BackgroundSpherical	  Mirror	  Projection	  for	  Large	  domes
New	  ideas	  for	  maximizing	  brightness,	  resolution,	  and	  contrast	  for	  spherical	  mirror	  projection.
Presented by Bentley Ousley• Gottlieb	  Planetarium	  – 18.5	  M	  dome	  in	  Kansas	  City
• Gottlieb	  built	  in	  1999	  as	  part	  of	  Science	  City	  in	  Kansas	  City’s	  Union	  Station• Nov.	  2009	  – Partnership	  is	  formed	  between	  Union	  Station	  and	  ASKC	  to	  re-­‐ignite	  interest	  in	  the	  venue

Additional images:
Loch Ness Productions
Jack Dunn
Paul Bourke

Thank	   you	   for	   allowing	   me	   the	   opportunity	   to	   present	   at	   this	  conference.	   My	   name	   is	   Bentley	   Ousley.	   I’m	   a	   member	   of	   the	  Astronomical	   Society	   of	   Kansas	   City	   and	   a	   volunteer	   for	   the	   Arvin	  Gottlieb	  Planetarium	  in	  Kansas	  City.	  	  I	   was	   involved	   in	   helping	   to	   design	   and	   build	   a	   new	   full	   dome	  projection	   system	   for	   the	   Gottlieb	   Planetarium	   and	   wanted	   to	   offer	  our	  experiences	  to	  those	  who	  are	  interested.	  	  Gottlieb	  Planetarium	  was	  built	  in	  1999	  as	  part	  of	  a	  restoration	  project	  for	  Kansas	  City's	  Union	  Station.	  It	  houses	  an	  18.5	  Meter	  dome	  and	  is	  one	  of	  the	  attractions	  in	  Science	  City.	  	  In	   November	   2009	   I	   attended	   a	   meeting	   that	   lead	   to	   a	   partnership	  being	   formed	   between	   the	   Astronomical	   Society	   of	   Kansas	   City	   and	  Union	  Station.	  
The	   purpose	   of	   the	   new	   partnership	   was	   to	   increase	   ASKC's	  involvement	   in	   raising	   awareness	   of	   the	   planetarium	   and	   develop	  ideas	  to	  re-­‐engineer	  or	  replace	  the	  mostly	  non-­‐functional	  or	  outdated	  projection	  equipment	  with	  a	  system	  that	  could	  re-­‐ignite	  interest	  in	  the	  venue.	  Challenges

New	   projection	  system	  requirements• Not	  enough	  working	  equipment	  to	  present	  shows• Very	  limited	  money• Very	  low	  public	  awareness• ASKC	  “gun-­‐shy” due	  to	  previous	  partnership• A	  stable,	  reliable	  platform	  that	  would	  function	  day-­‐to-­‐day	  with	  few	  problems.
• Bright	  enough	  images,	  acceptable	  resolution,	  and	  good	  contrast	  on	  an	  18.5	  M	  dome.
• Compatibility	  with	  industry	  standard	  content• A	  reasonable	  looking	  night-­‐sky	  for	  star	  tours.
• Allows	  development	  of	  in-­‐house	  content.
• Must	  cost	  less	  than	  $35,000

At	   the	   beginning	   of	   the	   partnership	   we	   had	   several	   challenges	   to	  address.	  
• Not	   much	   was	   working	   in	   the	   planetarium,	   in	   fact	   there	   wasn’t	  enough	  working	  equipment	  to	  present	  shows.	  	  • Union	   Station	   had	   been	   operating	   in	   the	   red	   for	   several	   years	  and	  internal	  money	  was	  extremely	  difficult	  to	  get.	  	  	  • The	   Kansas	   City	   public	   was	   pretty	   much	   oblivious	   to	   the	  existence	  of	  the	  venue.	  
• ASKC	   members	   had	   previously	   been	   involved	   in	   a	   partnership	  that	   ended	   in	   a	   way	   that	   many	   in	   the	   club	   found	   grounds	   for	  being	  suspicious	  of	  any	  new	  agreement.	  

