NELLIE A BLACK MEMORIAL MEDALLION
3D WHITE LIGHT SCANNING
Belmont Village Lincoln Park, Chicago, IL, USA
Occasionally, Architectural Cast Stone is asked to reproduce existing architectural elements that have been damaged by weather and time. Sometimes we’re merely documenting existing architectural elements prior to a renovation so that they can be reproduced if they become damaged.
Using a portable hand-held white-light 3D scanner, I have literally hung from the sides of buildings and worked on the streets of Chicago, gathering detailed 3D information that enables me to create CAD models of existing infrastructure.
A white-light 3D scanner works by projecting a high-contrast checkerboard pattern onto a surface. Stereoscopic cameras capture the pattern, and software interprets the shape of the object in three dimensions.
Direct sunlight interferes with the process, sometimes catastrophically.
It’s not generally possible to completely avoid direct sunlight, but this medallion had already been removed from the building and placed on a pallet. Before I had it moved into shade, I thoroughly photographed it, to later help verify and refine the eventual computer model.
For accurate restoration work, I generally use the white-light scanner’s highest resolution setting: 0.5mm (0.02”).
At just over eight feet in width, this medallion scan would have quickly exceeded the capabilities of the laptop if I were to attempt to scan the entire object into a single file.
There are two solutions available:
Scan the object in several smaller scans, and later knit the smaller scans together using specialized software (FROGDecimate and FROGFusion). This can be exceedingly time-consuming, and the resulting stitched file may also be too large to efficiently utilize without sacrificing detail.
Scan only the portions of the artifact that may be too difficult to build in CAD (hand-sculpted reliefs, for example), and rebuild more regular geometry in Rhino, based on cross-sections of smaller scans and photographs. An added benefit is that geometric details that may have been lost to the elements can be cleanly restored easily.
LEFT: Randomly placed peel’n’stick positioning targets allow the software to distinguish between similar geometries.
White-light scanning is not a simple process, and it is definitely not fast.
Nooks and crannies are particularly hard to access with the light, and the operator must scan from multiple angles, stopping frequently to allow the software to compile new input, and checking the current model for holes in the data, before trying from yet another angle.
After about three hours of scanning, my poor little laptop was ready to explode. You can see that I very thoroughly covered the hand-sculpted center section, which would be exceedingly time-consuming to model in CAD. As well, I made sure to thoroughly scan a few of the repeating outer segments. Combined with my photographic documentation and physical tape measurements, this was enough to create an accurate model of the entire medallion.
The outlying geometry of the medallion was recreated in Rhino based on profiles taken from the scan and physical measurements.
Remodeling the text ensures that the lettering is crisp and clean.
Through my time at ACS, I have become quite adept at modeling neoclassical serif fonts in 3D.
The scan of the center panel merges seamlessly with the CAD model.
Based on scans, photographs, measurements, and the new medallion file itself, I was able to reproduce the cast stone surround pieces as well.