Photogrammetry Photography Guide: Shooting Underneath Objects

Journalism Technologist Ben Kreimer prepared a complete guide to photogrammetry photography for Journalism 360. We’ve shared portions of the guide, Shooting Underneath Objects and Ring Flash Photography, here with their permission. Join Ben for a virtual Q&A on May 21, 2020, to learn about all things photogrammetryThe use of photography to survey and map a location from a variety of angles..

How to Capture the Underside Of an Object Using Plexiglass

When doing photogrammetryThe use of photography to survey and map a location from a variety of angles., you can only reconstruct the surfaces visible to your camera during image capture. If for example the demonstration shoe is on a wood table, the bottom of the shoe is not visible, and if you move the shoe to capture the bottom the reconstruction will fail.

Transparent acrylic-based plexiglass, like that used in museum exhibits and aquariums, functions as an invisible support surface for photogrammetric captures, allowing you to photograph all sides of an object from above and below the object. Unlike glass, plexiglass can support significantly more weight than glass without breaking, easily a few pounds, but approach this technique with caution and common sense. This plexiglass technique was used to produce the 3D model of the shoe with its sole, as seen at right (on Sketchfab).

Shoe with Sole (Photographed on Plexiglass)
by Ben Kreimer
on Sketchfab

What you Need:

¼” or thicker transparent acrylic plexiglass sheet (available at home improvement stores and Amazon). Acrylic sheets can support significantly more weight than an equivalent-sized glass pane.

Sizing your sheet: The overall dimensions are based on the object it needs to support. The sheet must be large enough so your object sits in the middle of the sheet, ideally with about 10” of unobstructed material around the object on all sides, as shown on the right with the placement of the cardboard box. This 10” of unobstructed sheet around the object is required for capturing photos from beneath the sheet at oblique angles.

A support system strong enough to suspend the plexiglass sheet high enough so you can photograph underneath the sheet.

I use four camera tripods with two 6’ long 1”x2” boards for the plexiglass to rest on. Additional support material is needed based on object weight.

A linear polarized ring flash with a circular polarizer on the lens.

A Ring Flash is Required

A linear polarized ring flash with a circular polarizer on the lens. The ring flash will significantly reduce unwanted reflections. The polarization will reduce unwanted reflections and flare from the flash. The image on the left was captured from underneath the plexiglass using the room’s ambient light, showing reflections from the room that obscure the subject. The image on the right was captured from the same perspective, from underneath the plexiglass, but using the ringflash.

Caring for your Plexiglass

Plexiglass scratches easily, so get an art portfolio bag or tape together a cardboard sleeve to protect your plexiglass.
Don’t use Windex or any other kind of glass cleaner containing ammonia. Doing so will cause cracking in the plexiglass. A surface cleaning product like Brillianize, designed specifically for acrylic and similar materials, is required.

A Crucial Step

To successfully combine the sole with the top part of the shoe, capture overlapping images on all four sides, first from slightly above and then from slightly below the plexiglass. For the topside images, the camera lens is perpendicular to the object and just above the plexiglass sheet, its edge highlighted by the red lines in the right image. Below are a few images captured from this perspective.

Then capture images on the four sides of the plexiglass from just below the sheet, ensuring that the entirety of the object is visible through the plexiglass. In these images, the object will appear below the edge of the plexiglass, as shown in the image on the right with red lines highlighting the edge. Below are a few images captured in sequence from this perspective.