How Well Do Line Drawings Depict Shape? Forrester Cole Kevin Sanik Doug DeCarlo Adam Finkelstein Thomas Funkhouser Szymon Rusinkiewicz Manish Singh RutgersPrinceton

Line drawings [Matisse 1932] [Flaxman 1805] [US Patent 378,973]

Line drawings Occluding ContoursSharp creases

Line drawings Ridges and ValleysSuggestive Contours [DeCarlo et al 2003] Apparent Ridges [Judd et al 2007] Occluding ContoursSharp 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 ContoursApparent Ridges

Comparing models Ridges and ValleysSuggestive ContoursApparent Ridges

Comparing models Ridges and ValleysSuggestive ContoursApparent 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 ridgessuggestive contours …

Comparing models to artists Comparisons to drawings made under controlled conditions [Cole et al. 2008] artist drawingapparent ridgessuggestive contours … d line (, ) rendering artist drawing

rendering Comparing shapes 3D d 3D (, ) perceived shapeoriginal 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 stylesx 12 models- 2 duplicates= 70 prompts Contours

Shaded R. and V. Sug. C. App. R. Artist’s Study Setup 6 stylesx 12 models- 2 duplicates= 70 prompts Contours

Study Setup 70 x 90 gauges / prompt x 2 settings / opinion prompts ≈ 100,000 settings x 8 opinions / gauge

Study Setup x 4 seconds / setting 70 x 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 different people Assignments Completed # 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 Avg. Error (degrees)

Accuracy by Model Avg. Error (degrees)

Gauge Visualization: Screwdriver Contours Only Artist’s Drawing 090Error (deg.) 180 gauges

Local Errors: Screwdriver Contours OnlyArtist’s Drawing 15 gauges, 5 pixel spacing 090Error (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 090Error (deg.)

Local Errors: Flange Suggestive ContoursRidges and Valleys 15 gauges, 5 pixel spacing 090Error (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 090Error (deg.)

Non-Local Effects: Rockerarm -9090Error Difference (deg) Worse than RV Better than RV Apparent 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:

Thank You Thanks to Andrew Van Sant and John Wilder Support by NSF grants CCF , CCF , CCF , CCF , IIS , and IIS , and Google Models from VAKHUN, and Cyberware Data available at:

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 =?

Gauge Visualization: Flange, #2 Artist’s Drawing 180 gauges Ridges and Valleys 090Error (deg.)

Extra Line: Flange -9090Error Difference (deg) Worse than RV Better than RV Artist’s Drawing

Non-Local Effects: Flange -9090Error Difference (deg) Worse than RV Better than RV Apparent Ridges