rfc809.txt

RFC 相关的技术文档

     The  details of this technique are described in [3] and
     [4].

       It may also be required that a picture  be  enlarged.
     This enlargement can be done by simply duplicating each
     pixel in the picture.  For a  non-integral  ratio,  the
     picture  can  be expanded up to the nearest integer and
     then shrunk to the correct size.  However, this  method
     may degrade the image quality, e.g. the oblique contour
     may become stepped,  especially  when  the  picture  is
     enlarged  too much. This problem can be solved by using
     an iterative enlargement algorithm. Each time  a  pixel
     is  replaced  with a 2x2 array of pixels, whose pattern
     depends  on  the  original   pixel   and   the   pixels
     surrounding  it.  This  procedure is repeated until the
     requested ratio is reached. If  the  ration  is  not  a
     power  of 2's, the same method as that for non-integral
     ratios is used.




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       As a side effect of  developing  this  technique,  we
     could  freely  change  the  size and shape of an image.
     The picture can be expanded or shrunk,  or  it  can  be
     distorted.   Distortion,  whereby  the  horizontal  and
     vertical dimensions of the  image  may  be  changed  by
     different amounts, is often useful in image editing.

       The immediate consequence of this ability  to  change
     the image size meant that we could display the image on
     a screen as well as output the  image  on  a  facsimile
     machine.  To  a user of a computerised facsimile system
     this could be a very  useful  feature:  images  can  be
     displayed  on  screen  much  faster than on a facsimile
     machine, and displays are  significantly  cheaper  than
     the  facsimile machines as well. It is possible that an
     installation could have many screen displays where  the
     image  could  be viewed, but perhaps only one facsimile
     machine would be available for hard copy. This would be
     similar to many computer configurations today where the
     number of printers is limited due to  their  cost,  and
     display screens are far more numerous.


     2.3 Image Enhancement

       One aspect of computer processing that we  wanted  to
     investigate  was  that  of image enhancement. Enhancing
     the image is a  very  tricky  operation;  as  the  name
     implies  it  means  that  the image is improved in some
     sense.  Under program  control  this  is  difficult  to
     achieve: what the program thinks is an improvement, the
     human might judge to be distinctly worse.

       Our enhancement attempts were aimed  particularly  at
     printed  documents  and  other forms of typed text. The
     experiment was double pronged: we  hoped  to  make  the
     image  easier  to  read by humans while also making the
     image easier for the computer to handle.

       In our earlier experiments we had  noticed  that  the
     encoding  of  printed  matter was often very poor. This
     was especially noticeable when we  enlarged  an  image.
     Rather  than  each  character having smooth edges as on
     the original  document,  the  edges  were  very  rough,
     unexpected notches and excrescences being caused by the
     facsimile scanner.  They not  only  degrade  the  image
     quality but also decrease the compression efficiency. A
     typical enlargement of several characters is  shown  in
     Fig. 5.






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             Fig 5.  An enlargement of an typed text


       The enhancement method we adopted was first  employed
     at  Loughborough  University  [5].  This method has the
     effect of smoothing the edges of the dark areas on  the
     image.  The  technique consists of considering each dot
     in the image in turn. The dot is either left as  it  is




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UCL FACSIMILE SYSTEM                              INDRA Note 1185

     or changed to the opposite colour (white  to  black  or
     black  to  white)  depending  upon  the eight dots that
     surround it. The particular pattern of surrounding dots
     that  are  required to change the inner dot's colour is
     used to control the harshness  of  the  algorithm  [6],
     [8].

