5 Toshiba MR 3.3.6 Hitachi MR 3.3.7 Shimadzu MR
3.3.8 Elscint MR
3.4 Proprietary Workstations
3.4.1 ISG Workstations
3.4.1.1 Gyroview
3.4.2 GE Workstations
3.4.2.1 GE Advantage Windows
3.5 Other Proprietary Formats
3.5.1 Analyze From Mayo
4. Host Machines
4.1 Data General
4.1.1 Data General Data
4.1.1.1 Data General Integers 4.1.1.2 Data General Floating Point
4.1.2 Data General Operating System
4.1.2.1 Data General RDOS 4.1.2.2 Data General AOS/VS
4.1.3 Data General Network
4.2 Vax
4.2.1 Vax Data
4.2.1.1 Vax Integers 4.2.1.2 Vax Floating Point 4.2.1.3 Vax
Strings
4.2.2 Vax Operating System
4.2.2.1 Vax VMS 4.2.2.2 ULTRIX 4.2.2.3 OSF
4.3 Sun - Sun3 68000 and Sun4 Sparc
4.3.1 Sun Data
4.3.1.1 Sun Integers 4.3.1.2 Sun Floating Point 4.3.1.3 Sun
Strings
4.3.2 Sun Operating System
5. Compression Schemes
5.1 Reversible Compression 5.2 Irreversible Compression
5.2.1 Perimeter Encoding
5.3 DICOM Compression
6. Getting Connected
6.1 Tapes 6.2 Ethernet 6.3 Serial Ports
7. Sources of Information
7.1 Contacts and Sites 7.2 Relevant FAQ's 7.3 Mailservers 7.4 References 7.5
Organizations and Societies 7.6 Usenet Newsgroups 7.7 DICOM Information
Sources
8. Acknowledgements
The next part is part1 - general information & standard formats.
1. Introduction
1.1 Objective
The goal of this FAQ is to facilitate access to medical images stored on
digital imaging modalities such as CT and MR scanners, and their
accompanying descriptive information. The document is designed
particularly for those who do not have access to the necessary
proprietary tools or descriptions, particularly in those moments when
inspiration strikes and one just can't wait for the local sales person
to track down the necessary authority and go through the cycle of
correspondence necessary to get a non-disclosure agreement in place, by
which time interest in the project has usually faded, and another great
research opportunity has passed! It may also be helpful for those keen
to experiment with home-grown PACS-like systems using their existing
equipment, and also for those who still have equipment that is still
useful but so old even the host computer vendor doesn't support it any
more!
There is of course no substitute for the genuine tools or descriptions
from the equipment vendors themselves, and pointers to helpful
individuals in various organizations, as well as names and catalog
numbers of various useful documents, are included here where known.
In addition there are several small companies that specialize in such
connectivity problems that have a good reputation and are well known.
Contact information is provided for them, though I personally have no
experience with their products and am not endorsing them.
Finally, great care has been taken not to include any information that
has been released under non-disclosure agreements. What is included
here is the result of either information freely released by vendors,
handy hints from others working in the field, or in many cases close
scrutiny of hex dumps and experimentation with scanner parameters and
study of the effects on the image files. The intent is to spread
hard-earned knowledge gained over many years amongst those new to the
field or a particular piece of equipment, not to threaten anyone's
proprietary interests, or to substitute for the technical support
available from vendors that ranges from free to extortionate, and
excellent to abysmal, depending on who your are dealing with and where
in the world you are located!
Please use this information in the spirit in which is intended, and
where possible contribute whatever you know in order to expand the
information to cover more vendors and equipment.
1.2 Types of Formats
Later sections will deal with the problems of getting the image files
from the modality to the workstation, but for the moment assume the
files are there and need to be deciphered.
Four types of information are generally present in these files:
- image data, which may be unmodified or compressed, - patient
identification and demographics, - technique information about the
exam, series, and slice/image.
Extracting the image information alone is usually straightforward and is
described in 1.3. Dealing with the descriptive information, for example
to make use of the data for dissemination in a PACS environment, or to
extract geometry details in order to combine images into 3D datasets, is
more difficult and requires deeper understanding of how the files are
constructed.
There are three basis families of formats that are in popular use:
- fixed format, where layout is identical in each file, - block
format, where the header contains pointers to information, - tag
based format, where each item contains its own length.
The block format is one of the most popular, though in most cases, the
early part of the header contains only a limited number of pointers to
large blocks, the blocks are almost always in the same place and a
constant length, for standard rather than reformatted images at least,
and if one doesn't know the specifics of the layout one can get by
assumming a fixed format. I presume this reflects the intent of the
designers to handle future expansion and revision of the format.
