Camera Angles: Tips on Photogrammetric Recording of Finds Part 2

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In part one of this guide I tried to explain the uses of different coloured backgrounds to achieve good Photogrammetric models of small finds. This however is only one of the things to considered, and hence today I’d like to talk a bit about camera angles.

The way I see it there are two parts to the Photogrammetric process, and these are the photography and the modelling. In order to create good models what you need is not only a good program or experience with how the software works, but Photogrammetry also requires good photographs to process, and the only way you can get exactly what you need is to learn where you should take the photographs from.

I often say that the best position to take photographs is every 45 degrees in a circle around the object, then the same from a different height and a final shot of the top. Generally this works well, but there is a reason for these positions. It allows every point of the surface to be included in at least seven shots, four more than technically required. This way no matter what happens, you will always have enough coverage. In addition to this the photo from to top will nearly without doubt be placed, and it can be used to manually stitch the others together.

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But let’s break it down even more. The ideal positions are described work perfectly if what you are photographing is a half-sphere, with the base placed on a surface, decorated with many different colours to allow the program to recognise points. The problem is that most archaeological objects are somewhat different from this description. A long object such as a sword reacts differently to Photogrammetry than a tall bulging object such as a pot. Because of this the methodology I described has to be adapted to fit these differences.

The sword will require some shots to be taken at a larger distance, or to have the long end photographed in two parts (not such a fan of this, but it seems to work). If it is engraved some close up shots of the decoration can help emphasise this, or if it is too shiny the camera may have to be moved to make the light more consistent. The pot instead can work easily with the consistent distance from the camera, but requires an additional circling around the top from a different height to process the inside. Handles may need extra photos to make sure all points are modelled, and a much lower circle may be necessary if the bulging is covering the bottom of it.

Similarly the lower spiral may need to be very low for a bowl, but in the case of the coin it can be as high as necessary, as the side is not as important.

Hence the main thing is to look at each object individually, realise the needs of the program and plan how to tackle any problems it may encounter. Realising that a certain area of a find could be a future issue before the photographs are taken means that the issue can be avoided before it even appears.

8 Reason Why We Should Be Using Photogrammetry in Archaeology

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If you are an archaeologist you should be using Photogrammetry because:

  1. It is easy to use: Unless you are dealing with something extremely large or extremely complex, Photogrammetry has an extremely high success rate. When it was still based on camera calibration, complex calculations and precise measuring was necessary, but with more modern programs often all that is needed is to take the photos and upload them. Decent models are easy to produce, and more complex ones are achievable without issue with some experience. Overall, anyone could potentially use it in small scale archaeology with no experience, and on large scale with limited training.
  2. It is quick: With a good internet connection I can probably model a single feature in under 10 minutes. And by single feature I mean anything from a posthole to a stone spread. In situ finds could be recorded in no time, cutting back on the need to plan everything by hand. A complex stone wall could be preserved for the archaeological record simply by taking a few dozen photographs, and sections can be recorded with much more realism than any hand drawn plan can achieve. A rough sketch of the section would of course help the interpretation, but the measuring time would considerably go down, as it would be possible to measure on the model using Meshlab.
  3. It is practical: Laser scanning is the current fashion in archaeology, but the problem with laser scanning is that you need to provide expensive equipment, you need to carry that equipment around and need to train specific people to use the machines and the software. Photogrammetry requires nothing more than a camera and a laptop, which are usually much more accessible on site. If a delicate object is found, that may not survive excavation, it is much easier to take some photographs with the site camera, to then edit later, than to bring in the equipment to laser scan it.
  4. It is accurate: As shown in one of my recent posts, the accuracy of 123D Catch is extremely good for the type of process. Although it cannot compete with that provided by laser scanning, an error margin of less than 1% means that any task required for interpretation can be carried out without having to worry of the results. The level of accuracy is ideal for presentation, for interpretation and for recording.
  5. It is photorealistic: No other technology gives you the photorealism that can be achieved by Photogrammetry. Due to the fact that at the base of the models we have the photos, and that the finished product contains .mtl files that record the exact position of the photographs, the surfaces of the features can be recorded as they are in real life. The models seem realistic because they are not a simple collection of points, but a combination of points and images.Image
  6. It is entertaining: Archaeology is not simply about recording the past, it is also about getting the information out there, to the general public. It is important that anyone interested in an archaeological has the opportunity to learn about the site itself. Academic texts are amazing when carrying out research, but for the average archaeological enthusiast, who lives in a now mostly digital world, texts can be seen as confusing. Photogrammetry provides a visual component to the archaeological record, making it possible for people to see from their own living room the archaeology, as if they were actually present at the site.
  7. It is constantly improving: At the moment there are some problems and flaws with the programs that may cause concerns to more traditional archaeologists. These problems however are only temporary. With such a great interest in the digital world, teams of developers are constantly trying to update and improve all software, and if at present programs like 123D Catch are not perfect, they can only get better. Also, 10 years ago this level of accuracy in Photogrammetry was unheard of, yet today it has got to this point. In another 10 years how much will the programs change for the better?
  8. It is not as simple as it looks, in a good way: There are different levels to Photogrammetry. The basic level is the simple recording of features and artefacts for the only reasons of recording and presenting. There is however a second level, which uses the models created to analyse archaeology, like I show in my previous post about finding inscriptions in coins. There is a third level, which alters the way the programs are used, by changing a part of the process to get greater results. An example is the attempt I did on reconstructing the Sphinx using tourists’ photographs, or the idea of using series of photographs in archaeological archives to reconstruct features long gone. Finally the fourth level is the more interesting one. It’s using many Photogrammetric models to create a single model, i.e. recreating pots by putting fragments together digitally or entire sites by gluing together individual features. So it is not only pretty models of features, it is much more.Image

