Clemens Weijkamp’s scientific contributions

High fidelity reproductions of paintings provide new opportunities to museums in preserving and providing access to cultural heritage. This article presents an integrated system that is able to capture and fabricate color, topography and gloss of a painting, of which gloss capturing forms the most important contribution. A 3D imaging system, utilizing stereo imaging combined with fringe projection, is extended to capture spatially varying gloss, based on the effect of specular reflectance polarization. The gloss is measured by sampling the specular reflection around Brewster’s angle, where these reflections are effectively polarized and can be separated from the unpolarized, diffuse reflectance. Off-center gloss measurements are calibrated relative to the center measurement. Off-specular gloss measurements, following from local variation of the surface normal, are masked based on the height map and corrected. Shadowed regions, caused by the 3D relief, are treated similarly. The area of a single capture is approximately 180 × 90 mm at a resolution of 25 × 25 µm. Aligned color, height, and gloss tiles are stitched together off-line, registering overlapping color regions. The resulting color, height, and gloss maps are inputs for the poly-jet 3D printer. Two paintings were reproduced to verify the effectiveness and efficiency of the proposed system. One painting was scanned four times, consecutively rotated by 90 degrees, to evaluate the influence of the scanning system geometric configuration on the gloss measurement. Experimental results show that the method is sufficiently fast for practical application, i.e., to scan a whole painting within eight hours, during closing hours of a museum. The results can well be used for the purpose of physical reproduction and other applications needing first-order estimates of the appearance (e.g., conservation diagnostics and condition reports). Our method to extend appearance scanning with gloss measurements is a valuable addition in the quest for realistic reproductions, in terms of its practical applicability—number of images needed for reconstruction and speed—and its perceptual added value, when added to color and topography reproduction.

High fidelity reproductions of paintings provide new opportunities to museums in preserving and providing access to cultural heritage. This paper presents an integrated system which is able to capture and fabricate color, topography and gloss of a painting, of which gloss capturing forms the most important contribution. A 3D imaging system, using fringe-encoded stereo imaging, is extended to capture spatially-varying gloss, utilizing specular reflectance polarization. The gloss is measured by sampling the specular reflection around Brewster's angle, where these reflections are effectively polarized, and can be separated from the unpolarized, diffuse reflectance. Off-center gloss measurements are calibrated relative to the center measurement. Off-specular gloss measurements, following from local variation of the surface normal, are masked based on the height map and corrected. Shadowed regions, caused by the 3D relief, are treated similarly. The area of a single capture is approximately 180x90mm at a resolution of 25x25micron. Aligned color, height, and gloss tiles are stitched together, registering overlapping color areas. These maps are inputs for a 3D printer. Two paintings were reproduced to verify the effectiveness and efficiency of the proposed system. One painting was scanned four times, consecutively rotated by 90 degrees, to evaluate the influence of the scanning system geometric configuration on the gloss measurement. Experimental results show that the method is sufficiently fast for practical application. The results can well be used for the purpose of physical reproduction and other applications needing first-order estimates of the appearance. Our method to extend appearance scanning with gloss measurements is a valuable addition in the quest for realistic reproductions, in terms of its practical applicability and its perceptual added value, when added to color and topography.

High fidelity reproductions of paintings provide new opportunities to museums in preserving and providing access to cultural heritage. This paper presents an integrated system which is able to capture and fabricate color, relief and gloss of a painting's surface, emphasizing on gloss capturing. To measure the spatially varying gloss, the specular reflection of the center of the scanned area is sampled at the Brewster angle, utilizing the effect of reflectance polarization at this angle. The off-center gloss measurements are corrected using the perpendicular and parallel reflectance coefficients, relative to the center measurement. Shadows in the gloss map, which are caused by 3D relief of the surface, are masked based on the height map and then filled by interpolating surrounding gloss information. The captured color image, height map and gloss map are inputs for the 3D printer. A painting "Two wrestling figures in the style of Van Gogh" was reproduced to verify the effectiveness and efficiency of the proposed system. Experiment results indicate that the proposed system gives accurate enough gloss measurement of the painting's surface for the purpose of gloss fabrication.

In 2015 the Mauritshuis (The Netherlands) hosted a special exhibition titled Rembrandt? The Case of Saul and David, which presented the results of eight years of research into a single painting Saul and David - a painting with a complex history - since then reattributed to Rembrandt himself. In the 19th century the painting was cut into pieces and later reassembled. Using advanced 3D scanning and 3D printing technologies a 3D printed reconstruction was made of the painting showing its original format. During the restoration process - after the varnish and previous retouches were removed - the colour and surface texture (relief) of the painting was scanned, using a high resolution 3D scanner. These scans were digitally restored, in conjunction with the restoration of the painting itself. Finally the restored pieces of the painting were printed in their assumed original configuration. Saul and David and 3D print were on shown together at the exhibition. The 3D reconstruction of Saul and David was evaluated by fine art experts, on its material appearance, and the role of 3D reproductions in museums. This paper presents the digital reconstruction process as well as the evaluation of the reproduction by experts.

Capturing and printing spatially varying gloss of a painting.