Dr Mike J Smith

Plymouth University

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Studio of Objects: point clouds from photogrammetry (Guest Post: James O'Connor)

Mon, 02 Feb 2015

(Ed: this is 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.

SfM paolozzi

An example of one corner of the room has been processed and can be interactively viewed here (using the fabulous Potree). The imperfections weren’t processed out to show the product without any manual intervention, and one can imagine with further processing (outlier removal, surface meshing) you will get a pretty good idea of what this part of the studio was like. Unfortunately areas remain without points as appropriate imagery couldn’t be obtained due to both time and physical constraints, but one exciting prospect is that images of Paolozzi’s studio could be mined from the internet to fill in the gaps, which would be very interesting to see!

Watch this space in the future for developments and other applications of this technology, as whilst competing with laser scanning is tricky, the low-cost and accessibility of photogrammetry means it won’t be going anywhere for a while, and with cameras becoming so ubiquitous in modern society (BIG data!) one can only imagine where we’ll be in ten years!

in close association with hijack and Dacapo

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