towards automatic non-metric traits analysis on 3d models

Towards automatic non-metric traits analysis on 3D

models of skulls

Paulo Dias, Bruno Andrade, Catarina Coelho, João Coelho, David Navega, Maria Teresa Ferreira, Sofia Wasterlain, Beatriz Sousa Santos

University of Aveiro, University of Coimbra

IV'2018

Craniometry

• Anthropology methodology useful in Archeology and Forensic

sciences to identify:

– Ancestry

– Sex

– Variations in populations, …

• Based on interest points and measures

• Disadvantages of the traditional approach:

– Low repeatability (intra- and inter-observer)

– Specimen wear (due to contact)

– Impossible to analyse fragments

2

CraMs–3D model-based Craniometric Measurements

• New approach

• “Computer-assisted Craniometry” • Interdisciplinary work

• Participatory design

• Overcoming some limitations of the

traditional approach

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The CraMs approach

• Laser scanning specimens, 3D models, model alignment, interest points and measures

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Specimens 3D models Aligned models Interest 3D points and measures

• User-validated

• Semi-automated

Ancestry estimation beyond metric methods

• To determine the ancestry metric and morphological methods may be used

• Many ancestry methods are based on a qualitative analysis of the skull characteristics

• Difficult and time consuming to analyse and describe shapes objectively

• Subjective approach and much influenced by the experience of the analyst

5

Extending the CraMs Approach

• Scanning, 3D models, interest points and measures, structures, estimation of ancestry

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Acquiring specimen 3D aligned model Interest 3D points Interest 3D structures

Extracted 2D structures Compared to template Ancestry estimation

Hefner´s method (2009) for Ancestry classification

• Anterior Nasal Spine (ANS)

• Inferior Nasal Aperture (INA)

• Interorbital Breadth (IOB)

• Malar Tubercle (MT)

• Nasal Aperture Width (NAW)

• Nasal Bone Contour (NBC)

• Nasal growth (NO)

• Post-Bregmatic Depression (PBD)

• Supranasal Suture (SPS)

• Transversal Palatine Suture (TPS)

• Zigomaxilar Suture (ZS)

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Groups of populations: Africans, Asians, Europeans and Native-Americans

Eleven morphological characteristics to estimate ancestry:

1. Nasal Aperture Width (NAW)

• Automatic extraction of interest points, contour and width

• Using a priori information provided by Anthropologists

• Comparison with Hefner’s templates for classification

• Automatic classification in 3 types

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1.1 Finding reference points

• Reference points:

– rhinion (rhi);

– nasospinale (ns)

• Analysing the skull curvature on a 2D section by the sagittal (XZ) plane

• Using a priori information concerning skull anatomy

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Sagittal plane

1.2 Detecting other points along the nasal aperture

• Angular search

• Finding points on the surface with the highest zz coordinate along each plane

• Other rules to solve specific anatomic cases

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1.3 Closing the nasal aperture

• Number of seed points: empirically obtained

• Curvature analysis of the neighbourhood of seed points

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1.4 Classifying the Nasal Aperture Width

• Project the 3D curve on a coronal plane

• Align and resize to compare with the Hefner’s contours

• Estimate the most similar Hefner contour

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Coronal plane

Sagittal plane

2. Anterior Nasal Spine (ANS)

• Automatic extraction of structure


• Comparison with Hefner’s classification

• Automatic classification in 3 types (absent, medium, long)

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• Analysing the nasospinale neighbourhood on a section by the sagittal (XZ) plane


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Sagittal plane

3. Post-Bregmatic Depression (PBD)

• Automatic extraction of the structure


• Comparison with Hefner’s classification

• Automatic classification in 2 types (absent or present)

15

16

A

B

A

B

• Analysing the section on the sagittal (XZ) plane between points A and B


B A

Evaluation

• Two anthropologists classified 51 specimens

• Regarding each characteristic (NAW, NAS, PBD)

(they disagreed on one situation)

• Two Collections:

– 23 specimens • Portugal, XV to XVII centuries

• Age >18

– 28 specimens • Portugal, XX century

• Age 29 to 99 17

Results and Discussion

• NAW - Nasal Aperture Width – 85% success rate

– 15% incorrect classifications (type 1 identified as type 2)

– 9% rhinion and 4% nasospinale manually marked;

• NAS - Nasal Anterior Spine – 93% success

– 7% incorrect classifications (type 3 identified as type 2)

– Empirically established parameters;

• PBD - Post-Bregmatic Depression – 100 % success;

– Few specimens had this characteristic present.

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“Ancestry Estimator” in CraMs

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• Provides a likelihood of a specimen belonging to each type

• Using the classification of several features automatically or manually obtained

Conclusion

• Automatic detection of 3 morphological characteristics important for

ancestry estimation

• Potentially making the process easier and more repeatable

• Evaluation with 51 specimens suggest similar results to traditional methods

• Possible limitations concerning specimens of different population groups

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Future Work

• Further validation with more specimens (of different population groups)

• More characteristics for ancestry estimation

• Sex classification

• “3D puzzle” with existing fragments allowing the analysis of severely damaged skulls

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towards automatic non-metric traits analysis on 3d models

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