We present an algorithm for automatically selecting and matching control points for the purpose of registering
images acquired using different imaging modalities. The modulus maxima of the wavelet transform were used to
define a criterion for identifying control points. This criterion is capable of selecting points based on the size of
features in the image. This technique can be tailored, by adjusting the scale of the filters in the modulus calculation,
to the specific objects or structures known to occur in each image being registered. The control-point matching
technique includes an iterative method for reducing the set of control-point pairs using the horizontal and vertical
disparities between the matched pairs of points. Least-squares planes are fit to the horizontal and vertical disparity
data, and control-point pairings are deleted based on their distances from those planes. The remaining points are
used to recompute the planes. The process is iterated until the remaining points fall within a certain distance from
the planes. Finally, a spatial transformation is performed on the template image to bring it into alignment with the
reference image. The result of the control-point pair reduction is a more accurate alignment than what would have
been produced using the initial control-point pairs. These techniques are applicable to medical images, but
examples are given using images of paintings.
As high-resolution images of paintings, acquired using various imaging modalities (e.g. X-ray, luminescence, visible and
infrared reflection) become more available, it is increasingly useful to have accurate registration between them. Accurate
registration allows new information to be compiled from the several multimodal images. This leads to a better
understanding of how the painting was constructed and of any compositional changes that have occurred. To that end,
we have produced an automatic image registration algorithm that is capable of aligning X-ray, color, and infrared
images, as well as multispectral luminescence and reflectance image sets, or cubes. The key steps of the algorithm
include identifying large sets of candidate control points in the reference image, then pairing them with potential points
in a second image using cross-correlation. Finally, after selecting the best set of control point pairs, the second image is
transformed to be in register with the reference image. Tests show the algorithm to be capable of achieving sub-pixel
registration across these various image modalities.
Site specific, in situ techniques such as X-ray fluorescence (XRF) and Raman spectroscopy are commonly used to
identify pigments on illuminated manuscripts. With both techniques, spectra are usually acquired on visually identified
sites thought to be representative of the pigments and mixtures used for the illumination. Such visual inspection may not
always ensure an adequate representation of the pigment diversity. Here we report on the application of multispectral
(MSI) visible/infrared reflectance and luminescence imaging spectroscopy, along with fiber optics reflectance
spectroscopy (FORS) to help determine and map the primary pigments in a late 14th century miniature on vellum,
attributed to Niccolo da Bologna and representing the birth of John the Baptist. XRF analyses of visually selected sites
found elements consistent with azurite, ultramarine, vermillion, lead white, "mosaic gold" and yellow earth pigments.
Visible/infrared FORS analyses confirmed these assignments and showed evidence for the use of organic dyes. The
spectral analysis of the MSI-reflectance images gave distribution maps for these pigments (i.e., regions of azurite,
ultramarine, vermillion) along with some indication of pigment layering not identified visually. The luminescence image
gave a probable map of the organic dye(s). Images acquired in the near- and shortwave-infrared (NIR and SWIR, 750 to
2400 nm) revealed preparatory sketches and illumination techniques. These results show, like those of a prior study
carried out on another 14th century Italian miniature, that the combination of low light multi-spectral imaging
spectroscopy with FORS provides improved in situ mapping and identification of pigments on illuminated manuscripts.
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