GRAPHIC  SYSTEMS  

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Lecture-3

Display  Systems          

Random  Scan  Displays  

Raster  Scan  Displays  

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Random  Scan  Displays  

Raster-­‐Scan  Display  

•  Raster:  A  rectangular  array  of  points  or  dots  

•  Pixel:  One  dot  or  picture  element  (pel)  of  the  raster  

•  Scan  line:  A  row  of  pixels  

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Raster-­‐Scan  Display…1  •  The    electron    beam    is    swept    across   the    screen,    one    row    at    a  

Eme  from  top  to  bottom.  As  the  electron  beam  moves  across  each  row,  the  beam  intensity  is  turned  on  and  off  to  create  a  pattern  of  illuminated  spots.  

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Raster-­‐Scan  Display…2  •  Picture      definiEon     is     stored      in      a      memory      area      called      the  

refresh  buffer  or  frame  buffer,  which  holds  the  set  of  intensity  values  for  all  the  screen  points.  

•  Stored    intensity    values    are    then    retrieved    from    the    refresh  buffer  and  painted  on  the  screen  one  row  at  a  time  as  shown  in  the  figure.  

Display of an object as a set of discrete points across each scan line

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Raster-­‐Scan  Display…3  •  The   intensity   range  of  pixel  posiEons  define  capability  

of  raster  system.  The  frame  buffer  may  be  called:  –  Bitmap:  with  one  bit  per  pixel,  ex.  Black  and  white  system,  a  bit  value  of  1  indicates  that  the  electron  beam  is  to  be  turned  on  and  0  indicates  off.  

–  Pixmap:  with  mulEple  bits  per  pixel,  24  bits  per  pixel  in  high  definiEon  systems  

 

A  system  with  24  bits  per  pixel  and  a  screen  resoluEon  1024  by  1024  requires  3  MB  (24*1024*1024)  storage  for  frame  buffer.  

 

•  Refreshing  on  raster-­‐scan  display  is  carried  out  at  a  rate  of    60    to    80    frames    per    second    (Hz).    This    creates    new  image.  Refresh  rate  1/60  to  1/80  second.  

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Raster-­‐Scan  Display…4  

•  Scanning  (leW  to  right,  top  to  bottom)  –  Vertical Sync Pulse: Signals the start of the next field –  Vertical Retrace: Time needed to get from the bottom of

the current field to the top of the next field –  Horizontal Sync Pulse: Signals the start of the new scan

line  

–  Horizontal Retrace: The time needed to get from the end of the current scan line to the start of the next scan line

         

Refresh rate for 512×512 monochrome raster with an access rate of 200 nanosecond per pixel =1/(512*512*200*10-9)=19 frame/s (approx.)

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Raster-­‐Scan  Display…5  

Raster-­‐Scan  Display…6  §  Interlace procedure is used in slow refreshing rate display

device like TV

§  Flickers are noticed for scanning all the pixels of screen at low refreshing rate like 30 frames/second.

 

§  To reduce flicker, divide each frame into two “fields” of odd and even lines. This reduces the scan time of screen from top to bottom to half as the beam will sweep every alternate scan line in one pass in 1/60s.

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1/60  Sec   1/60  Sec  Field  1   Field  2  

Frame  

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PIXELS

•  An  image  is  composed  of  discrete  pixels  or  picture  elements.  

•  These    are    arranged    in    a    row-­‐column    fashion    to    form    a  rectangular  area  (raster).  

•  The  total  number  of  pixels  in  a  image  is  a  funcEon  of  the  size  of  the  image  and  number  of  pixels  per  unit  length  in  the  horizontal  as  well  as  verEcal  direcEon  (ResoluEon).  

•  Image    size    is    defined    as    total    number    of    pixels    in    the  horizontal    direcEon    Emes    the    total    number    of    pixels    in  the  verEcal  direcEon  (512  x  512,  640  x  480,  1024  x  768).  

Pixels…1  

•  Size  of  image,  at  400  pixels  per  inch  for  640  x  480  image  =  640/400  x  480/400  =  1.6  x  1.2  inch  

 

•  Aspect  raEo,  for  640  x  480  image  =  640/480  =  4/3  •  Pixel  at  lower  leW  corner  of  an  image  is  considered  to  be  at  

the  origin  (0,0)  of  a  pixel  coordinate  system.  