We	   determined	   that	   a	   new	   projection	   system	   was	   needed.	   We	   put	  together	   a	   list	   of	   minimum	   “must-­‐haves”	   and	   set	   about	   researching	  what	  we	  might	  be	  able	  to	  do	  to	  satisfy	  these	  requirements.	  
Here	  is	  what	  was	  required:	  • A	  stable,	  reliable	  platform	  that	  would	  function	  day-­‐to-­‐day	  with	  few	  problems.	  
• A	   system	   with	   bright	   enough	   images,	   acceptable	   resolution,	  and	  good	  contrast	  on	  an	  18.5	  M	  dome.	  
• Compatibility	  with	  industry	  standard	  content	  • A	  reasonable	  looking	  night-­‐sky	  for	  star	  tours.	  
• Allows	  development	  of	  in-­‐house	  content.	  
• Must	  cost	  less	  than	  $35,000

Development	  path
• Researched	  projection	  systems• Contacted	  Paul	  Bourke• Referred	  to	  Jack	  Dunn
• Jack	  lent	  us	  a	  spherical	  mirror	  and	  encouraged	  us	  to	  experiment• The	  new	  system	  was	  launched	  in	  Spring	  of	  2010
• We	  are	  pleased	  to	  offer	  what	  we	  learned	  from	  our	  experience

In	  our	  research	  for	  possible	  projection	  systems,	  we	  became	  intrigued	  with	   the	   idea	   of	   Digital	   Spherical	   Mirror	   Projection	   as	   developed	   by	  Mr.	  Paul	  Bourke	  at	  the	  University	  of	  Western	  Australia	  in	  2003.	  There	  was	   a	   wealth	   of	   information	   online	   and	   Paul	   courteously	   agreed	   to	  advise	  us	  on	  our	  project.	  
Paul	   recommended	   we	   contact	   Jack	   Dunn,	   longtime	   Director	   of	   the	  Meuller	  Planetarium	  at	  the	  University	  of	  Nebraska	  at	  Lincoln.	  
Mr.	   Dunn	   had	   helped	   set-­‐up	   several	   new	   spherical	   mirror	   projection	  systems	  and	  was	  a	  wealth	  of	  information	  on	  the	  topic.	  No	  planetarium	  as	   large	   as	   Gottlieb	   had	   successfully	   implemented	   this	   type	   of	  projection	   system	   in	   the	   past.	   In	   fact,	   we	   were	   advised	   by	   some	  vendors	   that	   this	   type	   of	   system	   was	   impractical	   in	   a	   venue	   as	   large	   as	  Gottlieb.	  	  [Comment	   by	   Paul	   Bourke:	   Actually	   the	   Bangalore	   planetarium	  implemented	   spherical	   mirror	   projection	   in	   2005,	   it	   is	   a	   20m	   dome.	  
And	  the	  Gravitational	  Discovery	  Centre	  in	  Western	  Australia	  installed	  a	  spherical	  mirror	  system	  in	  2010,	  it	  is	  also	  very	  close	  to	  20m	  diameter].	  	  Mr.	   Dunn	   lent	   us	   a	   spherical	   mirror	   and	   encouraged	   us	   to	   experiment.	  
His	  advice	  has	  been	  invaluable	  and	  we	  are	  greatly	  indebted	  to	  him	  for	  his	   help.	   After	   some	   experimentation,	   we	   made	   the	   determination	  that	   we	   could	   meet	   the	   minimum	   requirements	   we	   had	   outlined	  previously.	  In	  the	  implementation	  of	  this	  system,	  we	  discovered	  some	  ideas	  that	  made	  the	  system	  perform	  considerably	  better	  than	  we	  had	  expected.	  
Even	  though	  many	  of	  these	  concepts	  were	  developed	  to	  improve	  the	  performance	  of	  a	  large	  dome	  system.	  It	  is	  our	  hope	  that	  you	  find	  some	  of	  these	  ideas	  helpful.	  	  Concepts:	  Fisheye	  Lens	  Projection

