A guest post by James O’Connor 

Whilst the main aim of the technical side of the Studio of Objects project was the use of laser scanning to reconstruct and archive the 3D geometry of Paolozzi’s studio, as conserved at the Scottish National Gallery of Modern Art, we were interested in, at least partially, trialling the use of photogrammetry to achieve the same goal and in particular Structure from motion.

Structure from motion (SfM) is a strategy of image matching and 3D point cloud reconstruction based on minimal input information, namely no prior information about camera settings/shooting position. Whilst these added parameters add uncertainty to the solutions within the equations (notoriously non-linear!) for a well-designed survey the image matching for this strategy can minimise errors in the solutions stage if things are as consistent as possible. Another nuance to SfM, as opposed to traditional photogrammetry, is that the more angles the better. Furukawa’s dense matching algorithm has demonstrated its versatility in this regard, and so we set out to capture a mixture of orthogonal imagery with some oblique photos worked in.

Having a Nikon D810 to hand isn’t something you could hope for every day, so for the few hours I was let loose in Paolozzzi’s studio I knew a well-designed survey design was key. Whilst we discussed some strategies that were post-processing and data heavy, such as focus-stacking and bracketing (High Dynamic Range Imaging is something I’ve become fascinated by!) the majority of the imagery captured was taken using predefined settings with two different prime lenses (Nikon 24mm and 50mm) in order to ensure every part of the room was in focus from every position imagery was captured from. This meant shooting from 15 different positions facing in either direction using the 50mm lens and focusing at shorter distances for near field objects from 40 or so positions using the 24mm lens.

Imagery was captured in both RAW and JPG formats, RAW for its higher dynamic range and JPG for its ease of use. Some early results were produced using the very easy to use VisualSfM (see image below), a free to use SfM solution that was born out of work done by Noah Snavely in the late noughties. It’s my first port of call to exhibit the versatility of modern photogrammetry, as simple models conserving geometry can be captured from something as user-friendly as a video with a handheld camera and built into usable products (see this example). This also means historical archives could be data mined to generate meaningful 3D models of various culturally and scientifically important areas.
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Point Clouds from Photogrammetry – 2nd February 2015