The example par excellence of the tag based format is the ACR/NEMA style
of data stream, which, though never intended as a file format per se has
proven useful as model. See for example the sections dealing with the
ACR/NEMA standards as well as DICOM (whose creators are about to vote on
a media interchange format after all this time) and Papyrus. ACR/NEMA
style tags are described in more detail elsewhere, but each is
self-contained and self-describing (at least if you have the appropriate
data dictionary) and contains its own length, so if you can't interpret
it you can skip it! Very convenient. Most file formats based on this
scheme are just concatenated series of tags, and apart from having to
guess the byte order, which is not specified (unlike TIFF which is a
similar deal for those in the "real" imaging world), and sometimes skip
a fixed length but short header, are dead easy to handle.
To identify such a file just do a "strings <file | grep 'ACR-NEMA'" - if
it is such a file, just look through the start of the hex dump until you
start to see the characteristic sequentially ordered pairs of 16 bit
words that identify ACR/NEMA attributes, decide the byte order, et
voila, you can pipe it into any general ACR/NEMA dumping program to see
what it contains. If you see even group tags, they will be described in
the standard. If you see odd group tags then they are vendor specific
and you will have to ask the vendor or correlate them with
identification information printed on the film until you figure out the
ones that are important to you.
1.3 In Desperation - Quick & Dirty Tricks
Because radiologists, radiographers, technologists, physicists and
imaging programmers are dedicated long suffering creatures who work long
hours under adverse conditions for little reward, the vendors in their
generosity have seen fit to make life a little easier, by almost
universally putting the image data at the end of the file. Rarely you
will see files that are padded out to fixed record size boundaries (eg.
Vax VMS 512 byte records), and sometimes overlay plane data may be
stored after the image data. Furthermore there is almost always an
option at archive time to allow for storage in an uncompressed and
totally unadulterated form. Even in ACR/NEMA the tag for image pixel
data is numerically the highest and hence the last to appear in the
sequence which is guaranteed to be sorted.
They could have screwed us up totally by gratuitously adding variable
length blocks of other stuff at the end, but the only time I have
encountered this was on a Siemens Impact with the ACR/NEMA based SPI
format padded out to 512 bytes.
In other words, if an image is 256 by 256, uncompressed, and 12-16 bits
deep (and hence usually, though not always, stored as two bytes per
pixel), then we all know that the file is going to contain
256*256*2=131072 bytes of pixel data at the end of the file. If the
file is say 145408 bytes long, as all GE Signa 3X/4X files are for
example, then you need to skip 14336 bytes of header before you get to
the data. Presume row by row starting with top left hand corner raster
order, try both alternatives for byte order, deal with the 16 to 8 bit
windowing problem, and very soon you have your image on the screen of
your workstation.
This technique is so useful, even NIH Image for the Macintosh (an
excellent must-have free program BTW.) provides a raw import tool to do
this, and describes it in the manual using the 14336 byte offset! This
tool is something that is sadly lacking in most commercial image
handling programs for non-medical applications, which can't import
images with more than 8 bits per channel.
Of course you have to live without the identification, demographic and
technique information (other than what can be derived from the file name
in some cases), but for many research and presentation purposes this is
quite adequate.
Occasionally one runs into clever files where four 12 bit words are
packed into three 16 bit words and one goes crazy trying to figure out
the logic of how they are packed. The back of the old ACR/NEMA standard
describes somewhere one way in which this is done. One should still be
able to calculate the length easily enough.
I haven't yet encountered a format that did nasty things like have
strips of rows seperated by padding ... I guess we are lucky that most
images are nice powers of two or even multiples thereof (256,320,512).
Of course the GE CT 9800 uses perimeter encoding even when DPCM
compression is not selected, so this technique won't work.
2. Standard Formats
2.1 ACR/NEMA 1.0 and 2.0
ACR/NEMA Standards Publication No. 300-1985 <- ACR/NEMA 1.0 ACR/NEMA
Standards Publication No. 300-1988 <- ACR/NEMA 2.0 ACR/NEMA Standards
Publication PS2-1989 <- data compression
The American College of Radiologists (ACR) and the National Electrical
Manufacturers Association (NEMA) recognized some time ago the need for
standards to facilitate multi-vendor connectivity to promote the
development of PACS and what is now referred to as Wide Area Networking.