Spherical	  Mirror	  Projection

1080

1920
Total area (1920X1080) = 2,073,600 (pixels)
Area of Fish-Eye = 916,088

Concepts:	  Bear	  with	  me	  while	  I	  cover	  some	  background	  information.	  
Pictured	  is	  a	  radial	  grid	  representing	  the	  full	  dome	  projection	  area	  for	  a	   hemispheric	   180x360	   degree	   dome.	   The	   center	   of	   the	   grid	  represents	   the	   zenith	   of	   the	   dome.	   The	   circle	   inscribing	   the	   outside	  edge	  is	  the	  area	  of	  the	  dome	  nearest	  the	  cove.	  The	  bottom	  is	  the	  front	  of	   the	   dome.	   The	   top	   is	   the	   rear	   of	   the	   dome.	   Single	   Fisheye	   Lens	  fulldome	   projection	   involves	   sending	   a	   Fish-­‐Eye	   compensated	   image	  (such	   as	   this)	   through	   a	   digital	   projector	   fitted	   with	   a	   Fish-­‐Eye	   lens	  placed	  in	  the	  center	  of	  the	  theatre,	  aimed	  directly	  overhead.	  
The	  grey	  area	  represents	  the	  area	  of	  a	  standard	  1920	  X	  1080	  pixel	  HD	  projection.	  As	  you	  can	  see	  by	  this	  image,	  the	  number	  of	  pixels	  that	  are	  actually	   used	   to	   project	   the	   fisheye	   image	   account	   for	   less	   than	   half	  the	  total	  number	  of	  pixels	  available	  in	  the	  HD	  projection.	  	  Spherical	  Mirror	  Projection	  is	  typically	  done	  by	  a	  projecting	  a	  specially	  warped	  image	  onto	  a	  first-­‐surface	  Mirror	  in	  the	  form	  of	  a	  section	  of	  a	  sphere.	   The	   mirror	   and	   projector	   are	   typically	   located	   at	   the	   rear	   of	  the	  theatre	  near	  the	  cove	  at	  the	  bottom	  edge	  of	  the	  dome.	  	  	  	  Spherical	  Mirror	  Projection

Fisheye	  vs Spherical	  Mirror	  Projection

1080
540
Front half of dome = ~458,000 pixels

1920

Front half of dome = ~458,000 pixels

An equivalent number of pixels are used to project the front half of thedome for both Fisheye and Spherical Mirror Projection.

Total area (1920X1080) = 2,073,600
Area of spherical mirror projection = 1,664,000

The	   particular	   type	   of	   image	   warping	   that	   is	   used	   in	   spherical	   mirror	  projection	  allows	  more	  pixels	  to	  be	  projected	  on	  the	  dome.	  
At	  first	  glance,	  this	  looks	  ideal,	  more	  pixels	  on	  the	  dome	  mean	  brighter	  images	  and	  more	  resolution,	  right?	  	  The	   areas	   shown	   in	   red	   are	   used	   to	   project	   the	   front	   half	   of	   the	   dome.	  
Notice	  that	  about	  the	  same	  number	  of	  pixels	  are	  used	  to	  project	  the	  front	  half	  of	  the	  dome	  for	  both	  Spherical	  Mirror	  and	  Fisheye	  setups.	  
Although	   more	   pixels	   are	   brought	   to	   bear	   with	   spherical	   mirror	  projection,	  the	  pixels	  that	  are	  gained	  by	  warping	  are	  projected	  at	  the	  rear	  of	  the	  dome.	  As	  you	  can	  see,	  the	  situation	  is	  not	  ideal	  -­‐	  the	  areas	  of	  greatest	  resolution	  are	  projected	  behind	  the	  audience.	  
Another	  issue	  -­‐	  the	  rear	  of	  the	  dome	  is	  where	  the	  warping	  artifacts	  of	  spherical	  mirror	  projection	  are	  most	  pronounced.	  	  	  	  	  	  Fisheye	  Projection	  with	  truncated	  image

Fisheye projection image truncation gives more resolution but
sacrifices the area at the rear of the dome.

Fisheye	   projection	   truncation	   can	   be	   used	   to	   make	   better	   use	   of	   the	  available	  pixels	  to	  increase	  brightness	  and	  resolution,	  but	  a	  substantial	  area	  of	  the	  back	  of	  the	  dome	  is	  lost	  as	  a	  result.	  In	  this	  scenario,	  around	  30%.	  	  Optimizing	  Spherical	  Mirror	  Projection

How do we get more resolution and brightness on the front of
the dome using spherical projection?

How	   can	   we	   get	   more	   resolution	   and	   brightness	   on	   the	   front	   of	   the	  dome?	  
Unlike	   Fisheye	   projection,	   there	   is	   not	   a	   one-­‐to-­‐one	   relationship	  between	   the	   pixels	   used	   to	   cover	   a	   specific	   area	   in	   the	   rear	   of	   the	  dome	  and	  the	  pixels	  used	  in	  the	  front	  of	  the	  dome.	  
For	   example:	   the	   red	   area	   is	   used	   to	   project	   only	   about	   7%	   of	   the	   rear	  of	  the	  dome.	  Yet	  it	  contains	  almost	  twice	  the	  number	  of	  pixels	  that	  are	  used	  to	  project	  the	  front	  50%	  of	  the	  dome!	  
So,	   how	   can	   we	   manipulate	   a	   spherical	   mirror	   projection	   system	   to	  give	  up	  the	  extremely	  pixel	  rich	  but	  small	  area	  immediately	  adjacent	  to	  the	   mirror	   in	   the	   rear	   of	   the	   dome	   in	   such	   a	   way	   as	   to	   greatly	   increase	  the	  pixel	  density	  in	  the	  front	  of	  the	  dome?	  Throw	  ratio

