[Back to FAQ SWAG index] [Back to Main SWAG index] [Original]

    Herbert Zarb <panther!jaguar!hzarb@relay.iunet.it>

           This Text File explains the format of Gif Files.

    ---------------------------------------------------------------------------
        G I F (tm)
           Graphics Interchange Format (tm)

            A standard defining a mechanism
           For the storage and transmission
         of raster-based Graphics information

              June 15, 1987

           (c) CompuServe Incorporated, 1987
           All rights reserved

         While this document is copyrighted, the information
       contained Within is made available For use in computer
       software Without royalties, or licensing restrictions.

       Gif and 'Graphics Interchange Format' are trademarks of
        CompuServe, Incorporated.
          an H&R Block Company

       5000 Arlington Centre Blvd.
          Columbus, Ohio 43220
             (614) 457-8600
                 Page 2

           Graphics Interchange Format (GIF) Specification

            Table of Contents

     INTRODUCTION . . . . . . . . . . . . . . . . . page 3
     GENERAL File FORMAT  . . . . . . . . . . . . . page 3
     Gif SIGNATURE  . . . . . . . . . . . . . . . . page 4
     SCREEN DESCRIPTOR  . . . . . . . . . . . . . . page 4
     GLOBAL COLOR MAP . . . . . . . . . . . . . . . page 5
     IMAGE DESCRIPTOR . . . . . . . . . . . . . . . page 6
     LOCAL COLOR MAP  . . . . . . . . . . . . . . . page 7
     RASTER DATA  . . . . . . . . . . . . . . . . . page 7
     Gif TERMINATOR . . . . . . . . . . . . . . . . page 8
     Gif EXTENSION BLOCKS . . . . . . . . . . . . . page 8
     APPendIX A - GLOSSARY  . . . . . . . . . . . . page 9
     APPendIX B - INTERACTIVE SEQUENCES . . . . . . page 10
     APPendIX C - IMAGE PACKAGING & COMPRESSION . . page 12
     APPendIX D - MULTIPLE IMAGE PROCESSING . . . . page 15

    INTRODUCTION

     'GIF' (tm) is CompuServe's standard For defining generalized  color
       raster   images.    This   'Graphics  Interchange  Format'  (tm)  allows
       high-quality, high-resolution Graphics to be displayed on a  Variety  of
       Graphics  hardware  and is intended as an exchange and display mechanism
       For Graphics images.  The image format described  in  this  document  is
       designed  to  support  current  and  future image technology and will in
       addition serve as a basis For future CompuServe Graphics products.

     The main focus  of  this  document  is  to  provide  the  technical
       information  necessary  For  a  Programmer to implement Gif encoders and
       decoders.  As such, some assumptions are made as to terminology relavent
       to Graphics and Programming in general.

     The first section of this document describes the  Gif  data  format
       and its components and applies to all Gif decoders, either as standalone
       Programs or as part of  a  communications  package.   Appendix  B  is  a
       section  relavent to decoders that are part of a communications software
       package and describes the protocol requirements For entering and Exiting
       Gif mode, and responding to host interrogations.  A glossary in Appendix
       A defines some of the terminology used in  this  document.   Appendix  C
       gives  a  detailed  explanation  of  how  the  Graphics  image itself is
       packaged as a series of data Bytes.

      Graphics Interchange Format Data Definition

     GENERAL File FORMAT

     +-----------------------+
     | +-------------------+ |
     | |   Gif Signature   | |
     | +-------------------+ |
     | +-------------------+ |
     | | Screen Descriptor | |
     | +-------------------+ |
     | +-------------------+ |
     | | Global Color Map  | |
     | +-------------------+ |
     . . .               . . .
     | +-------------------+ |    ---+
     | |  Image Descriptor | |       |
     | +-------------------+ |       |
     | +-------------------+ |       |
     | |  Local Color Map  | |       |-   Repeated 1 to n times
     | +-------------------+ |       |
     | +-------------------+ |       |
     | |    Raster Data    | |       |
     | +-------------------+ |    ---+
     . . .               . . .
     |-    Gif Terminator   -|
     +-----------------------+

     Gif SIGNATURE

     The following Gif Signature identifies  the  data  following  as  a
       valid Gif image stream.  It consists of the following six Characters:

          G I F 8 7 a

     The last three Characters '87a' may be viewed as a  version  number
       For  this  particular  Gif  definition  and will be used in general as a
       reference  in  documents  regarding  Gif  that   address   any   version
       dependencies.