Back or No Back: Tips on Photogrammetric Recording of Finds Part 1

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Photographing finds in order to make a 3d model can be a quick and easy process, but on a few occasions you will find that even the most accurately placed camera positions can create blurry or incomplete models. There are many reasons for this problem, and even with all precautions taken there is no 100% certainty that the results will be good. Still, there is no harm in trying to create the ideal environment for the photography.

Lighting is certainly the bigger problem, but there is also another aspect of the photograph that is easy to forget when using Photogrammetry, which is the background. We usually ignore the background in models as what we are interested in is the object itself, but for 123D Catch (or whatever the program you use is), there is no actual difference between what the focus of the process is and what is to be discarded. Therefore the program will treat both equally, and if it so happens that the points of the background are clearer than those of the object, the background comes out perfectly and the find does not.

Now, in most cases, provided the object is not moved around there will be consistency in the background, so no problem rises. But say you are in a studio in which movement is limited, due to lights or simply space, is it still possible to make the object rotate without this affecting the results? The simple solution I found is to use a rotatable table. You can get really expensive ones, but even a rotatable tv stand does a perfect job. If you place the object on top and make sure the camera shots only include the stand and the object then you should have no problems with the background.

As an addition to this I have found that colouring the surface of the table of a bright colour (such as blue or green), significantly helps the program to find the edges of the object, due to the increase in contrast. The only downside is that occasionally you may get a thin blue line appearing at the corners and sides, which however can be removed with the paint tool in Blender.

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On a similar note, I remember reading somewhere that often a background full of stuff can really help in Photogrammetry, as it is easier to find manual points in the background and thus it allows images that would be hard to place to be aligned with the others. I’m of two minds on this front. Part of me wants to argue that this is only taking the focus away from the actual object, and that it will create a “shaky” points where you most need them. On the other hand, this has actually saved me on quite a few occasions. I’d say that a good compromise is choosing the points in the background only if you are close enough to the object to reduce the chances of mismatch.

Finally, these ideas with rotating backgrounds do only work if lighting is consistent on the object. If the change within the shot is drastic, then changing the rotation can only have negative effects. For all other situations it can make a great difference.

Accuracy of 123D Catch

I always go on about how Photogrammetry should be used to record everything from small finds to entire sites, but just how accurate are these models? Are they good enough only for recording the objects as nice images or can they actually be used to gain more archaeological information? In essence, is it technology for technology’s sake or is there more to it?

In order to answer this I photographed three different objects and made models out of them. I then proceeded to measure the objects, and by using a set distance in Meshlab I then measured the same distances in the models. Finally I compared the two sets of data in order to see what the results suggested.