Changing the throw ratio of the projector in order to place all projected pixels on the mirror

Let’s	  take	  a	  look	  at	  a	  two	  step	  process	  for	  achieving	  this.	  	  Here	   is	   the	   image	   projected	   on	   the	   spherical	   mirror	   in	   a	   standard	  configuration.	   Notice	   the	   grey	   areas	   on	   the	   top	   corners.	   These	   areas	  are	  not	  projected	  on	  the	  mirror	  and	  thus	  are	  not	  part	  of	  the	  intended	  projected	  image.	  Basically,	  wasted	  pixels.	  
By	   changing	   the	   throw	   ratio	   of	   the	   projector	   such	   that	   all	   pixels	  produced	   by	   the	   projector	   are	   placed	   on	   the	   mirror,	   we	   are	   able	   to	  greatly	  increase	  the	  number	  of	  pixels	  projected	  on	  the	  front	  half	  of	  the	  dome.	   Notice	   how	   the	   corners	   of	   the	   projection	   just	   touch	   the	   outside	  edge	  of	  the	  mirror.	  All	  the	  pixels	  are	  now	  in	  use.	  
This	   adjustment,	   done	   by	   itself,	   causes	   the	   standard	   warping	   map	   to	  be	  considerably	  mis-­‐aligned.	  Let’s	  address	  this	  next.	  

Re-aligning the grid

Adjusting the mesh map to re-align the grid and restore the geometry

Using	   Paul	   Bourke’s	   Meshmapper	   software	   program,	   a	   new	   warping	  map	   is	   created	   that	   re-­‐warps	   the	   projection	   to	   make	   the	   images	  geometrically	  correct	  on	  the	  dome.	  
This	  new	  grid	  may	  look	  familiar.	  What	  we’ve	  done	  is:	  by	  not	  projecting	  the	   area	   in	   red	   (about	   7%	   of	   the	   dome	   in	   the	   rear),	   we	   have	   nearly	  doubled	  the	  pixel	  density	  in	  the	  front	  half	  of	  the	  dome.	  	  	  	  	  Optimizing	  resolution	  and	  brightness

Standard configuration

540

Front half of dome = ~458,000 pixels

New configuration

Optimizing	  contrast

740

Front half of dome = ~860,000 pixels

Pixel count is nearly doubled in the front half of the dome. With this,brightness and resolution are also nearly doubled.

The	  new	  configuration	  puts	  another	  400,000	  pixels	  in	  the	  front	  half	  of	  the	  dome	  while	  only	  giving	  up	  about	  7%	  of	  the	  projection	  area	  in	  the	  rear	   of	   the	   dome.	   This	   nearly	   doubles	   the	   resolution	   and	   contrast	   in	  the	  front	  half	  of	  the	  dome.	  	  

A	  Spherical	  mirror	  system	  projects	  an	  unusually	  large	  number	  of	  pixels	  near	  the	  mirror	  in	  the	  back	  of	  the	  theatre.	  This	  causes	  problems	  with	  balancing	   the	   brightness	   between	   areas	   of	   the	   dome	   with	   different	  concentrations	  of	  pixels	  per	  given	  area.	  
This	   is	   usually	   accomplished	   by	   an	   option	   in	   Paul	   Bourke’s	  Meshmapper	   which	   applies	   a	   gradual	   dimming	   to	   pixels	   near	   the	  mirror	  in	  order	  to	  evenly	  project	  brightness	  over	  the	  whole	  dome.	  
In	   a	   large	   dome,	   the	   projector	   is	   extremely	   bright	   and	   even	   with	   the	  pixels	   dimmed	   nearly	   to	   black	   at	   the	   closest	   point	   to	   the	   mirror,	   the	  projected	  black	  is	  actually	  a	  bright	  grey.	  When	  projecting	  the	  night	  sky,	  we	   found	   the	   area	   around	   the	   mirror	   was	   still	   causing	   a	   problem	   in	  contrast	  on	  the	  rest	  of	  the	  dome.	  	  	  	  	  	  	  Optimizing	  contrast
A Baffle is shaped to cast ashadow on the mirror that
evenly reduces excess lightin the rear of the theatre.

