2D Photogrammetry for Fabrication and Design

August 22, 2017  •  Leave a Comment

In the course of my career as a Fabricator and Designer, I've learned that often the most challenging aspect of the job isn't mastering fine precision of CNC Machinery or even creative fixturing in order to finish a part. Often, the most challenging aspect (unless you have extremely expensive 3D scanning tools tools at your disposal) is getting accurate data into the computer to work from.

How many hours have I spent with a set of calipers, a measuring tape, and a notepad, only to find human error on a measurement or simply overlooked something? Even if I'm perfectly thorough, this process is time-consuming. During my time at Blue Seas Fabrication I was lucky enough to be able to use a Proliner to scan boats with - this provided excellent accuracy, and made design and fabrication of SeaDek much more efficient.

At West Coast Customs, we don't have a 3D scanning device in-house (from time to time we rely on the services of a 3D scanning company), so for the day-to-day digitizing of parts and materials, I've had to find some creative workarounds. This is where my knowledge and experience as a fabricator/designer intersect with my knowledge and experience as a photographer/cinematographer, and the results serve me extremely well daily even on large objects of 10' or greater. It is my intention to share those with you here and now.

 

Option 1: 2D Scanning.

This is quite straightfoward. For smaller parts, use a flatbed scanner, like the one on all-in-one printers that you sometimes get for free. This works remarkably well for scanning small, flat parts and for smaller templates. If needed, one can always scan something larger in pieces, then recombine them using added registration marks. Critical to this (whether a single image or multiple), is adding marks for scaling (see ex 1, 2).

Example 1: Cardboard Template for a custom Spoiler bracket.

Example 2: part 2 of same Cardboard Template. The marks on these are almost impossible to see - miniscule blue dots. Can you spot them?

The final product

In whatever CAD program you prefer, bring the images in and scale them based on your measurement points, and align them using the alignment points - these points can serve multiple purposes, I like to use registration marks that are also measurement marks. My preferred method is a dot surrounded by a circle of dots, as concentric circles are easy for the human eye to spot and align.

Once the images are aligned, draw your part, check your measurements, adjust as needed. Done.

 

Option 2: 2D Photogrammertry.

Scanning is great, but isn't always practical. breaking apart a large template can take a really long time, and the more tasks a human has to undertake, the more room for error creeps into the equation, which could easily cost time and money for materials. If the piece in question is something that someone has already worked a long time on, the results could be disastrous.

It wasn't long before I realized just how important it was to be able to gather accurate and repeatable visual data here at West Coast Customs. Initial efforts with mobile-phones and measuring tapes have been mixed at best, usually requiring a generous amount of grinding and/or bondo to make work, if indeed a revision and re-cutting isn't required.

My solution for this was to find a spot in the shop I could use pretty reliably for Photogrammertry using my camera. Below you will find a basic write-up and tutorial on how to do this yourself, should you feel so inclined.

The supplies and gear I used are as follows:

Sony A7r
Konica Hexanon 50mm f1.7 prime lens with adapter
One 1/4-20 screw (available pretty much everywhere)
Some 3/4" MDF (wood) and tools to work it
glue
a plumb-bob (any sort of weight and string will do)
masking tape
tape measure
either a clear wall or elevated position (in my case I used a catwalk about 15' above a clear section of floor)

The first thing I did was assess where I was likely to do most scanning. Since perspective distortion reduces the further a camera is from the object, I picked the catwalk, a level, elevated position some 20' above a nice flat even surface. It'd be pretty easy to use a wall or pair of walls for this - in my opinion the key to good photogrammertry is repeatabilitity, so ideally, choose a spot where you can install a fixed camera mount or at the very least, someplace where you can make marks for consistent image acquisition.

With my location chosen, I took my camera with fixed 50mm lens and measuring tape upstairs. Holding it relatively level, I measured how far out from the rail it would need to rest in order for the field of view to clear the catwalk. As I was not using leveling tools, I was generous in my estimate - about 15" from the edge of the rail to the base of my camera.

After that, I designed a simple mount to hang on the rail, while sitting square and flush. I used the lower rails as bracing, measuring the standoff difference, and overcompensating for it in my design with the intention of sanding to fit. Below, you can see the completed mount with my camera affixed. As a bonus, it looks like a goat. \m/

Next, I used a plumb-bob to find the center-point of my lens, directly below where it sits on the mount. With a marker in place, I used Adobe's Lens Calibration Tool and associated calibration charts to create a lens calibration profile for my camera and lens. For this purpose, I set the lens to f5.6 to ensure crisp focus across the entire field of view and still let in enough light to keep noise levels low.

Once I knew what was directly below my lens, I used a sanding block to trim down the lower portion of the mount until my mark below was dead-center of my lens. This ensured that the corners of the frame would essentially all be equidistant from the lens on the floor, further reducing perspective distortion.

Once a repeatable, level camera position had been achieved with a clear level space in the field of view, it was time to create a lens calibration profile for my camera. For this, I used the Adobe Lens Profile Creator Tool, a free download for Windows or Mac. Granted, this will require Photoshop or Lightroom to use your created profile, but a subscription isn't cost-prohibitive, and full demo-versions can be downloaded and used for a full month from Adobe's website. Following the instructions of this tool via the included PDF was not a simple undertaking, but absolutely worth the effort. Stock lens profiles are not nearly as accurate as can be achieved via a custom profile. Using a measuring tape, measure the dimensions of your field of view. Compare these dimensions in inches to the resolution of your camera images. This will allow you to find an approximate measurement of pixels per inch. My setup has a resolution of about 78 PPI. For my purposes, this is the right blend of precision with overall size capacity. Using our large-format printer, I printed a nice big lens calibration chart (for my purposes I used "Landscape JIS_B0 - 40.60 In x 57.30 In (Square Print Dimension 54 Pts, Version 51 x 73).pdf" from the included charts). Make sure you choose a chart that is large enough to cover a significant portion of the frame and has small enough checkers to give the computer good data, but not less than 20 pixels across.

Initially, I had planned to move the chart in the frame, but the software will NOT work if you do this (I discovered this the hard way) - you'll need to move the camera itself, so loosen the screw on the mount enough to pivot the camera from its position enough to take a series of images as per the instructions, and run your calibration profile creator based on them. Make sure to take enough images to avoid having to do it twice - I recommend 13. Below is an image of me setting the initial position. Technically, this is a selfie.


 

Once your calibration profile is complete. use of the setup is quite simple. Place the piece to be scanned, extend your measuring tape and lock it, laying it on top of or next to the part, check focus and f-stop, photograph the piece, correct the geometry of the image in Photoshop of Lightroom, crop and export, import to CAD, draw your part, check measurements, adjusting as needed, and you're done! Enjoy drawing parts from accurate visual data!

A custom door trim piece for a Camaro. Photographed, corrected, and drawn. The final part was dead-on.

 

A final note on photography equipment:

It doesn't have to be expensive to gather accurate visual data! Not at all! The lens I'm using for this I bought at a Swap-Meet for $6. Of course the higher-resolution and better build quality camera you use will help you gather MORE data, but even an old used inexpensive mirrorless camera with a garage-sale fixed focal length (prime) lens will absolutely do the trick. You COULD use a variable zoom lens, but I would advise against it if you are attempting to make accurate scans. Good Luck!


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