The Digital Humanities Lab (DHLab) is a research group within the faculty of Humanities of the University of Basel. The group was founded in the 1920 as scientific photography lab. In the project “Digital Materiality”, we applied and enhanced Reflection Transformation Imaging (RTI) enabling this technique to capture artwork that is characterized by surfaces that strongly interact with light. Standard photographic approaches are unable to capture the dynamic component of the light-surface interdependence specific for these kinds of objects. RTI is a promising approach to go beyond the limitations of conventional photography. However, RTI has some drawbacks that are critical in the context of the reproduction of surfaces composed of materials of different kind. Typical RTI images reduce any mixture of materials with an average gloss for the whole surface. Our improvements of the technology take into account more sophisticated but still robust and simple reflection models for the realistic visualization of localized diffuse and specular surfaces within the same digital reproduction. The quality of a RTI rendering can be judged by various parameters. The most important is certainly the photorealism of a specific viewing situation. In photography one of the most important aspects is the visual appearance of color. Proper color transformation and reproduction is in a state-of-the-art digital photography workflow. However, when RTI was first developed color management was not a common process in photography workflows. Therefore, in typical implementations of RTI, color is handled as RGB data without any further explanation about what color primaries are chosen to span a color space. We will show what workflow can be used to optimize color representation in RTIs to enhance photorealism.

1. Enhanced Reflection Transformation Imaging (RTI)
for
Managed Color and Gloss Reproduction
Color Management for Cultural Heritage Objects using RTI
2D and 3D Photography
Rijksmuseum, Amsterdam
Dr. Peter Fornaro
Digital Humanities Lab, University of Basel

4. RTI
• Reflectance Transformation Imaging
• RTI is a set of computational photographic methods that
captures a subject’s surface shape and color and enables
interactive re-lighting.
• RTI permits the virtual enhancement of the subject’s surface
shape and color attributes.
• A series of images is necessary, each illuminated differently

5. Apparatus
• Light sources positioned in 30-60 points on a sphere
• Camera in a fixed position
• Data acquisition system for taking 30-60 photos in
controlled and calibrated conditions
• Software to analyse the photos (Fitter)

7. Reflectance
Transformation
Imaging (RTI)
11
33
44
55
66
22
11
33
44
55
66
22
Position of Light-source
Intensity

8. Light directionIntensityofreflectedlight
PTM fit

9. Regions defined by threshold on measured quantities
Gloss index

10. Standard PTM Gloss model 1 Gloss model 2

11. Color of Original Color of Reproduction

12. T

13. RAW
RTI
Processing
RTI
Processing
Capture RAW Converter Fitting of Modell Visualisation
Adobe RGB Adobe RGB
Adobe RGB
on appropriate
output device:
Wide Gamut Display P3
Optimal Workflow

14. Color-Primaries:
Definition of “physical meaning“ of
R, G, B values, e.g. sRGB
CIE 1931 Chromaticity Diagram
based on
Standard Observer Experiment

15. RAW
RTI
Processing
RTI
Processing
Capture RAW Converter Fitting of Modell
Visualisation
WebGL Canvas
in Browser
Adobe RGB RGB? sRGB
Typical Workflow

16. Color-Primaries:
Definition of “physical meaning“ of
R, G, B values, e.g. Adobe RGB or P3
Wrong Color Reproduction
The RGB values are misinterpreted.

18. Adobe RGB with correct interpretationAdobe RGB with correct interpretation
Adobe RGB with wrong interpretation,
in this case as sRGB
Adobe RGB with wrong interpretation,
in this case as sRGB

19. Correct Color Reproduction in
WebGL
• WebKit (macOS, iOS) offers proper color
renderings of images on Wide Gamut screens like
P3.
• WebGL is running in Canvas Elements in HTML.
Within canvas only sRGB is supported.
• Take care on proper profile conversion. All
images are interpreted as sRGB.
• Take care on proper camera
input profile.
• Even if no profile information is found, sRGB is
assumed.

20. Thank you
peter.fornaro@unibas.ch