We	   don’t	   use	   the	   option	   in	   Meshmapper	   to	   gradually	   electronically	  dim	  the	  pixels	  close	  to	  the	  mirror.	  We	  chose	  to	  use	  a	  mechanical	  baffle	  to	   accomplish	   the	   task	   of	   dimming	   the	   area	   associated	   with	   those	  pixels.	   Using	   this	   method,	   all	   levels	   of	   light	   are	   reduced	   equally	   in	  areas	   close	   to	   the	   mirror	   and	   contrast	   between	   light	   and	   dark	   are	  maintained.	   Conversely,	   with	   electronic	   dimming,	   the	   contrast	   is	  diminished	   between	   light	   and	   dark	   as	   a	   consequence	   of	   making	   the	  brightest	  light	  proportionally	  dimmer.	  Also,	  because	  the	  baffle	  dims	  all	  projector	   light	   near	   the	   mirror,	   less	   stray	   light	   is	   reflected	   on	   the	  remainder	  of	  the	  dome.	  
A	   specially	   shaped	   baffle	   is	   placed	   close	   to	   the	   projector	   lens.	   It	   blocks	  light	  from	  the	  tops	  and	  sides	  of	  the	  unfocused	  image.	  It	  casts	  a	  shadow	  on	   the	   mirror	   of	   a	   shape	   very	   similar	   to	   the	   effect	   produced	  electronically	  with	  Meshmapper.	  

Optimizing	  contrast
Moving the baffle
away from theprojector increases
the shadow on themirror.

The	   baffle	   can	   be	   set	   at	   different	   distances	   from	   the	   projector	   to	  adjust	   the	   amount	   of	   baffling.	   As	   the	   baffle	   is	   moved	   away	   from	   the	  projector,	   	   more	   of	   the	   light	   cone	   is	   intercepted	   and	   the	   shadow	   on	  the	  mirror	  becomes	  more	  pronounced.	  	  	  SummaryCaveats:• Only	  works	  for	  unidirectional	  seating• Standard	  pre-­‐warped	  programming	  won’t	  look	  right	  on	  the	  dome	  unless	  	  re-­‐warped	  with	  a	  custom	  meshmap.
• Not	  many	  digital	  projectors	  have	  the	  flexibility	  for	  this	  method	  to	  work.	  (An	  alternative	  could	  be	  using	  a	  larger	  diameter	  mirror.)Given	  the	  right	  equipment,	  dramatic	  increases	  in	  resolution,	  brightness,	  and	  contrast	  and	  be	  achieved	  in	  spherical	  mirror	  projection	  systems	  with	  a	  relatively	  small	  investment	  of	  time	  and	  money.

In	  summary:	  	  Some	  caveats:	  • Obviously,	   this	   method	   is	   only	   going	   to	   work	   well	   with	  unidirectional	  seating.	  
• If	  you	  are	  buying	  your	  programming	  pre-­‐warped,	  it	  won’t	  look	  right	  unless	  re-­‐warped	  with	  a	  custom	  meshmap.	  
• The	  Digital	  Projection	  dVision	  1080	  XL	  is	  used	  in	  the	  system	  at	  the	  Gottlieb.	  It	  incorporates	  a	  zoom	  feature	  which	  allows	  for	  a	  great	   deal	   of	   latitude	   in	   adjusting	   the	   throw	   ratio	   of	   the	  projector	  and	  is	  capable	  of	  focusing	  a	  very	  small	  image	  on	  the	  mirror.	  Not	  many	  digital	  projectors	  allow	  this	  much	  flexibility	  in	  image	  size	  and	  focus.	  One	  idea	  that	  might	  be	  explored	  to	  make	  this	   technique	   available	   to	   more	   projectors	   is	   using	   a	   larger	  diameter	  mirror.	  

Spherical	  Mirror	  Projection	  for	  Large	  domes
New	  ideas	  for	  maximizing	  brightness,	  resolution,	  and	  contrast	  for	  spherical	  mirror	  projection.
Presented by Bentley Ousley
Special thanks to:
Rick Henderson
Jeff Rosenblatt
Jack Dunn

Thank	  you	  very	  much	  for	  the	  opportunity	  to	  present.