The first such standard was version 1.0 which was released in 1985 as
ACR/NEMA Standards Publication No. 300-1985, subsequently revised
several times, then revised again and released as version 2.0 in 1988,
described in ACR/NEMA Standards Publication No. 300-1988. There it
remained until a radically revised and reorganized approach, preserving
backward compatibility, was released during 1992-1993 as ACR/NEMA
Standards Publication PS3, also referred to as DICOM 3.
In the interim, to facilitate the transfer of compressed images, another
standard described in ACR/NEMA Standards Publication PS2-1989, was
released which described various means fo extending standard 300-1985 to
handle compression utilizing a broad range of reversible and
irreversible schemes. Though this part of the standard was never
apparently implemented by anyone, and has been quietly bypassed by those
working on DICOM 3 compression, it makes very interesting reading and is
a nice summary of applicable techniques.
What does one need to know about ACR/NEMA 1.0 and 2.0 ? The standards
define a mechanism along the lines of the layered ISO-OSI (Open Systems
Interconnect) model, with physical, transport/network, session, and
presentation and application layers. Unless one actually wants to
physically connect to a device that supports the unique 50 pin
point-to-point electrical interface, then one really only needs to be
aware of how ACR/NEMA implements the presentation and application
layers, which are described in terms of a "message format". This
message format is important to many people, not because anyone seriously
wants to connect devices in the limited fashion envisaged by these early
standards, but because many proprietary formats and other de facto
standards have adopted the ACR/NEMA message format and its corresponding
data dictionary and extension mechanisms.
The message format is described in sections 4, 5 and 10 of ACR/NEMA SP
300-1988 which are summarized briefly here. Section 6 describes command
structure which is not really relevant other than that commands are also
structured in the same way as data and consume part of the data
dictionary. You will not encounter command tags in data streams
("messages") encapsulated in file formats though.
A message consists of a series of "data elements" each of which contains
a piece of information. Each element is described by an "element name"
consisting of a pair of 16 bit unsigned integers ("group number", "data
element number"). The data stream is ordered by ascending group number,
and within each group by ascending data element number. Each element
may occur only once in a message. Even numbered groups describe
elements defined by the standard. Odd numbered groups are available for
use by vendors or users, but must conform to the same structure as
standard elements. Following the (group number, data element number)
pair is a length field that is a 32 bit unsigned even integer that
describes the number of bytes from the end of the length field to the
beginning of the next data element.
The last part of a data element is its value, which is defined by the
data dictionary to be an ascii (numeric AN or text AT) or binary value
(BI 16 bit or BD 32 bit). The values may be single or multiple.
Multiple ascii values are delimited by the backslash (05CH) character.
Odd length ascii values are padded with a space (020H).
For example:
0008 0010 000C 0000 4341 2D52 454E 414D
3120 302E
is data element "Recognition Code" because that is what the dictionary
defines group 0008 element 0010 to be. The dictionary says it is of
type AT (ascii text), has a value multiplicity of single and only
enumerated values are allowed, in this case the ascii string "ACR-NEMA
2.0". It is of length 0000000C hex or 12 bytes long.
The electrical interface is a 16 bit one, and hence even though 32
binary values are defined to be transmitted least significant word first
(though the order for the 32 bit length is not actually specified),
there is no mention in the standard as to how to encapsulate the message
in an 8 bit world, hence different users and vendors have chosen little
or big endian schemes. The new DICOM standard assumes a default little
endian representation which seems to be the most appropriate considering
the old definition for 32 bit words, which specified that the least
significant 16 bit word be transmitted first.
Hence there are three likely possible byte orders that a vendor
interpreting the ACR/NEMA standard in a byte oriented world may have
used:
- little endian 16 and 32 bit words, as in DICOM 3, - big endian 16
and 32 bit words, as in DICOM 3, - big endian 16 bit words, but the
least significant half of
a 32 bit word is sent first (as per ACR/NEMA 2.0).
The choice seems to be made usually on the basis of the native byte
order of integers on the host processor. Most of the formats I have
encountered are one of the first two, but I did encounter one from
Philips that used the last scheme and it drove me crazy for a while,
until I appreciated the subtlety of it ! I call it "Big Bad Endian"
format in my implementation that recognizes it, but that may be a value
judgement on my part :)
Notice particularly how this design allows one to parse the message even
if the data dictionary is not complete. Consider an element that has an
unrecognized element name. One cannot interpret the content of the
element and so has to ignore it. One doesn't even know whether it
contains binary or ascii information (this is what DICOM later refers to
as "implicit representation". despite this, the length value allows one
to skip to the next element and proceed.
