Back or No Back: Tips on Photogrammetric Recording of Finds Part 1

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Photographing finds in order to make a 3d model can be a quick and easy process, but on a few occasions you will find that even the most accurately placed camera positions can create blurry or incomplete models. There are many reasons for this problem, and even with all precautions taken there is no 100% certainty that the results will be good. Still, there is no harm in trying to create the ideal environment for the photography.

Lighting is certainly the bigger problem, but there is also another aspect of the photograph that is easy to forget when using Photogrammetry, which is the background. We usually ignore the background in models as what we are interested in is the object itself, but for 123D Catch (or whatever the program you use is), there is no actual difference between what the focus of the process is and what is to be discarded. Therefore the program will treat both equally, and if it so happens that the points of the background are clearer than those of the object, the background comes out perfectly and the find does not.

Now, in most cases, provided the object is not moved around there will be consistency in the background, so no problem rises. But say you are in a studio in which movement is limited, due to lights or simply space, is it still possible to make the object rotate without this affecting the results? The simple solution I found is to use a rotatable table. You can get really expensive ones, but even a rotatable tv stand does a perfect job. If you place the object on top and make sure the camera shots only include the stand and the object then you should have no problems with the background.

As an addition to this I have found that colouring the surface of the table of a bright colour (such as blue or green), significantly helps the program to find the edges of the object, due to the increase in contrast. The only downside is that occasionally you may get a thin blue line appearing at the corners and sides, which however can be removed with the paint tool in Blender.

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On a similar note, I remember reading somewhere that often a background full of stuff can really help in Photogrammetry, as it is easier to find manual points in the background and thus it allows images that would be hard to place to be aligned with the others. I’m of two minds on this front. Part of me wants to argue that this is only taking the focus away from the actual object, and that it will create a “shaky” points where you most need them. On the other hand, this has actually saved me on quite a few occasions. I’d say that a good compromise is choosing the points in the background only if you are close enough to the object to reduce the chances of mismatch.

Finally, these ideas with rotating backgrounds do only work if lighting is consistent on the object. If the change within the shot is drastic, then changing the rotation can only have negative effects. For all other situations it can make a great difference.

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