As part of the first stage for the practical assignment in 3D recording we were given the task of capturing ancient Irish pottery bowls, dated as far back as 2500BC <http://www.museum.ie/en/collection/bronze-age.aspx>. These Bronze Age vessels were found in burial sites, or cists, and it is believed that they contained food as an offering to the dead for their journey to the afterlife.
In the process of photogrammetry, it is necessary to capture all sides of an object, in 360 degrees. Particular challenges involved in capturing these ceramic vessels was getting images of the inside or interior of the object, as well as the base, and making sure that the lighting was correct so to capture as accurately as possible for capturing the decorations and patterned incisions made around the exterior of the objects.
As aforementioned, it is important to capture the object in 360 degrees, on all sides, so that the software program can then extrapolate from the two-dimensional images a model in three dimensions. Since it is best to keep camera movement to a minimum, it is a good idea when working with smaller objects to rotate the object rather than to move around it with the camera. Rotating the object is relatively easy if placed on a turntable. In keeping the camera in the same position for a series of captures this would hopefully obviate the possibility of blurred images. Images that are blurred may not be of a quality that is good enough for the image processing stage in PhotoScan.
Lighting is yet another issue to think about in photogrammetry. Harsh lighting and the casting of shadows is best to be avoided, and so it is worth your while to light your object equally from all sides <https://dinosaurpalaeo.wordpress.com/2013/12/20/photogrammetry-tutorial-3-turntables/>. A well-lit object means surface detail is not distorted by shadow, and so the intricately incised patterns and shapes on the surface of the ceramic vessels, in this instance, would be clear and undistorted once the three-dimensional models are processed. There is also a very handy method to diffuse the light hitting your object: namely, the lightbox. The lightbox is essentially a white, thin box made of canvas material, with an opening on one side through which you can place your object inside to record. The object can be placed in the centre of the lightbox on the turntable. It is best to place two lights at an equal distance from the box on either side, with a third light source placed directly above, pointing directly down on the box. This arrangement of lights ensures that there is no area of the object is more lit up than any other areas, and so there are stark shadows distorting the surface detail.
We arrived at the museum early in the morning as five different objects were to be recorded, which we knew could take up to at least four hours between all of us. We chose a table on which to place the lightbox and set up our equipment. The camera was hooked up to the laptop in order take the pictures remotely, as it was thought that pressing down on the shutter-release button every time to take a picture could potentially cause the camera to shake. We used a Canon E0560D digital camera to take our pictures.
After choosing the vessel I could like to capture and once it was placed in position on the turntable, I inspected the image through the “live feed” on the laptop screen. I noticed that the image was a little dark, so I decided to turn the ISO up a little to 120. I needed to be careful not to turn the ISO up too much, since this could potentially cause the image to go quite grainy and therefore decrease its quality or sharpness. I also noticed that the rim of the vessel furthest from the position of the camera was a little out of focus, and I decided to turn the depth of field up to f22.
During the photographing stage, I was careful to rotate the turntable only a little between each take, maybe by only about 10 degrees each time, since there needs to be at least a 60 percent overlap between each image in order for PhotoScan to triangulate the images correctly. I made sure to capture my vessel from three different height positions. I had to make sure that, at my highest angle, the camera was able to capture the interior of the vessel. I then needed the bowl to be turned upside-down so to get the base, and then to repeat the process, taking the vessel from three different height position, 360 degrees around each time. The photographing process in total took about 40 minutes in total.
All bowls featured in this blog courtesy of the National Museum of Ireland.