Some Issues with Photogrammetry

This is based on some of the work I did for my dissertation. I realised that as it stands it isn’t likely to be published, so I thought I should at least share some of the concepts and ideas that I used for it.

Creating Photogrammetric models for archaeology can be a simple process, but there are some cases in which problems may rise, due to the shape of the objects or the type of surface. We’ve already seen how large features can require certain considerations when photographing, to simplify the work of 123D Catch.

But there are other issues as well:

  • Curved surfaces: If for example we are looking at a pot fragment and we are photographing it from the inside and the outside, the fact the surfaces are concave and convex rather than flat means we may have some difficulties. In general, convex surfaces are easier to manage than concave surfaces, as, provided that the angle of the two spirals of photos is big enough, the pattern of points is such that it provides sufficient information to the program to deal with the surface. If, however, the surface is concave then the same does not apply, and the outcome of the rendering depends on the size of the object. A large surface will generally provide sufficient detail for the program to recognise it and deal accordingly, but a smaller sample often does not have the same results, and the only course of action is to adjust the illumination to create as much contrast on the surface as possible, although this also does not always work. Similarly, if the object has elements which overlap in the photos, these can present some problems, as the program often assumes that they are on the same level. In this case, it may be necessary to increase the number of photos input to help clarify, as well as manually unstitching and stitching all photos that have not been properly placed.

Image

  • Translucent surfaces: These can cause problems with the camera, as they alter the contrast depending on the angle and lighting. In this case, it is important to make sure that the lighting is uniform and the object fixed, hence making it necessary to circle the object rather than rotate it.
  • Uniform texture: If the colour of the object is too similar throughout, thus not providing enough difference and contrast to allow the selection of keypoints, adjusting the light to bring out as much contrast as possible may assist in the creation of the model. One particular case of this is objects that have similar sides, like the image below that looked very similar from different angles. On the other hand, 123D Catch seems to cope quite well with this case, allowing the smallest of details to be picked out from the texture.

Image

There are of course many other cases in which we have problems using 123D Catch, but these described are some of the most common. The fact that these issue exist though doesn’t mean that the software is extremely faulty or useless, as we are dealing with some rare exceptions, while generally models should be produced easily and with none of these issues.

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