[Nhcoll-l] 3D scales in 2D images

[Callomon,Paul]

In our mollusk type imaging project, images from which are viewable at http://clade.ansp.org/malacology/collections/search.php?search=advanced, we include a scale bar laid on the center line of the specimen in the vertical plane where that is practicable. For things under a certain size, or where the specimen has enough depth to require image stacking, however, we record the maximum measured dimension as a number on the image itself.
Scale bars on images are largely there to indicate relative size, not for users to take accurate measurements from. Nowadays it's easy to include maximum-diameter measurements in image metadata or an associated database.



Paul Callomon
Collection Manager, Malacology and General Invertebrates
________________________________
Academy of Natural Sciences of Drexel University, Philadelphia
callomon at ansp.org Tel 215-405-5096 - Fax 215-299-1170


________________________________
From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu> on behalf of Gary W Shugart <gshugart at pugetsound.edu>
Sent: Friday, June 5, 2020 1:02 PM
To: nhcoll-l at mailman.yale.edu <nhcoll-l at mailman.yale.edu>
Subject: [Nhcoll-l] 3D scales in 2D images


External.

Hi All:  When I take a picture of a specimen I usually include a scale ruler on the stage or at the base. It didn't occur to me until recently that the scale will not apply for any part of the object not on the same plane as the base. The differences are substantial with eggs, nests, bones.  For example using a scale on a base plane compared to two scales above separated by pencils (7 mm) and the top scale is 10 mm = 11 mm at base (reference photo on Slater Museum FB page (https://tinyurl.com/ybnjkq2r​).  Searching Google and this appears to be something like perspective or forced perspective (not parallax view) .  There are explanations of angular size calculation online and calculators, but you have to know the distance between the base and plane to calculate a size. I noticed this especially in the new Birds of the World (formerly Birds of North American) account with eggs and nest with a scale.  Also recall the issue occurred in egg photographs.

How to deal with this?  This depends on the purpose of the scale.  If to just give a general idea of size it doesn't matter.  But if the idea is to use the scale to set the scale in imagej or other measuring software, it is a problem.  A pain to set up and redo for each object though. Or actually deal with specimens and measure them IRL.

Gary Shugart
Collection Manager
Slater Museum
Tacoma, WA

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