     SCREEN DESCRIPTOR

     The Screen Descriptor describes the overall parameters For all  GIF
       images  following.  It defines the overall dimensions of the image space
       or logical screen required, the existance of color mapping  information,
       background  screen color, and color depth information.  This information
       is stored in a series of 8-bit Bytes as described below.

           bits
      7 6 5 4 3 2 1 0  Byte #
     +---------------+
     |               |  1
     +-Screen Width -+      Raster width in pixels (LSB first)
     |               |  2
     +---------------+
     |               |  3
     +-Screen Height-+      Raster height in pixels (LSB first)
     |               |  4
     +-+-----+-+-----+      M = 1, Global color map follows Descriptor
     |M|  cr |0|pixel|  5   cr+1 = # bits of color resolution
     +-+-----+-+-----+      pixel+1 = # bits/pixel in image
     |   background  |  6   background=Color index of screen background
     +---------------+          (color is defined from the Global color
     |0 0 0 0 0 0 0 0|  7        map or default map if none specified)
     +---------------+

     The logical screen width and height can both  be  larger  than  the
       physical  display.   How  images  larger  than  the physical display are
       handled is Implementation dependent and can take advantage  of  hardware
       Characteristics  (e.g.   Macintosh scrolling Windows).  Otherwise images
       can be clipped to the edges of the display.

     The value of 'pixel' also defines  the  maximum  number  of  colors
       Within  an  image.   The  range  of  values  For 'pixel' is 0 to 7 which
       represents 1 to 8 bits.  This translates to a range of 2 (B & W) to  256
       colors.   Bit  3 of Word 5 is reserved For future definition and must be
       zero.

     GLOBAL COLOR MAP

     The Global Color Map is optional but recommended For  images  where
       accurate color rendition is desired.  The existence of this color map is
       indicated in the 'M' field of Byte 5 of the Screen Descriptor.  A  color
       map  can  also  be associated With each image in a Gif File as described
       later.  However this  global  map  will  normally  be  used  because  of
       hardware  restrictions  in equipment available today.  In the individual
       Image Descriptors the 'M' flag will normally be  zero.   if  the  Global
       Color  Map  is  present,  it's definition immediately follows the Screen
       Descriptor.   The  number  of  color  map  entries  following  a  Screen
       Descriptor  is equal to 2**(# bits per pixel), where each entry consists
       of three Byte values representing the relative intensities of red, green
       and blue respectively.  The structure of the Color Map block is:

           bits
      7 6 5 4 3 2 1 0  Byte #
     +---------------+
     | red intensity |  1    Red value For color index 0
     +---------------+
     |green intensity|  2    Green value For color index 0
     +---------------+
     | blue intensity|  3    Blue value For color index 0
     +---------------+
     | red intensity |  4    Red value For color index 1
     +---------------+
     |green intensity|  5    Green value For color index 1
     +---------------+
     | blue intensity|  6    Blue value For color index 1
     +---------------+
     :               :       (Continues For remaining colors)

     Each image pixel value received will be displayed according to  its
       closest match With an available color of the display based on this color
       map.  The color components represent a fractional intensity  value  from
       none  (0)  to  full (255).  White would be represented as (255,255,255),
       black as (0,0,0) and medium yellow as (180,180,0).  For display, if  the
       device  supports fewer than 8 bits per color component, the higher order
       bits of each component are used.  In the creation of  a  Gif  color  map
       entry  With  hardware  supporting  fewer  than 8 bits per component, the
       component values For the hardware  should  be  converted  to  the  8-bit
       format With the following calculation:

     <map_value> = <component_value>*255/(2**<nbits> -1)

     This assures accurate translation of colors For all  displays.   In
       the  cases  of  creating  Gif images from hardware Without color palette
       capability, a fixed palette should be created  based  on  the  available
       display  colors For that hardware.  if no Global Color Map is indicated,
       a default color map is generated internally  which  maps  each  possible
       incoming  color  index to the same hardware color index modulo <n> where
       <n> is the number of available hardware colors.