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Random  Scan  Display  

§  In  random  scan  display  unit,  a  CRT  has  the  electron  beam  directed  only  to  the  parts  of  the  screen  where  picture  is  to  be  drawn.  

§  Draws    a    picture    one    line    at    a    time:    also    called    vector    or  stroke-­‐wri-ng  or  calligraphic  displays.  

 

§  The    component     lines    of    a    picture    can    be    drawn    and  refreshed  in  a  specified  manner  (see  next  figure)  

§  Refresh      rate      depends      on      the      number      of      lines      to      be  displayed  

§  Refresh      display      file      or      buffer      or      list      or      program      is      a  memory        area        which        stores        a        set        of        line        drawing  commands.  

Random Scan Display

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In  random  scan  display  an  electron  beam  is  deflected  from  endpoint  to  end-­‐point.    The   order   of   deflecEon   is   dictated   by   the   arbitrary   order   of   the   display   commands.   The  display   must   be   refreshed   at   regular   intervals   –   minimum   of   30   Hz   (fps)   for   flicker-­‐free  display  

Random Scan Display

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Random  Scan  Display  

•  Advantages  

–  It  has  higher  resoluEon  than  raster-­‐scan  systems.  –  Lines    drawn    are    smoother    in    contrast    to    raster    system,    which  

produces  jagged  lines,  plobed  as  discrete  point  sets.  

•  Disadvantages  

–  Suitable  only  for  line  drawings  like  wireframes  and  can  not  display  realisEc  shaded  scenes.  

–  Complex  scenes  cause  visible  flicker  

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Raster  Scan  vs.  Random  Scan  

Base  of    Difference  

Raster  Scan  System   Random  Scan  System  

Electron  Beam  

The   electron   beam   is   swept   across   the  screen,  one   row  at  a  Eme,   from  top   to  bobom.  

The   electron   beam   is   directed   only  to   the   parts   of   screen   where   a  picture  is  to  be  drawn.  

ResoluEon   Its  resoluEon  is  poor  because  raster    system   produces   zig-­‐zag   lines   that   are  plobed  as  discrete  point  sets.  

Its   resoluEon   is   good   because   this  system   produces   smooth   lines  drawings   because   CRT   beam  directly  follows  the  line  path.  

Picture    DefiniEon  

Picture   definiEon   is   stored   as   a   set   of  intensity   values   for   all   screen   points,  called  pixels  in  a  refresh  buffer  area.  

Picture   definiEon   is   stored   as   a   set  of   line   drawing   instrucEons   in   a  display  file.  

RealisEc  Display  

The   capability   of   this   system   to   store    intensity   values   for   pixel   makes   it   well  suited  for  the  realisEc  display  of  scenes  contain  shadow  and  color  pabern.  

These  systems  are  designed  for  line-­‐drawing   and   can’t   display   realisEc  shaded  scenes.  

Draw  an  Image  

Screen   points/pixels   are   used   to   draw  an  image.  

MathemaEcal  funcEons  are  used  to    draw  an  image.  

Raster  Scan  Systems  

•  InteracEve  raster  graphics  systems  employ  several  processing  units:  

 

–  CPU  –  Display  controller  or  video  controller  

typically  

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Raster  Scan  Systems  

§  A  fixed  area  of  the  system  memory  is  reserved  for  the  frame  buffer.  

§  Video  controller  is  frame  buffer.  

given  direct  access  to  the  

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Raster  Scan  Systems  

§  Frame  buffer   locaEons  and  corresponding  screen  posiEons  are  referenced  in  Cartesian  coordinates.  

§  The  origin  of  the  coordinate  system  is  usually  specified  in  the  lower  leW  corner.  

 

§  Screen  surface  is  represented  as  first  quadrant  of  a  two  dimensional  system.  

   

§  Two    registers    (x    &    y)    are  used  to              store              the  coordinates    of    the    screen  pixels.  

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Image  File  Formats  

•  Standardized  means  of  organizing  and  storing  images.  •  These  are  digital  image  formats  basically  used  to  store  photographs  and  images.  Image  files  are  composed  of:  – Pixels  – Vector  (geometric)  data  rasterized  to  pixels  for  display  in  vector  graphic  display  

 

•  Pixels  composing  an  image  are  ordered  as  a  grid  (columns  and  rows)  and  its  number  represents  the  brightness  and  colour.  

