A Perceptual Heuristic

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A Perceptual Heuristic for Shadow Computation in
Photo-Realistic Images
Wednesday, 2 August 2006 Peter Vangorp Olivier
Dumont Toon Lenaerts Philip Dutré Katholieke
Universiteit Leuven
peter.vangorp,olivier.dumont,toon.lenaerts,philip
.dutre_at_cs.kuleuven.be
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Introduction

• 3 types of realistic rendering

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Introduction

• 3 types of realistic rendering
• radiometric accuracyrender everything a
  photometer can detect

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Introduction

• 3 types of realistic rendering
• radiometric accuracyrender everything a
  photometer can detect
• physiological perceptionrender only what the
  eye can see

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Introduction

• 3 types of realistic rendering
• radiometric accuracyrender everything a
  photometer can detect
• physiological perceptionrender only what the
  eye can see
• psychological perceptionrender only what the
  brain can see

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Radiometric accuracy

• measure shapes, light sources, materials, ...

photometer
Cornell box
Meyer et al. 1986
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Physiological perception

• use low-level limitations of human visual
  systemthreshold vs intensity, contrast
  sensitivity, ...

reference
threshold map
visibly indistinguishable adaptive rendering
Ramasubramanian et al. 1999
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Physiological perception

• use low-level limitations of human visual
  systemthreshold vs intensity, contrast
  sensitivity, ...

reference
threshold map
visibly indistinguishable adaptive rendering
Ramasubramanian et al. 1999
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Psychological perception

• use higher-level heuristics
• Is a shadow necessary for therealism of a
  scene?

Thompson et al. 1998
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Psychological perception

• use higher-level heuristics
• Is a shadow necessary for therealism of a
  scene?
• Do we need highlights to convey material
  properties?

Thompson et al. 1998
Fleming et al. 2004
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Psychological perception

• use higher-level heuristics
• Is a shadow necessary for therealism of a
  scene?
• Do we need highlights to convey material
  properties?
• How detailed should the geometry be?

Thompson et al. 1998
Fleming et al. 2004
Luebke 2001
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Motivation Goals

• Motivation
• shadows are important for perception of realism
• Goal
• detect perceptually important shadows in the
  scene
• render important shadows accurately
• approximate unimportant shadows

Kersten et al. 1997
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Methodology

1. Psycho-physical experiments
2. Derive a heuristic predicting shadow importance
3. Design a perceptually driven algorithm
4. Experimental validation

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Methodology

1. Psycho-physical experiments
2. Derive a heuristic predicting shadow importance
3. Design a perceptually driven algorithm
4. Experimental validation

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Methodology

1. Psycho-physical experiments
2. Derive a heuristic predicting shadow importance
3. Design a perceptually driven algorithm
4. Experimental validation

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Methodology

1. Psycho-physical experiments
2. Derive a heuristic predicting shadow importance
3. Design a perceptually driven algorithm
4. Experimental validation

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1. Perceptual experiments

• Test setup 162 images, varying sphere
  over 9 radii and 9 heights

correct shadow
no shadow (avg. illumination)
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1. Perceptual experiments

• Does the lighting in this image look realistic?
• single stimulus
• 5000 decisions
• avg. 2 sec / decision

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2. Heuristic
large difference in realism, e.g.
difference in realism
small difference in realism, e.g.
? sphere height
? sphere radius
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3. A perceptually driven algorithm

• Ray tracing

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3. A perceptually driven algorithm

• Ray tracing
• shoot viewing ray

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3. A perceptually driven algorithm

• Ray tracing
• shoot viewing ray
• evaluate heuristic in hit point to be shaded
• in function of distance and solid angle

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3. A perceptually driven algorithm

• Preprocessing step
• shadow photon map

Jensen and Christensen 1995
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3. A perceptually driven algorithm

• Preprocessing step
• shadow photon map
• shadow photons augmented with heuristic

Jensen and Christensen 1995
shadow photon map
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3. A perceptually driven algorithm
For each viewing ray
Gather nearest shadow photons
Calculate average perceptual value
yes
no
avg lt threshold
approximate photon map
render accurately shadow rays
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3. A perceptually driven algorithm
For each viewing ray
Gather nearest shadow photons
Calculate average perceptual value
yes
no
avg lt threshold
approximate photon map
render accurately shadow rays
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3. A perceptually driven algorithm
For each viewing ray
Gather nearest shadow photons
Calculate average perceptual value
yes
no
avg lt threshold
approximate photon map
render accurately shadow rays
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3. A perceptually driven algorithm
For each viewing ray
Gather nearest shadow photons
user-defined threshold
Calculate average perceptual value
yes
no
avg lt threshold
approximate photon map
render accurately shadow rays
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3. A perceptually driven algorithm
For each viewing ray
Gather nearest shadow photons
user-defined threshold
Calculate average perceptual value
yes
no
avg lt threshold
approximate photon map
render accurately shadow rays
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3. A perceptually driven algorithm
For each viewing ray
Gather nearest shadow photons
user-defined threshold
Calculate average perceptual value
yes
no
avg lt threshold
approximate photon map
render accurately shadow rays
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3. A perceptually driven algorithm
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4. Validation

• Similar perceptual experiment
• Do the lighting and the shadows look realistic?

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4. Validation

• Similar perceptual experiment
• Do the lighting and the shadows look realistic?
• Stimuli 6 scenes
• threshold 25, 50, 75
• reference rendering (threshold 0)

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4. Validation

• Similar perceptual experiment
• Do the lighting and the shadows look realistic?
• Stimuli 6 scenes
• threshold 25, 50, 75
• reference rendering (threshold 0)
• 15 subjects, almost 6000 decisions, avg. 5 seconds

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4. Validation
Threshold 25 Approx px 13
Threshold 50 Approx px 24
Threshold 75 Approx px 48

• Up to 50 of the pixels can be approximated,
  without loss of perceptual realism

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Results
reference
threshold 90
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Results
threshold 90
reference
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Results
reference
threshold 90
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Results
reference
threshold 90
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Results
reference
threshold 80
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Results
reference
threshold 80
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Results
reference
threshold 80
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Results
reference
threshold 80
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Results
reference
threshold 90
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Results
reference
threshold 90
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Results
reference
threshold 90
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Conclusions Future Work

• Intuitions confirmed by statistical data
• Rendering algorithm driven by perceptual
  information

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Conclusions Future Work

• Intuitions confirmed by statistical data
• Rendering algorithm driven by perceptual
  information
• Extend methodology to other phenomena
• Different questions than Does this look
  realistic?
• Better ways to incorporate perceptual information
• currently no significant acceleration yet

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Questions?
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