AIC's 41st Annual Meeting

Imaging artwork for documentation and reproduction has a long and rich history, resulting in images with both high resolution and quality for a specific viewing experience. When multiple experiences are desired, e.g., normal viewing and raking, separate images are captured.

Alternatively, computer graphics (CG) can be used to create a virtual experience where lighting and viewing are changed interactively, enabling multiple experiences from a single dataset, obtained by capturing multiple images using dozens of lighting directions, e.g., a dome or portable flash. The focus of such research is on the interactive experience as well as forensics where details might be obscured using conventional imaging.

Beginning in 2006 research was initiated to merge these two approaches, that is, to measure the physical properties of the artwork with high resolution and quality such that CG could be used to both interact with the object and render multiple experiences at high resolution and quality without re-shooting. A concomitant goal was for the image capture to be readily implemented in a museum imaging studio using existing equipment.

For paintings and drawings (assumed to be planar), the minimum physical properties for CG are color, surface macrostructure (depth or surface normal), and surface microstructure (gloss), all a function of xy position, that is, the artwork’s total appearance. If we further assume that the object’s gloss is consistent (e.g., varnished painting or matte drawing), a studio setup can be used to measure color and surface normal and the gloss defined by the user, either visually or from an artist material database.

Two digital cameras have been tested: a Canon Mark II with 85mm lens and a Sinar 86H rePro system with Sinaron 100mm lens, each affixed with a glass linear polarizer. Four Broncolor Pulso G 1600 J strobes with P70 reflectors, barn doors, and affixed triacetate film linear polarizers were used as light sources with each light positioned symmetrically 90° apart within an annulus and 45° from the object plane. Thus conventional 45° illumination from two strobes is augmented by two additional strobes, one above and one below. Calibration requires imaging a glossy black ball to define lighting geometries, setting cross polarization, imaging a diffuse white board, and imaging a color target. Automated software outputs sRGB encoded diffuse color and surface normal floating-point images (PFM). For artwork, 6 images are collected: each cross-polarized strobe (4) and the left and right strobe with parallel polarization (2).

Software was written, “Artviewer,” to render images interactively for specific lighting conditions, either a point source or museum lighting. Once a view is defined, full-resolution 16-bit Tiff files can be created. “Isee” was also written to view the floating-point images, similar to HDR Shop.

The system has been tested in the conservation studio at the Museum of Modern Art for paintings of Pollack, Van Gogh, and Magritte. One of the advantages of this system is that it can be readily implemented in a museum imaging studio, providing both conventional high-resolution images (via parallel polarization) and total-appearance image data.