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Here are the results:

Iron Buckle

Pot Fragment

Arrow Head

Object

 Model

Object

Model

Object

Model

80*

80

74*

74

76*

76

49

49

61

61

81

80

49

50

86

86

54

54

25

26

81

82

63

63

34

34.5

72

73

64

64

33

33

Error

0.6%

Error

0.6%

78

79

Error

1%

The asterisk indicates the measurement I used as a reference and all the numbers are in mm.

Overall the results suggest a maximum error of 1%, which considering the size of the objects is more than acceptable. With this data it seems that analysis of the surfaces can be done without false results appearing, and as such it does seem like Photogrammetry can have many more uses than simply pretty pictures.

A more careful analysis of accuracy on a larger scale has still to be done, although Chandler and Fryer (2011) may be of some help: http://homepages.lboro.ac.uk/~cvjhc/otherfiles/accuracy%20of%20123dcatch.htm

Still, the results seem to greatly favour this technology for the purpose of archaeological recording.

Community Engagement and Online Galleries

One of the main aims of the 2013 excavation at Caerau was to engage the community as much as possible, so that this amazing hill fort would become part of the people living in Cardiff and particularly Ely, rather than something separate from them. During the four weeks spent on site hundreds of school children were shown around the site and asked to perform tasks such as writing letters to the Iron Age, washing finds and making pottery.

In particular the kids were asked to make some “celtic heads” using clay, basically faces decorated in any way they liked. The results are extremely artistic and are worthy of being put on display, something which I realised may be possible to do using Photogrammetry.

Hence having selected 40 or so of the most well preserved heads (some had cracked due to the sun) I proceeded to 3D them and the results can be found at http://robbarratt.sketchfab.me/folders/celtic-heads . This way everyone can appreciate the great work these kids put into them.

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At the same time this shows another potential for Photogrammetry, which is creating online galleries, potentially entire museum collections stored on the web and accessible to all.

Potential Method to Emphasise Inscriptions with 123D Catch?

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Although this requires much more work and more tests with different objects, it could be a somewhat interesting method to better distinguish inscriptions present on coins or small object.

The picture above is of a Medieval lead stopper or weight found at the site of Caerau, Cardiff, which has been made into a 3D model using 123D Catch and then run through a filter in Meshlab (all freeware).

Meshlab itself is an interesting but somewhat limited program, but the Colorize Curvature (APSS) filter seems promising, as it changes the texture colour based on the inclination of the surface. The papers regarding the filter are extremely technical, which at the moment make it difficult for me to understand the process in detail, but with an extremely high MSL – Spherical Parameter (even 10000000000) and an approximate mean Curvature Type the results above come out.

The stopper seems to bear three letters on the top (EXQ?) and maybe something at the bottom too.

At present the only other object I’ve tried is a piece of pot with decoration, but the natural curve of the object itself made it hard to achieve anything. However the way the filter works seems to suggest that there is indeed correlation between the inscription above and the filter rather than a random effect that just so happened to produce the image. Research continues…

Photogrammetric Pottery Reassembling (PPR) Preview

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This is part of a little project I’ve been working on, and which I’m hoping to write an article about soon. What you see here is a pot which was originally in fragments, and that I’ve reassembled virtually, in order to avoid having to glue it together.

I used 123D Catch to digitise the pieces of pot, and Blender to reassemble them and correct some of the textures.

The most fragments I’ve been able to put together at the moment is 9, but there is certainly the potential for making much larger and more complex pots come back to life.

The aim of this method is to preserve the pottery as much as possible as well as provide an online copy of the object that is visible to the general public, thus making archaeology even more accessible to all.

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Introduction

Hi everyone, my name is Rob Barratt and I’m currently finishing my third year in archaeology at Cardiff University. During my time here I have developed a passion in anything computer related, and in particular with 3D modelling. As part of my dissertation I’m exploring the advantages of the technique called Photogrammetry in the archaeological field, based on a number of models I have created over the past two years. In order to make this information accessible to the general public (which is the main aim of archaeology) I have decided to create this blog to run you through my work and keep you updated with anything else I come up with. I hope you enjoy your visit.

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