     IMAGE DESCRIPTOR

     The Image Descriptor defines the actual placement  and  extents  of
       the  following  image Within the space defined in the Screen Descriptor.
       Also defined are flags to indicate the presence of a local color  lookup
       map, and to define the pixel display sequence.  Each Image Descriptor is
       introduced by an image separator  Character.   The  role  of  the  Image
       Separator  is simply to provide a synchronization Character to introduce
       an Image Descriptor.  This is desirable if a Gif File happens to contain
       more  than  one  image.   This  Character  is defined as 0x2C hex or ','
       (comma).  When this Character is encountered between images,  the  Image
       Descriptor will follow immediately.

     Any Characters encountered between the end of a previous image  and
       the image separator Character are to be ignored.  This allows future GIF
       enhancements to be present in newer image formats and yet ignored safely
       by older software decoders.

           bits
      7 6 5 4 3 2 1 0  Byte #
     +---------------+
     |0 0 1 0 1 1 0 0|  1    ',' - Image separator Character
     +---------------+
     |               |  2    Start of image in pixels from the
     +-  Image Left -+       left side of the screen (LSB first)
     |               |  3
     +---------------+
     |               |  4
     +-  Image Top  -+       Start of image in pixels from the
     |               |  5    top of the screen (LSB first)
     +---------------+
     |               |  6
     +- Image Width -+       Width of the image in pixels (LSB first)
     |               |  7
     +---------------+
     |               |  8
     +- Image Height-+       Height of the image in pixels (LSB first)
     |               |  9
     +-+-+-+-+-+-----+       M=0 - Use global color map, ignore 'pixel'
     |M|I|0|0|0|pixel| 10    M=1 - Local color map follows, use 'pixel'
     +-+-+-+-+-+-----+       I=0 - Image formatted in Sequential order
        I=1 - Image formatted in Interlaced order
        pixel+1 - # bits per pixel For this image

     The specifications For the image position and size must be confined
       to  the  dimensions defined by the Screen Descriptor.  On the other hand
       it is not necessary that the image fill the entire screen defined.

     LOCAL COLOR MAP

     A Local Color Map is optional and defined here For future use.   If
       the  'M' bit of Byte 10 of the Image Descriptor is set, then a color map
       follows the Image Descriptor that applies only to the  following  image.
       At the end of the image, the color map will revert to that defined after
       the Screen Descriptor.  Note that the 'pixel' field of Byte  10  of  the
       Image  Descriptor  is used only if a Local Color Map is indicated.  This
       defines the parameters not only For the image pixel size, but determines
       the  number  of color map entries that follow.  The bits per pixel value
       will also revert to the value specified in the  Screen  Descriptor  when
       processing of the image is complete.

     RASTER DATA

     The format of the actual image is defined as the  series  of  pixel
       color  index  values that make up the image.  The pixels are stored left
       to right sequentially For an image row.  By default each  image  row  is
       written  sequentially, top to bottom.  In the Case that the Interlace or
       'I' bit is set in Byte 10 of the Image Descriptor then the row order  of
       the  image  display  follows  a  four-pass process in which the image is
       filled in by widely spaced rows.  The first pass Writes every  8th  row,
       starting  With  the top row of the image Window.  The second pass Writes
       every 8th row starting at the fifth row from the top.   The  third  pass
       Writes every 4th row starting at the third row from the top.  The fourth
       pass completes the image, writing  every  other  row,  starting  at  the
       second row from the top.  A Graphic description of this process follows:

       Image
       Row  Pass 1  Pass 2  Pass 3  Pass 4          Result
       ---------------------------------------------------
         0  **1a**                                  **1a**
         1                          **4a**          **4a**
         2                  **3a**                  **3a**
         3                          **4b**          **4b**
         4          **2a**                          **2a**
         5                          **4c**          **4c**
         6                  **3b**                  **3b**
         7                          **4d**          **4d**
         8  **1b**                                  **1b**
         9                          **4e**          **4e**
        10                  **3c**                  **3c**
        11                          **4f**          **4f**
        12          **2b**                          **2b**
       . . .

     The image pixel values are processed as a series of  color  indices
       which  map  into the existing color map.  The resulting color value from
       the map is what is actually displayed.  This series  of  pixel  indices,
       the  number  of  which  is equal to image-width*image-height pixels, are
       passed to the Gif image data stream one value per pixel, compressed  and
       packaged  according  to  a  version  of the LZW compression algorithm as
       defined in Appendix C.

