Photogrammetry – Taking the pictures

This is a list of links compiled as I searched for advice on how to take pictures to use for photogrammetry. Each link is followed by an excerpt from the post.

By far, the biggest impact on the final output file is what happens in the shooting phase. In fact, it is usually easier to re-shoot a new series of source photos of your subject than try to save a computed capture that’s not working right away. I recommend loading up the images and see how well the images align as soon as possible. If certain images are off or are confusing the software, re-shoot them while you still have access to your subject. It may be necessary to re-shoot multiple times for one model.

Technique:  The first is to put the camera on a tripod, and rotate your subject using a turntable or office chair. The second is to put the subject in the center, and you then move with the camera around the outside taking pictures. Both techniques have their pros and cons. And both are appropriate for different scenarios. Always shoot extra photos to make sure you have enough angles. Too many images may overwhelm the software (especially if you don’t have enough system RAM)

The Turntable method will be easier to setup if you are using artificial lighting. It will also be faster to turn the object than move the camera. It also makes it easier to use a green screen, since it stays in one place.

The Walk-around Method requires less setup if you are shooting outdoors or otherwise don’t need to set up lights. If you are scanning a person, they will have an easier time keeping their eyes fixed if they are not spinning around on a chair.

Camera sensors.
What you’d better keep in mind when
picking your gear:
camera performance is not provided by its megapixels resolution;
high end cameras do not always lead to better result;
A good camera must allow a full light management in order to control each exposition
value. That’s why DSLR cameras (reflex) are usually recommended even if both

mirrorless and bridge camera can fit that need.

Pixels size must be higher than 2 μm; therefore it is strongly recommended to exploit
camera sensor bigger than 1/2.3″ even if smaller sensors may be used depending on
the accuracy you want to reach; You’d better draw your attention to pixels size rather than pixels mount for each picture. Full frame cameras: use lenses with focal length value between 25 and 50 mm; Aps cameras or lower sensor size: use lenses with focal length value between 18 and 35 mm considering a crop factor value between 1.5 e 1.6

You can use your phone or a cheap (or expensive) point&shoot camera. But if you want really good models, there is no way around a really good DSLR camera. A method for getting equal and good lighting onto the object is a ring flash. Not too expensive, and very useful for smaller objects, especially on a turntable.

Here’s a not-so-short guide on how to ensure that your photos will get you a good model. Often, much less effort is required, so please experiment on your own. This part 1 will give you a list of tools you need.

So, once you have the light you need, are you all set to shoot? Nope! You need to add something to the scene, or your model will be a limited usability. That something is a scale object. Something that will come out in the 3D model and has a feature of exactly known length, so that you can scale your model correctly. You can place a caliper on the turntable, a scale bar, you can add a business card, whatever! Ideally, you use something big, because there will be a measuring error. The larger the distance, the less the proportional influence of the error.

So, now finally ready to rock&roll? Theoretically, yes! But there is the issue looming over your work how you are going to combine the models you get from your several sets of photos into one 3D file! How can you register scans or photos sets?

There are basically three ways of registering scans:

  1. based on points found on the object itself,
  2. on points that you mark on the object, and
  3. on points on the background.

If the photos aren’t good, then it’s going to put a ceiling on the quality of your 3D model, no matter how good the software is. That’s why photogrammetry is really about taking good photos.

No Information is Better than Bad Information

Give the software only high-confidence information. If you don’t need the background, mask it out. If you can’t track a subject’s hair, cover it up. If one image isn’t aligning correctly, get rid of it. You’re smarter than the software at filtering this out before it gets to work, and you want to make its job as smooth as possible.

It’s All in The Picture

If its not in the picture, then It’s not in your mesh. Get underneath your subject to take photos, get above it as well. For heads, take a few extra pictures behind the ear. Make sure you have the coverage you need to get all the details you want, because it’s difficult to go back and reshoot in the exact same conditions.

This forced me to learn how to make good models using the software and maximizing the quality of how I took the imagery. Most of the problems in SfM come from bad imagery, not from having a cheap camera.

Step 8: What Is the Best Camera to Use?

When you are getting started the best camera to use is the best camera you have.

The DSLR is the gold standard. If you want to buy a camera for serious photo scanning, or serious photography in general, this is what you want.

But remember, what it comes down to is the quality of your photos, not the quality of your camera.

With the right skills and the right conditions you can take good photos with a bad camera. But if you don’t know what you are doing, it is easy to take bad photos with a good camera. If you want to invest in something, invest in your skills as a photographer. The camera is only as good as the photographer behind it.

Make sure you have good lighting. If you can swing it, working outdoors on an overcast day is perfect. You get lots of nice even diffuse light. If you need to shoot indoors set up as much light as you can and make it as diffuse as possible. Point your lights at a white painted ceiling or bounce cards, or those groovy silver umbrellas. The idea is to get as much light as possible with as few shadows as possible. On-camera flash is not generally useful here. It tends to cast shadows which appear in different places in each photo. Remote strobes are fine as long as they provide a very diffuse, even light. It is possible to shoot using a tripod, but it is so time consuming that it should be avoided if at all possible. The best plan is to get enough light going that you can shoot handheld. Aperture priority is the best mode to shoot with. You choose an aperture and the camera makes all the other adjustments for you.

Shutter speed plays a huge role in your quest for sharp pictures. If the exposure is longer than the reciprocal of the lens’ focal length you can’t hold the camera steady enough to get a sharp picture. In other words if you are shooting a 50mm lens you need to keep the shutter speed faster than 1/50th sec. Usually the only way to do this is by adding more light. As a last resort you can use a monopod or tripod to allow slower shutter speeds. Try to make the subject fill as much of the frame as possible. Background objects in the shot won’t hurt and they can help the software locate the camera positions if there aren’t enough features on the subject.

The quality of your scan depends entirely on the quality of your photos. If you fill the frame with all the details of the subject you will capture those details in your scan.The idea is to move around the subject taking photos from many different perspectives. Standing in one place and shooting a bunch of photos does nothing to capture the 3d shape. Don’t expect perfect scans without a lot of practice and a lot of patience. I have been working on this for more than 2 years and my scans don’t always come out, but at least I’ve learned 800 ways not the scan something.

The mesh captures the physical form of the object. This is all we care about if we will be printing the object on a single color 3d printer. If this is what you have in mind then turn off those fancy colors and take a long hard critical look at your mesh. An incomplete mesh can be repaired, but if the mesh looks like a marshmallow now, it pretty much always will. The colored skin is variously known as a color map, a diffuse map or sometimes, nonsensically, a texture. It is a regular 2d color image which is wrapped around your model. This layer is important if you want to 3d print your object in color.




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