How to use structure from motion to produce 3D models

Structure from motion (SfM) is a photogrammetric range imaging technique for estimating three-dimensional structures from two-dimensional image sequences that may be coupled with local motion signals, according to Wikipedia, which I think is reasonably accurate in this case.

Simply put, one can snap a series of photographs using the camera in your smartphone, unmanned aerial vehicle (UAV) or other photographic equipment and produce a 3D model using software that is built using the SfM technique.

To assist you in capturing the photos on your smartphone to generate a 3D model, Autodesk has a free mobile app called 123D Catch. Using the app, you can create a 3D model of nearly any object you can imagine. Following is a one-minute YouTube video from Autodesk that succinctly shows the process of capturing photos using an iPhone camera with Autodesk’s free 123D Catch app and how to generate a 3D model.

Today’s smartphone cameras offer incredible resolution. My Samsung Galaxy S7 has a 12-megapixel camera. The iPhone 7 offers the same resolution. Modern iPads have an 8-megapixel camera, which is fine for 3D modeling.

So, think about this for a minute. What value can you derive from shooting images from your smartphone? If you need to know the volume of a pile of material (e.g. construction), your smartphone running the 123D Catch app can do it. The requirements are straight-forward:

• You need to be able to walk completely around the pile and shoot many overlapping photos, filling the camera frame with the pile. Every surface you want modeled should be visible from at least four photos from different angles.
• Avoid shooting featureless photos (e.g. walls, water or snow surfaces). Or, if a background is featureless, add a feature to the surface like a small target (similar to photogrammetry ground control targets but much smaller).
• Avoid shooting reflective surfaces.
• Don’t shoot moving objects (e.g. vehicles).
• Try shooting in well-lit areas.

Taking it a step further, the camera doesn’t have to be in your smartphone. You could use a camera mounted on a vehicle that could provide a different perspective. Something like a … drone! Yes, SfM-based software like Agisoft Photoscan allows drone pilots to exploit photographs shot from their aircraft.

In the past few years, I’ve written a lot about drones and my adventures in using them. Following are a series of images as a result of one of my UAV flights. A total of 500 photos were shot at 80 percent overlap from my drone flying at 200 feet AGL (above ground level). The photos were imported and run through Agisoft Photoscan.

The first screenshot (Figure 1) is a 2D view of the 3D model generated from processing 500 digital photos through SfM.

Figure 1

Figure 2 shows the camera location of each photo. Remember, the UAV was flying at a consistent altitude (200 feet AGL) and was taking photos with an 80 percent overlap.

Figure 2

Figure 3 is an oblique view of the 3D model. If I wanted to improve the quality of the 3D model (e.g. the sides of the building), I would have flown the drone again in a flight pattern perpendicular to the first pattern. Note the pile of material at the lower part of the screen and to the right of the pond.

Figure 3

The final screenshot, Figure 4, is a zoomed in view of the pile of material.

Figure 4

Since a 3D model has been created, clearly a DEM (digital elevation model) and DSM (digital surface model) can be generated, as well as associated 3D products like elevation contours and volume calculations.

Enough of the drone talk.

With your smartphone, you’ve got everything you need to create a 3D model of your children, your Christmas tree, your pet, your vehicle or other valued object. Give it a try. It won’t cost you anything but some of your time.

Start by downloading the Autodesk 123D Catch app. You might want to view this six-minute video describing how to plan a shoot for best results.

https://youtu.be/D7Torjkfec4

To process the photos and create a 3D model, install the Autodesk ReMake free version.

Once you’ve installed ReMake, take a look at this less than four-minute quick start for importing photos and processing them in ReMake.

https://youtu.be/wRWo3r-woMI?list=PLUgaUX0Wfr-R1hXT9J1pTh6kKhUTwXLZR

If you get a chance, post a 3D model you’ve created in the comments section below.

Follow me on Twitter @GPSGIS_Eric

Photos: Agisoft