Over the years there has been much discussion amongst those who favour
such implicit dictionary driven schemes, and those who prefer explicit
representations, including explicit description of the element type
(binary or ascii, etc.) and even the element description itself! Some
would prefer the message to contain something like
"RecognitionCode='ACR-NEMA 2.0';" for example. The nuclear medicine
groups have adopted a de facto standard called Interfile that makes use
of ACR/NEMA data elements, but uses such a descriptive representation.
Their argument is that the data stream is much more readable which is
true enough, and more readily extensible.
The groups are organized as follows:
0000 Command 0008 Identifying 0010 Patient 0018 Acquisition 0020
Relationship 0028 Image Presentation 4000 Text 6000-601E (even)
Overlay 7FE0 Pixel Data
Some of the more interesting elements are:
(nnnn,0000) BD S Group Length # of bytes in group nnnn (nnnn,4000)
AT M Comments
(0008,0010) AT S Recognition Code # ACR-NEMA 1.0 or 2.0 (0008,0020)
AT S Study Date # yyyy.mm.dd (0008,0021) AT S Series Date #
yyyy.mm.dd (0008,0022) AT S Acquisition Date # yyyy.mm.dd
(0008,0023) AT S Image Date # yyyy.mm.dd (0008,0030) AT S Study Time
# hh.mm.ss.frac (0008,0031) AT S Series Time # hh.mm.ss.frac
(0008,0032) AT S Acquisition Time # hh.mm.ss.frac (0008,0033) AT S
Image Time # hh.mm.ss.frac (0008,0060) AT S Modality #
CT,NM,MR,DS,DR,US,OT
(0010,0010) AT S Patient Name (0010,0020) AT S Patient ID
(0010,0030) AT S Patient Birthdate # yyyy.mm.dd (0010,0040) AT S
Patient Sex # M, F, O for other (0010,1010) AT S Patient Age # xxxD
or W or M or Y
(0018,0010) AT M Contrast/Bolus Agent # or NONE (0018,0030) AT M
Radionuclide (0018,0050) AN S Slice Thickness # mm (0018,0060) AN M
KVP (0018,0080) AN S Repetition Time # ms (0018,0081) AN S Echo Time
# ms (0018,0082) AN S Inversion Time # ms (0018,1120) AN S Gantry
Tilt # degrees
(0020,1040) AT S Position Reference # eg. iliac crest (0020,1041)
AN S Slice Location # in mm (signed)
(0028,0010) BI S Rows (0028,0011) BI S Columns (0028,0030) AN M
Pixel Size # row\col in mm (0028,0100) BI S Bits Allocated # eg. 12
bit for CT (0028,0101) BI S Bits Stored # eg. 16 bit (0028,0102) BI
S High Bit # eg. 11 (0028,0103) BI S Pixel Representation # 1
signed, 0 unsigned
(7FE0,0010) BI M Pixel Data # as described by grp 0028
The way in which the pixel data is stored can vary tremendously, though
thankfully most users and vendors use the simple unimaginative scheme
that is shown above, ie. 1 12 bit pixel stored in the low order part of
a 16 bit word with no attempt at packing more compactly. Following are
some examples shown in Appendix E of the standard. Note that when one
adds the little/big endian question the permutations mount!
Bits Allocated = 16 Bits Stored = 12 High Bit = 11
|<------------------ pixel ----------------->|
______________ ______________ ______________ ______________
|XXXXXXXXXXXXXX| | | |
|______________|______________|______________|______________|
15 12 11 8 7 4 3 0
---------------------------
Bits Allocated = 16 Bits Stored = 12 High Bit = 15
|<------------------ pixel ----------------->|
______________ ______________ ______________ ______________
| | | |XXXXXXXXXXXXXX|
|______________|______________|______________|______________|
15 12 11 8 7 4 3 0
---------------------------
Bits Allocated = 12 Bits Stored = 12 High Bit = 11
------ 2 ----->|<------------------ pixel 1 --------------->|
______________ ______________ ______________ ______________
| | | | |
|______________|______________|______________|______________|
15 12 11 8 7 4 3 0
-------------- 3 ------------>|<------------ 2 --------------
______________ ______________ ______________ ______________
| | | | |
|______________|______________|______________|______________|
15 12 11 8 7 4 3 0
|<------------------ pixel 4 --------------->|<----- 3 ------
______________ ______________ ______________ ______________
| | | | |
|______________|______________|______________|______________|
15 12 11 8 7 4 3 0
---------------------------
And so on ... refer to the standard itself for more detail.
The next part is part2 - standard formats (continued).