     Gif TERMINATOR

     In order to provide a synchronization For the termination of a  GIF
       image  File,  a  Gif  decoder  will process the end of Gif mode when the
       Character 0x3B hex or ';' is found after an image  has  been  processed.
       By  convention  the  decoding software will pause and wait For an action
       indicating that the user is ready to continue.  This may be  a  carriage
       return  entered  at  the  keyboard  or  a  mouse click.  For interactive
       applications this user action must  be  passed  on  to  the  host  as  a
       carriage  return  Character  so  that the host application can continue.
       The decoding software will then typically leave Graphics mode and resume
       any previous process.

     Gif EXTENSION BLOCKS

     To provide For orderly extension of the Gif definition, a mechanism
       For  defining  the  packaging  of extensions Within a Gif data stream is
       necessary.  Specific Gif extensions are to be defined and documented  by
       CompuServe in order to provide a controlled enhancement path.

     Gif Extension Blocks are packaged in a manner similar to that  used
       by the raster data though not compressed.  The basic structure is:

      7 6 5 4 3 2 1 0  Byte #
     +---------------+
     |0 0 1 0 0 0 0 1|  1       '!' - Gif Extension Block Introducer
     +---------------+
     | Function code |  2       Extension Function code (0 to 255)
     +---------------+    ---+
     |  Byte count   |       |
     +---------------+       |
     :               :       +-- Repeated as many times as necessary
     |func data Bytes|       |
     :               :       |
     +---------------+    ---+
     . . .       . . .
     +---------------+
     |0 0 0 0 0 0 0 0|       zero Byte count (terminates block)
     +---------------+

     A Gif Extension Block may immediately preceed any Image  Descriptor
       or occur before the Gif Terminator.

     All Gif decoders must be able to recognize  the  existence  of  GIF
       Extension  Blocks  and  read past them if unable to process the Function
       code.  This ensures that older decoders will be able to process extended
       Gif   image   Files   in  the  future,  though  Without  the  additional
       Functionality.

         GLOSSARY

    Pixel - The smallest picture element of a  Graphics  image.   This  usually
       corresponds  to  a single dot on a Graphics screen.  Image resolution is
       typically given in Units of  pixels.   For  example  a  fairly  standard
       Graphics  screen  format  is  one 320 pixels across and 200 pixels high.
       Each pixel can  appear  as  one  of  several  colors  depending  on  the
       capabilities of the Graphics hardware.

    Raster - A horizontal row of pixels representing one line of an  image.   A
       typical method of working With images since most hardware is oriented to
       work most efficiently in this manner.

    LSB - Least Significant Byte.  Refers to a convention For two Byte  numeric
       values in which the less significant Byte of the value preceeds the more
       significant Byte.  This convention is typical on many microcomputers.

    Color Map - The list of definitions of each color  used  in  a  Gif  image.
       These  desired  colors are converted to available colors through a table
       which is derived by assigning an incoming color index (from  the  image)
       to  an  output  color  index  (of  the  hardware).   While the color map
       definitons are specified in a Gif image, the output  pixel  colors  will
       Vary  based  on  the  hardware used and its ability to match the defined
       color.

    Interlace - The method of displaying a Gif image in which  multiple  passes
       are  made,  outputting  raster  lines  spaced  apart to provide a way of
       visualizing the general content of an entire image  before  all  of  the
       data has been processed.

    B Protocol - A CompuServe-developed error-correcting File transfer protocol
       available  in  the  public  domain  and implemented in CompuServe VIDTEX
       products.  This error checking mechanism will be used  in  transfers  of
       Gif images For interactive applications.

    LZW - A sophisticated data compression algorithm  based  on  work  done  by
       Lempel-Ziv  &  Welch  which  has  the feature of very efficient one-pass
       encoding and decoding.  This allows the image  to  be  decompressed  and
       displayed  at  the  same  time.   The  original  article from which this
       technique was adapted is:

       Terry  A.   Welch,  "A  Technique  For  High   Performance   Data
       Compression", IEEE Computer, vol 17 no 6 (June 1984)

     This basic algorithm is also used in the  public  domain  ARC  File
       compression  utilities.   The  CompuServe  adaptation  of LZW For Gif is
       described in Appendix C.

