How Well Do Line Drawings Depict Shape?

Forrester ColeKevin Sanik

Doug DeCarloAdam FinkelsteinThomas FunkhouserSzymon Rusinkiewicz

Manish Singh

RutgersPrinceton

Line drawings

[Matisse 1932]

[Flaxman 1805]

[US Patent 378,973]

Line drawings

Occluding Contours Sharp creases

Line drawings

Ridges and Valleys Suggestive Contours[DeCarlo et al 2003]

Apparent Ridges[Judd et al 2007]

Occluding Contours Sharp creases

Assessing Line Drawings

• Goals– Artistry, abstraction, etc.– Leads to accurate perception of shape

Assessing Line Drawings

• Goals– Artistry, abstraction, etc.– Leads to accurate perception of shape

• Methodology– Qualitative (examples, comparison to artists)– Quantitative comparison to artists’ drawings– Direct measurement of perceived shape

Comparing models

Ridges and ValleysSuggestive Contours Apparent Ridges

Comparing models

Ridges and ValleysSuggestive Contours Apparent Ridges

Comparing models

Ridges and ValleysSuggestive Contours Apparent Ridges

Comparing models to artists

© Estate of Roy Lichtenstein Suggestive contours and

suggestive highlights [DeCarlo and Rusinkiewicz 2007]

“Golf Ball” [Lichtenstein 1962]

Comparing models to artists

• Argument for ridge-like features [Judd et al. 2007]

[Brancusi 1910][Matisse 1932]

Comparing models to artists

• Comparisons to drawings made under controlled conditions [Cole et al. 2008]

artist drawingapparent ridges suggestive contours

…

Comparing models to artists

• Comparisons to drawings made under controlled conditions [Cole et al. 2008]

artist drawingapparent ridges suggestive contours

…

dline( , ) rendering artist drawing

rendering

Comparing shapes

3D

d3D( , )perceived shape original shape

Measuring perceived shape

Local measurements of shape geometry• Gauge figure adjustment

[Koenderink et al. 1992]

Measuring perceived shape

Local measurements of shape geometry• Gauge figure adjustment

[Koenderink et al. 1992]• Studied shaded surfaces

and one artist line drawing[Koenderink et al. 1996]

Questions

Do artist and CG drawings effectively convey shape?– how accurate are they?– how do they compare to a shaded rendering?

Do different viewers perceive the same shape?When are particular line types most effective?

Study Methodology

1. Measure percepts– Both artist and CG drawings– Range of models– Many participants

2. Compare against ground truth– 3D shape and shaded image– Accuracy and precision

Orienting a Gauge

Example Session

Study Setup

All 12 models from [Cole et al. 2008]

Shaded

R. and V.

Sug. C.

App. R.

Artist’s

Study Setup

6 styles x 12 models - 2 duplicates = 70 prompts

Contours

Shaded

R. and V.

Sug. C.

App. R.

Artist’s

Study Setup

6 styles x 12 models - 2 duplicates = 70 prompts

Contours

Study Setup

70x 90 gauges / prompt

x 2 settings / opinion

prompts

≈ 100,000 settings

x 8 opinions / gauge

Study Setup

x 4 seconds / setting

111 hours

70x 90 gauges / prompt

x 2 settings / opinion

prompts

≈ 100,000 settings

x 8 opinions / gauge

So Much Data…

• Amazon Mechanical Turk to the rescue!• Turker sets 60 gauges, gets paid $0.20• Efficient even after throwing away garbage– “Garbage” is inconsistent data– About 80% of data is consistent

Data Collection

• 275,000 gauge settings• 4 models x 180 gauges + 8 models x 90 gauges• Each gauge 9 to 29 opinions, average 15• 560 different people

1 53 1051572092613133654174695215730

102030405060

AssignmentsCompleted

# Participants

Global Accuracy

Error from ground (accuracy)

Global Accuracy

Error from ground (accuracy) Distribution of errors for shaded

Finding:

On average, turkers did a good job

Aggregating Per-Gauge Data

What is the most representative direction?– “Mean” is most obvious choice– “Median” more robust to outliers

mean median

Global Accuracy and Precision

Error from Ground (Accuracy) Error from Median (Precision)

Results:

• Precision greater than accuracy• Accuracy varies with style, precision does not

Finding:

Peoples’ interpretations of shape are similar, even when those interpretations do not match

ground truth.

Question:

Where are the errors?

Accuracy by Model

Vertebra

Cervical

Tooth

Bumps

Lumpcloth

Pulley

Femur

Screwdriver

Twoboxcloth

Flange

Rockerarm

Cubehole

0 10 20 30 40

Shaded Best Drawing

Avg. Error (degrees)

Accuracy by Model

Vertebra

Cervical

Tooth

Bumps

Lumpcloth

Pulley

Femur

Screwdriver

Twoboxcloth

Flange

Rockerarm

Cubehole

0 10 20 30 40

Shaded Best Drawing

Avg. Error (degrees)

Gauge Visualization: Screwdriver

Contours Only

Artist’s Drawing

0 90Error (deg.)180 gauges

Local Errors: Screwdriver

Contours OnlyArtist’s Drawing

15 gauges, 5 pixel spacing 0 90Error (deg.)

Curvature: Screwdriver

Contours Only

Artist’s Drawing

Contours Only

Artist’s Drawing

Ground Truth

Zero Curvature

Gauge Visualization: Flange

Suggestive Contours

180 gauges

Ridges and Valleys

0 90Error (deg.)

Local Errors: Flange

Suggestive ContoursRidges and Valleys

15 gauges, 5 pixel spacing 0 90Error (deg.)

Curvature: Flange

Suggestive Contours

Ridges and Valleys

Ground Truth

R. and V.

Sug. Contours

Gauge Visualization: Rockerarm

Apparent Ridges

90 gauges

Ridges and Valleys

0 90Error (deg.)

Non-Local Effects: Rockerarm

-90 90Error Difference (deg)

Worse than RVBetter than RVApparent Ridges

Conclusions

• Different people interpret drawings similarly• Some drawings almost match shaded images• Line drawings vary in effectiveness– Errors can be traced to specific lines

Future Work

• More analysis of collected data– Towards interpretation model for lines

• Further investigation of study methodology

Data available at:http://lineshape.cs.princeton.edu

Thank You• Thanks to Andrew Van Sant and John Wilder• Support by NSF grants CCF-0347427, CCF-0541185,

CCF- 0702672, CCF-0702580, IIS-0511965, and IIS-0612231, and Google

• Models from Aim@Shape, VAKHUN, and Cyberware

Data available at:http://lineshape.cs.princeton.edu

Global Accuracy and Precision

Before bas-relief fitting

Error from Ground (Accuracy) Error from Median (Precision)

Global Accuracy and Precision

After bas-relief fitting

Error from Ground (Accuracy) Error from Median (Precision)

Bas-Relief Ambiguity

Ambiguity in perception of shaded shapes [Koenderink 2001]

= ?

Line Drawing Ambiguity

Line drawings are even less constrained

= ?