Image mosaicking with the Phase OneWritten by Paul Bourke
The following documents some experiments aimed at evaluating the high fidelity capture of artworks using, at the time, the newly released Phase One camera. The Phase One is the most recent medium format camera from Sony with a 100MPixel back. There are a number of reasons why the Phase One was chosen for these tests, these include the high resolution sensor so fewer images need to be acquired for the same final stitched resolution, high dynamic range, availability of a high quality macro lens (in this case a 120mm macro lens). There are some other unique characteristics of the camera such as the seismometer to ensure photographs are only taken when the camera is steady.
Two artworks works were chosen, the first is a relatively flat painting (700mm across) but one with strong repeating patterns. This was chosen to test for any stitching issues. The aim in this case was to achieve a scan resolution of 1200DPI (approximately 50dots per mm). The final mosaic consisted of 4 photographs wide and 3 photographs high, each photograph from the Phase One is 11600x8700 pixels and an approximate 30% overlap was aimed for.
Global and zoomed in detail of dot painting
The second example is a smaller piece, 300mm on each side, but it consisted of some degree of roughness in the form of up to 5mm raised oil paint strokes. The aim in this example was to scan at 2400DPI (approximately 100 dots per mm), 15 photographs in a 3x5 grid were acquired.
Phase One and two area lights (turned off here)
A discussion of the lighting is out of the scope of this document and indeed was not optimised for the tests conducted here.
Global photograph and zoomed in view of second example
There are a number of ways to deal with the narrow depth of focus of these macro lenses, noting that these photographs were already taken at f16 which is considered the limit for the lens in question before one strikes diffraction issues. One approach is focus stacking, for notes on evaluating this with the same camera see here.
As the surface depth increases a tiled mosaic style capture becomes a less complete means for capturing the artwork. One is essentially recording a planar projection of a 2.5D surface. Not only does increased depth have implications for focus but also on lighting. One approach to address this is to capture the geometry of the surface through photogrammetric methods. The following is a surface representation acquired using the same camera, during the same shooting process. 15 photographs were taken, all from different positions and angles to the artwork.
Surface representation with side lighting to illustrate the surface features
The pipeline is to mosaic the high resolution image with highly diffuse lighting. This high resolution image is then co-registered with the surface model from the 3D reconstruction. Lighting of the artwork by different lighting scenarios can then be simulated by standard computer graphics lighting models and rendering methods.
An outstanding missing acquisition are material properties, for example, diffuse and reflective components that in general are also angle dependent. Capturing BDRF (Bidirectional reflectance distribution functions) at the scale of a large art work is highly problematic.