       In our  first  set  of  experiments  the  result  was
     definitely  worse  than  the original. Although square-
     like characters such as H, L, and T came out very well,
     anything  with slope (M, V, W, or S) became so bad that
     the oblique  contours  were  stepped.  The  method  was
     subsequently  modified to produce a result that was far
     more acceptable; the image looked a  lot  cleaner  than
     the  original.  Fig.  6  shows the same text as that in
     Fig. 5, but after it has been cleaned.







































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                     Fig. 6  A cleaned text


       The effect of these can be difficult to see  clearly.
     We have used the colour on our Grinnell display to show
     the original picture and the outcome of various picture
     processing  operations superposed in different colours.
     This brings out  the  effect  of  the  operations  very




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UCL FACSIMILE SYSTEM                              INDRA Note 1185

     vividly.

       It was mentioned above that the enhancement was  done
     not  only to improve the image for reading but also for
     easier  processing  by  the  computer.   As   described
     earlier,  the  image  from  the  facsimile  machine  is
     compressed in order to reduce the amount of data.   The
     cleaning  allows a higher compression rate so that more
     efficient transmission and/or storage can be achieved.

       We  learned   some   important   lessons   from   the
     enhancement  exercise.   Originally we thought that the
     main attraction in enhancement would be to improve  the
     readability.  In  the  end, we found that improving the
     readability was very difficult, especially because  the
     facsimile  image was so poor. Instead we found that the
     effect of  reducing  the  compressed  output  was  more
     important.  By reducing the data to be transmitted by a
     quarter, significant savings could be made. But  before
     such  a  technique  could be used in a live system, the
     time it  takes  to  produce  the  enhancement  must  be
     weighed  against  the  time  that  would  be  saved  in
     transmission.


     2.4 Image Editing

       By editing we mean that the facsimile picture can  be
     changed,  or  combined with other pictures, while it is
     stored inside the computer.  In  previous  sections  it
     was  mentioned  that we could change the size and shape
     of a facsimile image. This technique was later combined
     with  an  overlaying method that enabled one picture to
     be combined with another [12].

       In order to perform any editing it  is  necessary  to
     have  the picture displayed for the user to see. In our
     case we displayed the picture on  the  bit-map  screen.
     The image took up the left-hand side of the screen, the
     right side being reserved  for  the  picture  that  was
     being  built.   The  user  could  select an area of the
     left-hand screen and move  it  to  a  position  on  the
     right-hand  screen.   Several images could be displayed
     in succession on the left, and areas selected and moved
     to  the right.  Finally, the right-hand screen could be
     printed on the facsimile machine.

       The selection of an area of the picture was  done  by
     the   use   of   a   coloured  rectangular  subsection,
     controlled by a program in the computer, that could  be
     moved  around on the screen. The rectangular subsection




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     was moved with instructions typed in by  the  operator;
     it  could  be  moved  up  or  down,  and  increased  or
     decreased in size. When the  appropriate  area  of  the
     screen  had  been  selected, the program remembered the
     coordinates  and   moved   the   coloured   rectangular
     subsection  to  the  right-hand side of the screen. The
     user then selected an area again, in a similar  manner.
     When the user finished the editing, the program removed
     the part of the picture  selected  from  the  left-hand
     screen  and  converted  it  to  fit  the  shape  of the
     rectangular subsection on the  right-hand  screen.  The
     result was then displayed for the user to see.

       When an image was being edited,  the  editor  had  to
     keep  another  scaled  copy for display. This is due to
     the fact that the screen had a different  dimension  to
     that  of the facsimile machine. The editing operations,
     e.g.  chopping  and  merging,  were  performed  on  the
     original  image  data  files  with  the full resolution
     available on the facsimile machine.


     2.5 Integration with Other Data Types

       The facsimile  machine  can  be  viewed  in  a  wider
     context than merely a facsimile input/output device. It
     can work as a printer  for  other  data  representation
     types,  such  as  coded  character  text  and geometric
     graphics.  At  present,  text  can  be  converted  into
     facsimile  format and printed on the facsimile machine.
     Moreover, mixed pages containing pictures and text  can
     be  manipulated  by  our  system.  The  integration  of
     facsimile images with geometric graphics is a topic  of
     future research.

       In order to  convert  a  character  string  into  its