        Gif Sequence Exchanges For an Interactive Environment

     The following sequences are defined For use  in  mediating  control
       between a Gif sender and Gif receiver over an interactive communications
       line.  These  sequences  do  not  apply  to  applications  that  involve
       downloading  of  static  Gif  Files and are not considered part of a GIF
       File.

     Gif CAPABILITIES ENQUIRY

     The GCE sequence is issued from a host and requests an  interactive
       Gif  decoder  to  return  a  response  message that defines the Graphics
       parameters For the decoder.  This involves returning  information  about
       available screen sizes, number of bits/color supported and the amount of
       color detail supported.  The escape sequence For the GCE is defined as:

     ESC [ > 0 g     (g is lower case, spaces inserted For clarity)
        (0x1B 0x5B 0x3E 0x30 0x67)

     Gif CAPABILITIES RESPONSE

     The Gif Capabilities Response message is returned by an interactive
       Gif  decoder  and  defines  the  decoder's  display capabilities For all
       Graphics modes that are supported by the software.  Note that  this  can
       also include Graphics Printers as well as a monitor screen.  The general
       format of this message is:

         #version;protocol{;dev, width, height, color-bits, color-res}... <CR>

       '#'          - GCR identifier Character (Number Sign)
       version      - Gif format version number;  initially '87a'
       protocol='0' - No end-to-end protocol supported by decoder
        Transfer as direct 8-bit data stream.
       protocol='1' - Can use an error correction protocol to transfer Gif data
            interactively from the host directly to the display.

       dev = '0'    - Screen parameter set follows
       dev = '1'    - Printer parameter set follows

       width        - Maximum supported display width in pixels
       height       - Maximum supported display height in pixels
       color-bits   - Number of  bits  per  pixel  supported.   The  number  of
            supported colors is therefore 2**color-bits.
       color-res    - Number of bits  per  color  component  supported  in  the
            hardware  color  palette.   if  color-res  is  '0'  then  no
            hardware palette table is available.

     Note that all values in the  GCR  are  returned  as  ASCII  decimal
       numbers and the message is terminated by a Carriage Return Character.

     The  following   GCR   message   describes   three   standard   EGA
       configurations  With  no  Printer;  the Gif data stream can be processed
       Within an error correcting protocol:

     #87a;1 ;0,320,200,4,0 ;0,640,200,2,2 ;0,640,350,4,2<CR>

     ENTER Gif GraphICS MODE

     Two sequences are currently defined to invoke  an  interactive  GIF
       decoder into action.  The only difference between them is that different
       output media are selected.  These sequences are:

         ESC [ > 1 g   Display Gif image on screen
         (0x1B 0x5B 0x3E 0x31 0x67)

         ESC [ > 2 g   Display image directly to an attached Graphics  Printer.
         The  image  may optionally be displayed on the screen as
         well.
         (0x1B 0x5B 0x3E 0x32 0x67)

     Note that the 'g' Character terminating each sequence is  in  lower
       case.

     INTERACTIVE ENVIRONMENT

     The assumed environment For the transmission of Gif image data from
       an  interactive  application  is  a  full 8-bit data stream from host to
       micro.  All 256 Character codes must be transferrable.  The establishing
       of  an 8-bit data path For communications will normally be taken care of
       by the host application Programs.  It is however  up  to  the  receiving
       communications Programs supporting Gif to be able to receive and pass on
       all 256 8-bit codes to the Gif decoder software.

     The Raster Data stream that represents the actual output image  can
       be represented as:

      7 6 5 4 3 2 1 0
     +---------------+
     |   code size   |
     +---------------+     ---+
     |blok Byte count|        |
     +---------------+        |
     :               :        +-- Repeated as many times as necessary
     |  data Bytes   |        |
     :               :        |
     +---------------+     ---+
     . . .       . . .
     +---------------+
     |0 0 0 0 0 0 0 0|       zero Byte count (terminates data stream)
     +---------------+

     The conversion of the image from a series  of  pixel  values  to  a
       transmitted or stored Character stream involves several steps.  In brief
       these steps are:

       1.  Establish the Code Size -  Define  the  number  of  bits  needed  to
           represent the actual data.

       2.  Compress the Data - Compress the series of image pixels to a  series
           of compression codes.

       3.  Build a Series of Bytes - Take the  set  of  compression  codes  and
           convert to a String of 8-bit Bytes.

       4.  Package the Bytes - Package sets of Bytes into blocks  preceeded  by
           Character counts and output.

    ESTABLISH CODE SIZE

     The first Byte of the Gif Raster Data stream is a value  indicating
       the minimum number of bits required to represent the set of actual pixel
       values.  Normally this will be the same as the  number  of  color  bits.
       Because  of  some  algorithmic Constraints however, black & white images
       which have one color bit must be indicated as having a code size  of  2.
       This  code size value also implies that the compression codes must start
       out one bit longer.

