Photogrammetry Explained
(How Many Photos And More)
Photogrammetry – if you think this word is new, you’re mistaken! Photogrammetry has been here for a long time, almost equal to the amount of time photography has been around. Understanding this process might be a bit difficult, especially if you’re new to the terminology. However, we’re here to help. So, let’s run you through this technology, which has been here for decades now!
For photogrammetry software to process any point on your object, each particular point needs to be clearly visible in at least two photos. To ensure you’ve captured an object well, you need generally from 20 to 250 photos. The higher the number of photos, the better you can recreate the photographs when you run it through the photogrammetry software.
We will discuss this in the later part of the post. But first, let’s get started with what photogrammetry is.
What Is Photogrammetry?
Photogrammetry is the process followed to obtain, record, interpret, and replicate accurate information about 3D physical objects using photographs. Courtesy of the numerous aspects involved in this process, photogrammetry is an impressive technology. However, if you ask us, it is definitely way beyond that. It is an art, a science, and above all, it is magic! While photogrammetry was initially associated with the production of topographic maps through aerial surveys, since then its application has widely diversified. It is now used in any situation that requires the generation of accurate three-dimensional data or drawings. You will find photogrammetry used in architecture, archaeology, engineering, geology, forensic work and even underwater!
How many photos do you need for accurate recreation in photogrammetry?
Technically, each particular point on your object needs to be clearly visible in at least two photos. But yes, the requirement of photos varies on the basis of a whole range of factors, which we have listed below:
Complexity of the subject
The number of images is directly proportionate to the complexity of the subject. If your subject is layered and complex, try capturing multiple photographs from different angles. On the other hand, if your subject is simple, a few images would work well too.
The number of photos you need to capture varies based on the size, shape, scale, and type of shooting. However, you need to remember that the higher the number of images, the better will be the quality of the reconstructed images once you run it through the photogrammetry software.
Format Of Photos
Shoot the images in both JPEG (or PNG) and RAW formats. RAW files are the ‘digital negatives.’ This means you cannot use them as images for processing, but they hold all the data to make an image. These files also do not degrade over time, as they are the original data for an image.
JPEG images tend to degrade over time – once they have been opened, edited and saved, the quality keeps deteriorating. Every edit or copy of a JPEG image simply adds to the degradation of image quality.
While photogrammetry software purely uses the JPEG or PNG format of images, the RAW images are only to ensure you have pure files to convert into JPEG, without worrying about the depleting quality.
Resolution Of Photos
An important aspect is the resolution of the camera you will be using to capture the input images for photogrammetry. The number of pixels directly impacts the number of points a given photogrammetry algorithm can analyse. This also means that the quality of your input images determines the quality of the final 3D model.
While smartphone cameras can be used to capture these images, it is best to opt for DSLR cameras. These cameras deliver top-notch image quality, which are further converted into accurate and precise 3D models.
Image Overlap
Planning your movement as per your subject is a critical aspect while capturing images. As you move around your subject capturing photographs in a sequence, keep in mind that the sequence should overlap by at least 60%. An inadequate overlap will not build correctly, leading to a distorted final 3D model.
Is video-based photography possible?
It is totally possible to create a 3D model from a video instead of from multiple photos, as a video helps you record all aspects of your subject with precision. The great news is that all major and popular photogrammetry software supports creating a 3D model from a video. This method is time-saving too. All you have to do is ensure that the image captured is not blurred.
This method was not preferred earlier when video cameras had low resolution. But now, with top quality video cameras, you can also opt for video-based photogrammetry for certain use cases.
What kind of objects can be scanned?
One of the main things you need to consider is the physical form of the subject. This is because photogrammetry software works best with rough, multi-colored, and organic photos. If your subject is smooth and shiny, the software will face difficulties to reconstruct the image properly. Likewise, single-colored or mirrored subjects, too, are difficult for the software to process and reconstruct. The software works wonderfully otherwise, on structures as mentioned above.
How does photogrammetry software work?
Photogrammetry software is what makes it possible to process all the pictures taken and convert them into a 3D model.
There are four main steps that go into this conversion. Let’s have a look at them:
Alignment of photographs
Here, the points identified in the previous step are assigned coordinates. In three-dimensional space, these points will be given X, Y, and Z coordinates. In this stage, you have the option of discarding irrelevant points.
Building a dense cloud
In this process, the positions of the photographs are identified. All the images are adjusted between adjacent identified photos, and arranged into a ‘sparse point cloud’. This is a critical stage, as this is when you can permanently discard parts of the images that are completely irrelevant to the further process.
Building a mesh
Once the points are assigned coordinates, they are then linked into a composite ‘mesh’ of triangular polygons. You have the alternative to edit the mesh as you wish for desired outcomes. You can do the editing either in the mesh in the same software program or choose to do it in another editing software and then re-import it back to the original program once for further processing.
Building the texture
The texture is an essential aspect when it comes to photogrammetry. In this stage, the original two-dimensional images are used to create texture. The texture is nothing but a photographic surface on the mesh created in the previous stage. It gives the final 3D model a photorealistic surface.
These steps are in reality not as simple as we have described them here – they involve a lot of calculations, resources, and, of course, time. However, the advantages are worth the effort.
Why photogrammetry?
If you’re still wondering whether you need to use photogrammetry, here’s a couple of reasons to convince you right away:
Permanent photographic record
With the help of photogrammetry, you can permanently capture the photographic record of a particular condition prevailing while capturing the images. This is not merely a pictorial record, but it is accurate, precise, and measured, too.
Time-saving
As this information is permanent, there is no requirement of re-surveying the given area. This saves on a lot of time, effort as well as money that would otherwise be required to carry out the fieldwork over again.
Quick access
Beneficial in unsafe field areas
In locations where it is difficult to send out a crew, photogrammetry comes to the rescue. Without risking the lives of the crew, you can use this surveying method to fetch data as per your requirements. It is particularly helpful in locations that are unsafe, difficult, or impossible to access.
Re-use the data
Photogrammetry allows you to view and analyze the same captured data from different points of view. All you have to do is alter the coordinates of the mapping field, and you’re good to go! You can use the same data over again, without any additional costs or efforts.
Summing up
So there it is – photogrammetry photos explained. We hope this information helped you understand the process of photogrammetry. Despite it being an age-old process, it took a lot of time for it to gain the attention it deserves. Now that it has got the mainstream focus, there’s no turning back. Without a doubt, the process is complicated and requires time, effort, and money. However, remember, photogrammetry is a permanent process! So, every tiny effort that you in this process take is on its way to eternity! All in all, the process is a fun mix of fieldwork and computer work, but the results are simply amazing!
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