ImageSpace Dynamic Transparency for Improved Object Discovery in 3D Environments

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Image-Space Dynamic Transparency for Improved
Object Discovery in 3D Environments
orBecoming Superman

• Ulf Assarsson ltuffe_at_ce.chalmers.segt
• Niklas Elmqvist ltelm_at_cs.chalmers.segt
• Philippas Tsigas lttsigas_at_cs.chalmers.segt

Winter Meeting 2006 Jan 11-13, Alingsås
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Why Become Superman?

• What if we could endow all human users with
  Superman-like powers of observation?
• Difficult in the real world
• Possible in the computer world
• Idea Give the users super-human vision
• See through walls
• See things far away
• See things too small to see with the naked eye
• Discover visualizations anew

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Example Supermans X-Ray Vision
"Where we come from everyone has see-through
vision, extra-strength and extra-speed!S No.
65/3 "Three Supermen from Krypton!
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On Superhero X-Ray Vision
Today's Superman possesses a wide range of
optical super-powers, including X-ray vision,
which enables him to see through all substances
except lead telescopic vision, which enables him
to focus on objects millions of miles away
super-vision, a combination of X-ray vision and
telescopic vision, which enables him to perform
such optical feats as peering through the wall of
a house thousands of miles away micro-scopic
vision, which enables him to examine the tiniest
atomic particles

• Sources Supermanica (supermanica.info) and
  theSuperman Encyclopaedia (theages.superman.ws/En
  cyclopaedia/)
• Major components
• X-ray vision see through all substances and
  materials except lead
• Telescopic vision see (very) distant objects
• Supervision combination of x-ray and telescopic
  vision
• Microscopic vision see on a microscopic scale

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Benefits

• Superhero vision has a very important benefit
• Avoids problems with visibility and legibility in
  3D environments
• Can easily pinpoint important targets despite
  occluding distractors
• Main stumbling block of 3D information
  visualization
• Does not appear in 2D visualization
• Objects that do not intersect do not occlude each
  other
• Caused by the nature of the human vision system
• (But not the superhuman vision system...?)
• Example Cant see the forest for the trees.

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Problem Occlusion in 3D Environments

• Major problem inter-object occlusion
• Affects visual tasks related to object retrieval
• Object discovery finding all info-bearing
  objects
• Governed by object visibility
• Affects correctness may miss some information
  completely
• Object access retrieving info from objects
• Governed by object legibility
• Affects productivity effort needed to retrieve
  information
• Properties of the environment determines severity
  of the occlusion effects
• Object density, interaction and complexity

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Example Occlusion
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Image-Space Dynamic Transparency

• Idea Adjust transparency of surfaces to make
  targets visible through occluding distractors
• Observation The image space is perfect for
  detecting instances of occluded targets and
  dynamically adjusting transparency to allow the
  user to see through surfaces
• Can employ fragment and vertex shader
  capabilities of modern programmable graphics
  hardware
• Achieve Superman-like cutaway effect of
  surfaces to retain depth cues and spatial
  information
• In essence, we want to implement Supermans
  X-ray vision in our visualization
• Need to define a consistent model for dynamic
  transparency

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Dynamic Transparency Model

• We define our model for dynamic transparency as a
  set of rules
• R1 All important objects (targets) in a scene
  should be visible from any given viewpoint
• R2 Objects are made visible by changing the
  transparency level of occluding surfaces from
  opaque (? 100) to transparent (? ?t gt 0)
  within a cutout area enclosing the object
• R3 Some surfaces are impenetrable and will never
  be made transparent (cf lead for Superman)
• R4 Objects are allowed to self-occlude
  themselves
• Cutout area convex hull (circle) or outline with
  a gradient transparency border

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Example Russian Dolls
Normal vision
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Dynamic Transparency Model (2)

• There are some additional aspects on dynamic
  transparency worth discussing further
• Operational mode
• Active the user controls a searchlight on the
  image space that reveals underlying occluded
  objects
• Passive all important objects are revealed at
  all times (possibly using a selection filter)
• See-through layer control
• Standard dynamic transparency peels off all
  necessary surfaces to make an object visible
• Sometimes you want to control how many layers
  should be peeled away (search depth)
• Example one-layer depth technical illustrations
  Diepstraten et al. 2002 and 2003

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Algorithm Description

• Algorithm that fulfills requirements and model
• Multiple rendering passes
• Sort objects in descending depth (from the view)
• Initialize framebuffer with alpha 1.0 every
  frame
• Render differently depending on target or
  distractor
• Distractor Render to depth and then color
  buffer, but blend using the alpha value of the
  framebuffer
• Target Render to depth and then color buffer.
  Update the framebuffer alpha values using an
  alpha mask
• Alpha mask is multiplicatively blended to the
  alpha channel of the frame buffer
• Frame buffer becomes a cumulative alpha mask
• Alpha mask rendering is of special interest

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Alpha Mask Rendering

• Render object to mask buffer (256x256 texture)
• Iteratively grow the smooth alpha gradient
  frame around the object
• Idea Add a single line of increasing alpha to
  the pixels surrounding the previous pass

Target object
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Implementation

• We have implemented this algorithm in C/C
• OpenGL for 3D graphics
• OpenGL ARB shader extensions
• Uses GLSL (GL Shading Language) for programmable
  fragment and vertex shaders
• Prototype implementation allows for the
  construction of arbitrarily complex 3D scenes of
  targets and distractors
• Impenetrable surfaces can be omitted from the
  method using the stencil buffer
• Excellent framerates on the implementation (180
  FPS on NVidia Geforce 6800)
• Only screenshots, currently no real-time demo
• Lack laptop hardware capable of PS 3.0

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Results Screenshots
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Screenshots (2)
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Results Usability

• Plans are underway for a formal user study of the
  method
• Idea Present the user with a number of 3D scenes
  of increasing complexity. Ask the user to search
  for certain targets in a set of distractors.
• Measure performance and correctness using dynamic
  transparency versus standard 3D camera controls
• Intuition (Much) better results using our new
  method

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Conclusions

• Superhero vision has an important benefit
• Avoids visibility and legibility problems by
  allowing for occluding surfaces to be made
  (semi-)transparent
• Our model for dynamic transparency supports this
  mechanism in visualization applications
• Targets are always visible through semi-opaque
  cutouts in occluding distractors
• Image-space computer graphics algorithm and
  implementation of this model
• Based on modern fragment shader programmability
• Promising results invite further research
• Including visual, performance, and usability
  results

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Future Work

• Formal user study to measure efficiency
• Cutout geometry
• Interest-based dynamic transparency
• Importance/interest scale from 0,1
• Automatically derive object importance
• Technical illustrations
• Additional Superman vision techniques?
• Microscopic telescopic

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Questions?

• Contact informationNiklas Elmqvistelm_at_cs.chalme
  rs.sePhone 46 31 772 1024Fax [phone]
• Dynamic Transparency websitehttp//www.cs.chalme
  rs.se/elm/projects/dyntrans/