Friday, April 11, 2014

Photographing an object

This is step 7 of 10 in the tutorial Collecting 3D shape data using StereoMorph

This section provides tips on how to photograph an object for shape data collection.

Recommended materials for this section:
  • A yard or less of black velvet

This is the step where the DLT method has a major advantage over other 3D morphometric methods. Once the cameras are calibrated, the number of objects that can be photographed is only limited by the time it takes to position and photograph each object.

It is best to have a uniform background that provides good contrast to your specimen. First, this can decrease the photo size by as much as half (encoding a large black space takes up less space than a multi-colored, noisy background). If you’re taking several hundred photographs this is advantageous for data storage. Second, it’s easier to discern points on the edge of the specimen when the edge is clearly distinguishable from the background. For light-colored specimens, black velvet works well. The cheapest stuff available at fabric stores works great and only costs about $10 a yard.

A shell on black velvet. Black velvet works great as a solid, black background.
If you need to collect landmarks from several different places on an object that are not visible in a single camera view, reposition the object a few times, taking a photograph from both camera views each time. For the StereoMorph functions, these different orientations of the object are referred to as “aspects”.

The tutorial data set contains landmarks and curves from these three different aspects of a Canada Goose skull. The first aspect provides views of the ventral aspect (underside) of the skull.

First aspect for digitizing landmarks on the ventral side of the skull. The left and right images are the first and second camera views. The specimen is in the same position in both images, just viewed from different perspectives.
From the second aspect, landmarks on the lateral or side aspect of the skull can be digitized.

Second aspect for digitizing landmarks on the ventral aspect of the skull.
And the third aspect offers views of the back of the skull.

Third aspect for digitizing landmarks on the lateral aspect of the skull.
Depending on the data you want to collect you might be able to get away with a single image of each specimen, in which case landmarks and curves only have to be digitized in two images. If you have multiple aspects, you will need some overlap in landmarks among the images (at least three, preferably five to six) in order to combine all of the points into a single 3D point set (detailed in the section “Unifying, Reflecting and Aligning”). However, you don't have to digitize the same landmarks in every aspect.

Lastly, sometimes it's necessary to use a reference marker if it's difficult to find the exact same point between two views. I do this when digitizing beaks with a broad, flat tip. If using museum specimens, one should of course use tape that does not leave a residue.

No-residue tape can be used for points that are difficult to identify exactly in two different views.
Go to the next step: Digitizing photographs
Go back to the previous step: Testing the calibration accuracy

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