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Image  File  Sizes  

•  Expressed  as  number  of  bytes,  the  size  increasing  with  the  number  of  pixels  and  the  colour  depth  of  a  pixel.  –  Greater  pixels  means  greater  columns  and  rows,  defining  image  resoluEon  and  size  

–  Greater  colour  depth  is  defined  by  bits  and  bytes  of  a  pixel.  Single  byte,  8  bit  pixel  stores  256  colours  and  3  byte,  24  bit  pixel  stores  16,000,000  colours  (termed  as  true  colours)  

 

–  E.g.  8  megapixel  digital  camera  requires  24,000,000  bytes  of  memory  to  store  an  image  in  true  colour  (each  pixel  using  3  bytes  to  store  true  colour)  

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Image  Compression  

•  Compression  is  used  to  reduce  the  size  of  an  image  and  hence  the  storage  requirement.  

•  This  is  based  on  image  file  compression  algorithms.  These  are  of  two  types  –  

 

–  Lossless  Compression:  When  image  quality  is  valued  above  file  size,  lossless  algorithms  are  typically  chosen.  

–  Lossy  Compression:  Lossy  compression  algorithms  take  advantage  of  the  inherent  limitaEons  of  the  human  eye  and  discard  invisible  informaEon.  Most  lossy  compression  algorithms  allow  for  variable  quality  levels  (compression)  and  as  these  levels  are  increased,  file  size  is  reduced.  

•  At  the  highest  compression  levels,  image  deterioraEon  becomes  noEceable  as  "compression  arEfacEng“  in  the  case  of  lossy  compression.  

Image  Compression…2    

LOSSLESS COMPRESSION

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Image  Compression…3    

LOSSY COMPRESSION

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Major  File  Formats  

•  Raster  formats  –  These  formats  store  images  as  bitmaps  –  E.g.  JPEG,  PNG,  TIFF,  RAW,  GIF,  BMP,  PPG,  PPM,  PBM,  PNM  

•  Vector  formats  –  These  formats  contain  a  geometric  descripEon  that  can  be  rendered  smoothly  at  any  desired  display  size.  These  formats  can  contain  pixel  data  as  well  (texture  mapping  of  3-­‐D  images)  

–  E.g.  CGM,  SVG,  Encapsulated  Post  script,  PDF,  SWF,  Window  Metafile  

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Raster  Formats  

•  JPEG-­‐  Joint  Photographic  Experts  Group  •  TIFF  -­‐  Tagged  Image  File  Format  •  GIF-­‐Graphics  Interchange  Format  •  BMP-­‐bitmap  •  PNG-­‐  Portable  Network  Graphics  •  Raw  –Raw  Image  formats  •  PPM-­‐portable  pixmap  •  PGM-­‐portable  graymap  •  PBM-­‐portable  bitmap  •  PNM  -­‐portable  anymap  

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Vector  Formats  

•  CGM-­‐  Computer  Graphics  Metafile  •  SVG-­‐  Scalable  Vector  Graphics  •  EPS-­‐  Encapsulated  PostScript  •  PDF  -­‐  Portable  Document  Format  •  SWF-­‐Shock  Wave  Flash  •  WMF-­‐  Windows  Meta  File  

Image  File  Structure  

•  Consist  of  two  parts:  –  Header  –  Footer  

Format  /  version  identification  Image  width  and  height  in  pixels  Image  type  Image  data  format  Compression   type  Etc.  

Colour  map  (if  any)  Pixel  values  

Header

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Image data

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Image  File  Structure…1  

•  Header  –  IdenEficaEon  of  format  is  in  the  form  of  a  binary  code  or  ASCII  string  

–  Number  of  pixels  define  the  width  and  height  of  an  image  –  Common  image  types  are  black  and  white  (1  bit  per  pixel),  8-­‐bit  grey  scale  (256  levels  along  the  grey  axis),  8-­‐bit  colour,  and  24-­‐bit  colour  

–  Image  data  format  specifies  the  order  in  which  pixel  values  are  stored  in  the  image  data  secEon  

•  Common  order  is  leW  to  right  and  top  to  bobom  /  bobom  to  top  •  It  also  specifies  whether  the  RGB  values  are  interlaced  (R,G,B,R,G,B,..)  or  non-­‐interlaced  (R,R,R,..,G,G,G,..,B,B,B,..)