    COMPRESSION

     The LZW algorithm converts a series of data values into a series of
       codes  which may be raw values or a code designating a series of values.
       Using Text Characters as an analogy,  the  output  code  consists  of  a
       Character or a code representing a String of Characters.

     The LZW algorithm used in  Gif  matches  algorithmically  With  the
       standard LZW algorithm With the following differences:

       1.  A   special   Clear   code   is    defined    which    resets    all
           compression/decompression parameters and tables to a start-up state.
           The value of this code is 2**<code size>.  For example if  the  code
           size  indicated  was 4 (image was 4 bits/pixel) the Clear code value
           would be 16 (10000 binary).  The Clear code can appear at any  point
           in the image data stream and therefore requires the LZW algorithm to
           process succeeding codes as if  a  new  data  stream  was  starting.
           Encoders  should output a Clear code as the first code of each image
           data stream.

       2.  An end of Information code is defined that explicitly indicates  the
           end  of  the image data stream.  LZW processing terminates when this
           code is encountered.  It must be the last code output by the encoder
           For an image.  The value of this code is <Clear code>+1.

       3.  The first available compression code value is <Clear code>+2.

       4.  The output codes are of Variable length, starting  at  <code size>+1
           bits  per code, up to 12 bits per code.  This defines a maximum code
           value of 4095 (hex FFF).  Whenever the LZW code value  would  exceed
           the  current  code length, the code length is increased by one.  The
           packing/unpacking of these codes must then be altered to reflect the
           new code length.

    BUILD 8-BIT ByteS

     Because the LZW compression  used  For  Gif  creates  a  series  of
       Variable  length  codes, of between 3 and 12 bits each, these codes must
       be reformed into a series of 8-bit Bytes that  will  be  the  Characters
       actually stored or transmitted.  This provides additional compression of
       the image.  The codes are formed into a stream of bits as if  they  were
       packed  right to left and then picked off 8 bits at a time to be output.
       Assuming a Character Array of 8 bits per Character and using 5 bit codes
       to be packed, an example layout would be similar to:

      Byte n       Byte 5   Byte 4   Byte 3   Byte 2   Byte 1
     +-.....-----+--------+--------+--------+--------+--------+
     | and so on |hhhhhggg|ggfffffe|eeeedddd|dcccccbb|bbbaaaaa|
     +-.....-----+--------+--------+--------+--------+--------+

     Note that the physical  packing  arrangement  will  change  as  the
       number  of  bits per compression code change but the concept remains the
       same.

    PACKAGE THE ByteS

     Once the Bytes have been created, they are grouped into blocks  for
       output by preceeding each block of 0 to 255 Bytes With a Character count
       Byte.  A block With a zero Byte count terminates the Raster Data  stream
       For  a  given  image.  These blocks are what are actually output For the
       Gif image.  This block format has the side effect of allowing a decoding
       Program  the  ability to read past the actual image data if necessary by
       reading block counts and then skipping over the data.
    Graphics Interchange Format (GIF)                                   Page 15
    Appendix D - Multiple Image Processing

     Since a  Gif  data  stream  can  contain  multiple  images,  it  is
       necessary  to  describe  processing and display of such a File.  Because
       the image descriptor allows  For  placement  of  the  image  Within  the
       logical  screen,  it is possible to define a sequence of images that may
       each be a partial screen, but in total  fill  the  entire  screen.   The
       guidelines For handling the multiple image situation are:

       1.  There is no pause between images.  Each is processed immediately  as
           seen by the decoder.

       2.  Each image explicitly overWrites any image  already  on  the  screen
           inside  of  its Window.  The only screen clears are at the beginning
           and end of the  Gif  image  process.   See  discussion  on  the  GIF
           terminator.

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