Executive Summary – 3D Photo Imaging for Condition Detection, Monitoring and Documentation

This project studied whether existing three-dimensional (3D) digital photo imaging technologies can be easily adopted by historic sites and collections for documenting and monitoring the condition of buildings and objects.  It also aimed to determine whether workflows could be established that would result in consistent and scientifically reliable 3D condition data capture.

The Problem: For over a century, conservators have relied on 2-dimensional (2D) photography to document and monitor the condition of heritage sites and objects.  When a sudden and shocking visual indicator appeared (e.g., a sudden crack in a surface), conservators took notice of what had actually been thousands of slow, tiny, incremental condition changes (e.g., tiny fissures and failures in slowly aging materials).  Conservators then photographed and monitored the area to try to determine how rapidly the deterioration was advancing.  Two factors limited our ability to detect and monitor deterioration: our ability detect slow and small “micro” changes, and our ability to compare what humans see using 3D stereo vision and what we recorded in 2D photographs.

Digital 3D images or surrogates allow us to see and measure 3D volumes and contours and they allow us to use a computer to detect and highlight very small and slowly occurring changes.  Detailed 3D models can be made using laser scanning and 2D digital photography (Photogrammetry, Reflectance Transformation Imaging –RTI- and Structured light). Each technology has condition-imaging strengths and weaknesses. In each, the finer the resolution – that is, the smaller the area captured by either a laser point reflection or a digital camera sensor pixel, the larger the data set for an entire object will be. In practice, the larger the object you want to image – an entire historic house or landscape versus a single door or wall, for example – the lower your resolution or detail will need to be or the more expensive and customized your assembly computer will have to be. Laser scanning equipment uses proprietary laser projectors, collection cameras, software and digital file formats. The files are huge – tens to hundreds of gigabytes each, and require very specialized computing hardware and software packages.

The Technological Solution: This study found that two 3D digital photographic technologies –Highlight RTI and Photogrammetry – promise to change the condition-detecting and monitoring paradigm in important and beneficial ways:

  • 3D photo images allow us to record and study surfaces and volumes in ways that mimic the stereo view of a real-time human examiner.
  • Because specular, color and texture data is digital, a viewer can selectively highlight and study details by removing the color data, changing the virtual light source and reflectivity or colorize contour levels.
  • Photography can be done by anyone with a basic knowledge of digital camera operation and a lap-top computer. Laser scanning engineers are not required.
  • Photography uses an ordinary, consumer-professional grade, digital camera. Expensive and complex laser scanning equipment is not required.
  • Capture photographs are the same, open-format 2D photos you have always taken –RAW, TIFF and JPEG. The capture data is not a proprietary laser-scan file format that requires special, brand-name software to use and view.
  • The user owns their own images and controls their naming, archiving, meta-data and digital management protocols.
  • Assembled 3D, computational meshes, solids, and RTI images and their viewers are open-source software – there are no hidden steps and every transformation of the data is documented and visible.
  • Because the capture photos are regular, open-format digital images, 3D images can be assembled with whatever future enhanced versions of 3D assembly software happen to come along. You are not dependent upon the life of one assembly software or manufacturer.
  • Capture photography can use the best digital camera the heritage site can afford. While 21 megapixel, full frame CMOS sensors with prime lenses and RAW capture capability give the highest resolution, less expensive APS-C sensor cameras that have manual capture modes (allowing the user to fix the lens aperture and focus) give reliable 3D condition documentation.

We created an interactive, web-platform blog site that documented our evolving working methodologies, equipment and resulting images. We specifically targeted the historic preservation field and publicized the project blog, videos and workflows using historic preservation groups on Linked-In, Facebook and Twitter social media outlets. While we anticipated somewhere near 3000 views of the website content during the course of the project, we were astonished by how rapidly an international audience, heritage preservation audience for the project was developed:

9000 views were from the USA and Caribbean.

4000 views were from Europe

700 views were from Central and South America.

500 views were from Asia (China does not report by Chinese government regulation)

400 views from Australia and New Zealand

325 views were from Canada

200 views were from the Middle East

70 views were from India

35 views were from Africa

15,230 views, total

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Does RTI give repeatable and reliable normals of objects taken at different times and positions to facilitate detection of changes?

On the Linked-In discussion group Cultural Heritage Conservation Science. Research and practice’s discussion on 3-D digital imaging and photogrammetry for scientific documentation of heritage sites and collections http://linkd.in/RZMpFj , Greg Bearman wrote the following question:

“Does RTI give repeatable and quantitative set of normals good enough for looking for change? If I take an RTI set, rotate the object, let it warp a bit (flexible substrate), what do I get the second time? How do I align the datasets for comparison?

what is the system uncertainty? ie if I just take repeated images of the same object without moving anything, how well does the RTI data line up. Second, suppose I take something with some topography but is totally inflexible and cannot distort(make up test object here!) and I do repeated RTI on it in different orientations? Can I make the data all the same? If you are going to use an imaging method to determine changes in an object, the first thing to do is understand what is in inherent noise and uncertainty in the measuring system. It could be some combination of software, camera or inherent issues with the method itself”

I wrote back: “Hey Greg – tried sending response earlier last week but I do not see it!? Sorry. I’m on vacation until the 22nd – trying to recover and recharge. It is going well but I wanted to jot down my initial thoughts. One of my interns – Greg Williamson – is working on aberration recognition software that can recognize and highlight changes in condition captured by different H-RTI computational image assemblies – obviously taken at different times, but also with different equipment and with randomly different highlight flash positions. It seems, initally, that normal reflection is normal reflection, regardless of object or flash position and that the software correctly interpolates 3D positions of surface characteristics regardless of the precise position of the flash, because it is accustomed to calculating the highlights both the capture points and everywhere in between! Likewise, we have had promise with photogrammetry when the resolution of the images used to create the mesh and solids are similar. What may turn out to be key is a calibration set that will allow correction of the various lens distortions that would naturally come from different lenses. I know Mark Mudge at Cultural Heritage Imaging has suggested that we begin taking a calibration set before RTI capture, as we had before Photogrammetry. He may be working on incorporating a calibration correction into the highlight RTI Builder that CHI has made available. I’m sending this discussion along to the CHI forum at http://forums.cultur…ageimaging.org/ to see what others might have to add. When I return to work, I’ll ask Greg to give this some additional thought”

Whadaya think, Greg?