Methods for creating digital 3D models
Currently, creating 3D models of objects mainly involves two methods: photogrammetry and laser scanning. The methods can be used separately or in combination. The methods can produce similar results, but have their respective pros and cons, depending on the type of object to be documented, the budget, the purpose, and the use of the final models.
The method of using two or more photos to create 3D images is almost as old as photography itself, and has been used since the end of the 1830s.
When working with digital photogrammetry today, you take a number of photos of the object from various angles, with overlaps which mean that the entire surface of the object is covered. These photos are then processed by software which identifies shared points in the photos. Using these points as references, the software then creates a 3D model.
The end results is a photorealistic model which can often give a more detailed view of the object from all angles than what you can achieve through the display glass in an exhibition. However, the technique does not work on glass objects and objects with very reflective surfaces (unless they are sprayed to create a matte coat), as these objects lack the reference points necessary for the software to create 3D models.
Laser scanning uses a laser beam to measure the distance between the measuring instrument and the object. It is a very precise process, even across large areas where the properties of the camera lens make it hard to use photogrammetry, but the equipment is extremely expensive. The visual representation of heavily textured surfaces such as textiles is less realistic and visually appealing than when using photogrammetry.
Choosing a technique – photogrammetry or laser scanning?
Laser scanning is best suited for work which requires a high degree of precision across large surfaces. Photogrammetry is preferable when looking to create photorealistic models, where precision across large surfaces is of no practical importance.
Photogrammetry is not just a useful technique for photographing objects, but is also well-suited for documenting buildings and smaller archaeological sites. A common approach to using photogrammetry on larger outdoor objects is to use a camera attached to a drone.
Laser scanning – benefits
• High precision even when dealing with large surfaces (such as carved rock surfaces).
• Works independent of external light sources.
• Automated process where margins of error are completely down to the capacity of the equipment. This reduces the risk of user errors.
• Quicker than photogrammetry.
Laser scanning – drawbacks
• Expensive equipment which quickly becomes obsolete.
• Unable to accurately reproduce textured surfaces.
• Photogrammetry – benefits
• Cheap equipment – you can quickly get started using the equipment normally available in a museum in the form of a system camera, tripod, and lamps.
• Upgrades are mainly done in the form of new software, meaning that unlike with laser scanning, there is no need for regular expensive hardware upgrades.
• Photorealistic and more visually appealing reproduction of surfaces and structures.
• Photogrammetry – drawbacks
• Less precise than laser scanning, mainly across large surfaces.
• A largely manual process, where a person’s knowledge and experiences are crucial for the end result.
• Time consuming, both in terms of the time needed to take hundreds of photos of each object, and in terms of the time needed to render the 3D models.
From photos to a completed 3D model
There are several software alternatives for creating a 3D model from photos using photogrammetry. These videos give an overview of how to use two common pieces of software: Agisoft Photoscan and Reality Capture: