snaphots

“snapshot” – that word is usually used to describe an amateurish photograph (unless you are talking about a screen capture on your computer). But the term is not so much tied to the low level of professional knowledge as rather the haste/speed/lack of preparation that goes into taking the photograph. So the word has carried over into military use for a hastily taken shot – even for such a shot by an expert marksman. The same is true, obviously, for photography by experts: if there is no time, if you just yank up your camera and hit the trigger, it’s a snapshot, even if you’re Helmut Newton!

Now, obviously, an expert photographer will more often take snapshots that actually look like something, whereas us amateurs tend to produce, well….. erh…. let me just say that it’s a good thing that digital photography costs only the price of electrical power, and that I love the inventor of the “delete” button! And that better outcome quality is surely in part linked to the on average better equipment professional photographers and serious experienced amateurs lug around. It also correlates with knowledge of how to set up your camera so you’re ready for anything at short notice, and with practice.  And, to be perfectly honest, with not being lazy ;) Let me explain how and why on an example, one that’s putting this post within the range of the impending Theropod Thursday:

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A subadult ringbilled gull (Larus delawarensis) I photographed in New York, with the Peking and the Wavertree serving as the near-perfect frame. I caught the bird in flight and “froze” it perfectly – but only a half-second before I took the shot my camera had been dangling down by my leg. How did I do this? Am I a magician?

Certainly not – no magic involved at all! But two simple tricks and, admittedly, an OK camera. First of all, my camera is a fairly new DSLR, with a fairly new lens, which means it has a fast autofocus. And a smart one, too, that doesn’t get hung up on a tiny speck, but focusses quickly and reliably on whatever I ask it to focus on. If I handle the settings right, it’s point&shoot and the image will be in focus. A cell phone cam simply can’t do this, and even good compact cameras are usually hard pressed!

And secondly, I put it on settings that made the task easy. Here’s what you should do when you expect to be around birds, little children or other potential victims for snapshots:

–  set you camera to time variable mode (Tv for Canon). In this mode, you manually select the exposure time, and the camera tries to make aperture work for an OK exposure.
– select a short exposure time (here 1/1000 s). The shorter the less motion blur will you see, both from your moving the camera and from the subject you photograph moving versus the camera.
– check against the background you expect to photograph: can the camera come up with a well-exposed image with the settings you chose? If not, consider increasing the ISO value. I went to ISO 400 here, as I know this value gives OK images. For the shot above ISO 200 would have been possible (the image is F13!), but I was also expecting to shoot in the direction of the FDR Drive, which is a raised road, so that it’s kinda dark under it. And very low F-stops are not that great if your subject may be close to the camera. You may end up having part of it sharp, and parts already out of focus. If in doubt, let the camera take photos that are underexposed. You can always brighten them up on the computer. You can’t un-blur an image blurry from motion!
– depending on what you intend to photograph, decide between continuous, single-shot or some other autofocus mode.

So, now the camera is set up properly. I am a bit of a snob with regards to battery live, as I can always afford to carry enough spares. Therefore, I’ve set my camera not to switch off properly, but only “slumber” when there’s no action. This means that it comes back, ready to shoot, within a split-second if I press any button.

The second thing I do to make sure I am always ready for a snapshot is NOT dangling my camera from a long strap around my neck. I have a wrist strap, a short thing that serves as insurance only, and hand-carry my camera whenever I am in situations where I might be tempted to a snapshot. This means two things: I do not need to reach for my camera when an opportunity for a good photo comes around, and I do not have anything else in my right hand at that time!

So, when I see something, up comes the camera, and as I bring it up I lightly press the trigger. This turns the camera on, and by the time my eye is on the viewfinder it is ready to shoot. SNAP!

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Another ringbill. Rats of the Air, some call them. Whatever – they are pretty ;)

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Oh, a final hint: do not run out your zoom lens to the max length! With fast moving things close to you, a long lens means you spend a lot of time waving it around trying to find and focus on your subject. Rather, keep it short, and if there is time run out the zoom after acquiring your “prey”.

Oh, and if you want to read this advice again, but coming from someone who (as opposed to me) really knows what he’s talking about, go visit www.tornadoropa.eu/, the blog of my esteemed colleague Christian Neumann ;)

 

Posted in Aves, Dinopics, Dinosauria, How to, photography, Theropoda | 2 Comments

Theropod Thursday 51: a Spechtschmiede

It’s been a very long time since the last Theropod Thursday post, and I promise to increase the posting ratio a bit in the near future. Theropods are cool – after all, they are the sister group to the really cool dinosaurs, the Sauropoda that include my beloved Plateosaurus! But they are also extra cool because we can study things on them we simply can’t study on any other dinosaur group. That’s because theropods are the sole branch of the dinosaur tree that is still around. People call them birds, because the necessity to give a name to these feathery flying animals is older than the realization that they are just a part of the great overall dinosaurs, but at the core of even the most stupid chicken or tiny songbird there is a vicious predator hidden away that’s just waiting for someone to beat up!

Also, many birds are surprisingly smart – think corvids, the crows, magpies and ravens clan! But even much more mundane birds have some neat tricks up their feathery sleeves.  Here’s one bird that does something cool that I have known about forever, that I have seen traces of, and that I have actually watched being done – but never so far been able to get good photos of.

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Great Spotted Woodpecker (Dendrocopos major). They are quite common in Germany, and there are always several around our house. The first three years after we moved here to Berlin there was a breeding pair nesting very close by; I found young barely able to fly in our garden. Now there is a breeding pair close by, maybe in one of the trees at the edge of the forest 50 yards away. The bird in the picture, however, lives in Stuttgart, where I happened to get the rare chance to take photos down into a tree from a window at my parent’s flat. That’s nice for several reasons, one being that for once I was not pointing my camera at a bright background, which made balancing the exposure easier than a typical against-the-sky shot would have been. Another reason is this:

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The bird was busy hacking away at something on the branch, and not with the typical staccato beating that’s related to communication or wood&bark removal (to get at food living in and under the bark or in rotting wood). It also wasn’t the powerful rapid-fire hacking that’s used to dig cavities for nesting. Rather, what I saw was a series of deliberate individual picks, the sound of which (a bit like single shots of a nail gun two yards away) was what had drawn me to the open window in the first place. In between the pecks the bird looked at the object it was hacking at, and twice picked it up and put it back down:

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A walnut! A tiny one, as the tree was not properly grafted and thus grows only tiny nuts. Still, the ratio of dead nuts from this tree is low, and there’s lots of nuts, so it offers a reliable source of food in late summer for all those capable of opening the nuts.

What I was seeing in action here was the use of a Spechtschmiede, literally a woodpecker smithy or anvil. There seems not to be a special English term for this – we Germans have a penchant for creating those combined nouns for technical terms. Interestingly, there’s not only no special term for this, but also no mention of the associated behaviour in the English wikipedia or anywhere else. Basically, a Spechtschmiede is a place where woodpeckers (and others) are able to lodge an object tightly so that they can peck it open. Woodpeckers use them for generations and carry items to them for quite large distances. In this case the bird was lucky, as the source tree of the nut itself happened to offer a good anvil, too. That’s not unusual for walnut trees, which have a very smooth bark on young branches, but a very rough and grooved one on older branches, and which when planted near buildings require regular generous trimming. Where you cut off a branch there is a high chance that over the years an area with ample broken bark is created at the base of the new shoots branching out. Also, such young shoots tend to be weak, bend down over time and rip open on the top side. In general, dead trees, breaks in the bark of living trees, but also has-been-dead-trees-turned-fence-posts are favourites for Spechtschmieden. Here’s what the place looked like after the bird had flown off.

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A typical case for a walnut tree: bottom left you can see where a bigger branch had been cut, and the young shoot was long and thin and directed into the crown (thus received little light – don’t be cheated by the sunlight in the pic; the inside of a densely grown walnut crown is quite dark!), and bent under the weight of its leaves. A tiny tear in the bark on top developed over the course of a few years into quite a large welt, which now can receive one of the small nuts and hold it fixed. It is also entirely possible that the woodpecker improved the quality of the Schmiede by removing some wood and bark to make it larger.

I’ll end this with a more typical perspective of a woodpecker: up into a tree with the bird theropod sitting on the stem. This one I took right in our front yard, where the artificially increased elevation of the yard versus the tree’s base plus the fact that statistically, there’s practically never anyone moving in our yard meant that the bird let me get very close. The situational awareness of many birds is quite astounding; they have a pretty good feel for what’s normal and what’s not normal and might mean a predator sneaking up on them! For example, the birds in our hedgerow will completely ignore us, grandma, our neighbours and our neighbours’ dog, but fly off immediately when a rare visitor like my parents come into the yard. This woodpecker quite clearly considered me and my approach route as mostly harmless, whereas a women coming down the sidewalk got it to fly off when she was still three time as far away as I was.

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Posted in anatomy, Aves, Dinopics, Dinosauria, Maniraptora, photography, Theropoda | Leave a comment

Measuring Giraffatitan’s limbs

TL; DR: loads cool pics of dinosaurs at end of post.

Recently, a colleague contacted me via email. He wanted to know if he could use the opportunity of his Berlin visit for SVP to climb up a ladder and have some fun with a tape measure and the limbs of Giraffatitan (ex Brachiosaurus). He was interested in the minimal shaft circumferences of humeri and femora, from which one can calculate a rough estimate of total body weight (see here). That’s been done for Giraffatitan, but apparently it all traces back to a personal communication from ages ago.

Well, access to the mounted skeleton is difficult under the best of circumstances. The platform the skeletons in the MfN’s Dinosaur Hall rest on is covered by a roughened surface that is rather delicate, so you can’t simply walk all over it. And walking in socks can be quite uncomfortable. And that is the least of your worries, if you want to access anything above elbow/knee level: Giraffatitan is rather tall, but placing ladders close to the skeleton is not that easy, given the touchy surface and the fact that there are railings and other skeletons and whatnot around it.

Additionally, the museum is open all week except Mondays, and we do not like having people climb all over the exhibits during visiting hours. Simply so that we do not give our museum visitors stupid ideas……. So you’re either limited to the early morning and late evening hours – if a curator is willing to come in really early or stay really late. Or you can show up on a Monday (obviously pre-announced) and have all day. Not just any Monday, though – occasionally, the museum is open to the public on Mondays, too. This was the case for the two weeks this fall due to school holidays.

Now add SVP to that (the annual meeting of the Society of Vertebrate Paleontology), with some 1300 crazy vertebrate palaeontologists descending on Berlin, and the corresponding demand to see the collections in the week before and after the meeting….. yes, there are quieter times to get into the Dinosaur Hall than those two weeks this fall. And the SVP meeting week’s Monday happened to be the first Monday after two weeks of the museum being open, which means that it was the Monday the cleaning crew really needed to get in there. Oh, and by sheer luck, it was the Monday on which an orchestra had the hall for a rehearsal for a concert later that week. It never rains, but it pours.

So, the Monday right before SVP was a total no-go. I couldn’t help my colleague either, even though quite obviously there is a bit more wiggling room for employees than for externals. But even if there had been a short time window for me to access the skeleton with a ladder, I was busy as hell with other stuff – SVP in Berlin meant colleagues coming over, and two came over early whom I really needed to sit down with and talk fossils. More about that later; it’s cool stuff we work on together.

However, when I mentioned the cleaning crew, I talked about the regular dusting and mopping. There’s more to keeping the Dinosaur Hall clean, including a yearly window cleaning. Yes, yearly is enough, because all the windows are indoors windows, in the walls between the Dinosaur Hall and neighbouring exhibition and collections rooms. But to access them one needs a lifting platform, a hydraulic hoist. And – guess what? – the annual window cleaning was up for the Monday after SVP. And the window cleaner is a really nice guy who doesn’t mind sacrificing part of his breaks in order to lift a crazy palaeontologist or two up into places they normally can’t get to.

So the next Monday, while I was horribly busy already managing (link to German news video) the Fraunhofer IDG automated 3D scanning test at the MfN, my colleague Matteo Belvedere and I used the lifting platform to get some measurements of the limb bones. And we used the opportunity to also take a lo of photos of the dinosaurs from viewpoints you usually can’t get to. Previously, I had used a ladder or took photos from 1st floor windows, but this was MUCH better :)

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Two photos of me in action, both taken by Matteo Belvedere. Later, we switched roles with him up there and me on the ground, running off to run the CultLab3D thingy.

So, what about getting tight and cosy with Giraffatitan? Below you can see Matteo with the humerus, which was marked with tape in the three places we measured. While he was up there measuring I was standing ready to make notes. Note the scale bar we fixed to the vertical strut! Once I finish the photogrammetric models we can use this 1 m bar to scale them correctly.

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Obviously, we now need to do some really hardcore science with that stuff, too – for now, I’ll just show you some of the pics we took.

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A pterosaur’s view of Dicraeosaurus, slightly anterior. In this view it is really clear how 2D the sauropod is: it is high, it is long, but it isn’t wide!

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A view from further away and to the left back. Note how the narrow silhouette get additional height from the tall neural spines.

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And a look along the side of the back, showing off those spines to full effect. Giraffatitan‘s puny ones in back.

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Riding Diplodocus this is the view you’d get at your 12. Note the used and unused rings for hanging the neck from the ceiling – obviously they are not supposed to take the full weight, but only part of it – and in the process dampen any oscillation of the neck in case it gets bumped.

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Reverse view.

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A bit further back you can see the entire trunk and neck. As with Dicraeosaurus, this view highlights the extreme length versus a very reduced width. A legged snake? Not that far off.

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Over to fat-bellied Giraffatitan! The wide transverse processes of the dorsals make this animal look like a snake on legs that just had a huge capybara for lunch. Or, rather, an entire elephant.

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A closer view…..
and now for something completely different:

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Tiny spiky Kentrosaurus! It is so small that a 4 m high ladder allows taking similar pictures. Thus, enough with ornithischians already, back to more chunky stuff:

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Better! This view puts the nasty bird-hipper into the correct context with regards to dinosaur size! Yes, you can see that Tendaguru’s monster hedgehog – capable of 60 mph strikes with it’s 8x-basball-bat-weight tail tip – is not even half as long as the tail of Diplodocus. But then, isn’t it the tiny dogs that bite, making up their puny size with extra aggression? ;)

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In fact, some sauropods did not have such huge, long tails! Check out the tail of Giraffatitan compared to Dicraeosaurus‘ and Diplodocus‘ tails and (part of Kentrosaurus) in this pic! It is still much longer than all of Kentrosaurus, but compared to the more regular, short-forelimbed and level-backed sauropods around it Giraffatitan has a short and slim tail.

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Here, you can appreciate that it still is quite a significant tail. But compare the width of the tail to the width across the hips and across the middle of the trunk. Much slimmer tail than either Diplodocus or Dicraeosaurus. In fact, it looks almost mammalian, somewhat similar to a kangaroo or even less substantial.IMG_9849_sm

To reiterate those points on narrowness of the trunk and ratio of tail to trunk width, here’s a view along the trunk and tail of Dicraeosaurus, taken from nearly directly above the neck base. See how the tail base is actually wider than the width across the tops of the ilia?

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And more fat Giraffatitan rump. This view is really weird, but it does show up three things quite nicely: the wide back, the narrowness of the neck and the U-shape of the shoulder girdle.

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Wwith the left forelimb in view you can’t see the back as well anymore, but the abrupt vertical front end is nicely on show. Compare to the horizontal orientation of a diplodocid – the long-armed, tilted-back sauropods are really weird!

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A bit higher up we can get a total view. The perspective distortion makes the head look not ridiculously small as it really is, but it does highlight the stick-like neck.

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OK, wrapping up with a portrait or two:

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antero-lateral and very slightly ventral from the left and

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lateral and very slightly ventral from the right. Say cheese!

 

Posted in 3D modeling, anatomy, Conferences, Digitizing, Dinopics, Dinosauria, Giraffatitan, MfN Berlin, photogrammetry, Sauropoda, Sauropodomorpha, SVP 2014 | 9 Comments

SVP2014 – the wind-up

Today is Sunday, November 2, 2014, which means that the 74th Annual Meeting of the venerable Society of Vertebrate Paleontology will start in less than 72 hours. Hashtag #SVP2014, by the way. Depending on how you count the public lecture on Tuesday evening, things start day after tomorrow in the evening or one day later – at 8 a.m.!

As is usually the case, the last minute preparations are crazy, but this year the crazy is extreme. SVP comes to Berlin, and I am on the Museum für Naturkunde’s Host Committee. That’s added stress to the usual, with the latter being two posters (one as first author, one as second who has to do most of the work due to our division of labour on the project). The first is finished, the second still in the making. Ugh! But both posters deal with cool stuff, and I am very much looking forward to my colleagues’ reactions. Obviously, I’ll post about both here, too.

What else will be up this week? For one thing, EVA Berlin 2014! Where I will give a talk on Wednesday morning (yes, ideal timing *sigh*) on how a lack of standards for documenting how you 3D-sscan something means that nobody documents anything, which invalidates many 3D-models for research. Ugly topic, and I hope my talk will be a sort of wake-up call for many players in the field.

And then, obviously, there will be a bit of free time to talk to all those many friends and colleagues I only see sporadically. And there will be beer, and dinosaur talk, and anecdotes about crazy professors, crazy students, crazy excavations – i.e., there will be vertpalaeo-fun :D

So, back to poster making…. here’s a sneak preview.

tail recon

Photogrammetrically derived 3D model of the tail of Citipati osmolskae with partial muscle reconstruction.

Posted in 3D modeling, Berlin, Citipati, classic CAD, Conferences, Digitizing, Dinosaur models, Dinosauria, Maniraptora, Oviraptorosauridae, photogrammetry, SVP 2014, Theropoda | 3 Comments

Photogrammetry tutorial 7: multi-chunk project handling

In palaeontology (and many other disciplines) you often deal with specimens that you wish to capture in what I term “720°” – all around (360° around it for the vertical axis), and bottom and top, too (360° around the transverse axis). Because physical objects tend not to float stably on their own in thin air, this usually means taking one or more sets of photos, flipping the specimen over, and taking more photos. As a consequence, you will capture a lot of background in your photos that moves in relation to your specimen between the photo sets, and this background can then cause trouble when you build the model.

As I keep pointing out, e.g. in that paper on photogrammetry I recently published with Oliver Wings, it’s all no problem if your background is featureless. The software will not find points and ignore it, so you can simply toss all your photos into one chunk and be done with it. Neat trick – but it needs to work! (bonus points if you recognize the quote)

But what if a featureless background is not possible? Let’s say you’re dealing with a sauropod femur! It is highly unlikely that you can just lug it around and position it at your will in front of a greenscreen. Normally, you’ll be lucky to be able to lean it against a wall in various positions (long bones should be kept upright if possible and if sturdy enough, because that minimizes the bending moments on the shaft). Or it will have to be laid flat on the ground or a table, with some styrofoam or sandbag supports. In either case, there will be lots of stuff very close to the bone that you can’t turn into something the photogrammetry software doesn’t find points on. Like this:

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The proximal end of MB.R.2694 (field number ST 291), a Giraffatitan brancai femur from Tendaguru, Tanzania, today to be found as a kind of stumbling block in the MfN’s famous bone cellar. This little chunk weighs (guestimate) 40 kg, so it really is not something I want to toss around a lot. Additionally, as hard as the fossil is, its own weight is sufficient to cause local damage when you put it on something hard, like the floor. So, the black mats you see in the photo are a must, and they are structured and dirty enough to give plenty of features for the software. Plus, they are necessarily so close to the bone that whatever I do, they will be in focus on the images. Tough luck.

So, what to do? I took two sets of photographs, one from each side, making sure that I get good images from the sides. That’s a thing to keep in mind: do NOT take a lot of photos from above, but rather lots from the sides. This gives you a lot of overlap near the margins of each model, so merging them into a complete model is easier.

To create a complete model I could now mask all the background in all the photos, toss them all into one chunk, and have the software align them all. Good, but not perfect, because masking is a lot of tedious manual work. With a smooth, uniform background it would be much easier, because the magic wand tool would make masking easy, but that’s not to be in this case.

Thus, I had Photoscan calculate the alignment and dense cloud for each set of pictures, which is a piece of cake. A key point: I did so using two chunks in one overall file. Each dense cloud got trimmed down to the bone, the equivalent of the masking I could have done on the images, but a lot faster! Basically, I rotated the models so that I was looking at the points representing the ground in perfectly lateral view (so they form a line on the display, not an area), then used the box selection to cut them away. Additionally, I cropped the rather rough margins of the bone model. All in all this takes about a minute or two per model, compared to some 15 or more minutes for masking the photos. And the more photos there are, the bigger the difference.

So, there I was with two half-shells of the bone, and in need of a neat way of matching and merging. And so the fiddling started: In both chunks, I went looking for three easy to find spots on the bone photos, and placed markers on them. In doing so, I made sure that the same spot on the bone got identically named markers in each chunk. That’s easily achieved by creating the markers in the same order, so they get named “point 1″, “point 2″, and “point 3″ automatically. It can help to place them in one chunk, then take a screenshot of a photo on which they can be easily seen (or of the dense cloud), and paste it in a graphics program. That way, you can go back and forth between the photos of the other chunk where you need to find the same spots again and an image of where they are without having to search around in Photoscan.

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Here’s an example of such a screenshot, you can see the green marker I put on a dark colored spot that I believe is easy to find on other photos.

In the end, the first half of the model, the first chunk, looked like this as a dense point cloud:

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You can see four markers for alignment here. What’s with all these “target” markers? That’s for another post…. For now, it is enough to see that I spread the makers around the circumference of the bone, which means that aligning by them will not leave any end of the bone far away from a marker and thus susceptible to errors caused by tiny inaccuracies far away that simply add up. Misalignments typically mean that things are off by a tiny angle, that the two triangles formed by the two sets of three markers do not match perfectly – and distance from the marker points thus means that the tiny angle inaccuracy adds up to a lot of local separation. So it is a good idea to use more than three points, and to place them at the corners of your specimen.

Next up, I let Photoscan align the chunks based on the markers. If I am lucky, all comes out perfectly. Then, I turn “Show aligned chunks” on, so that I can see what the two dense point clouds together look like. This tells me not only about the quality of the alignment, but also if I cut away enough of the rims of the models.

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Here, you can clearly see how well my markers matched (by the fact that you can’t see that there are two of each marker), and how the two models match nearly perfectly. There are a bunch of erroneous points that needed trimming (easier done in the individual chunks BEFORE merging), but I was rather lazy – Photoscan Pro is very good at ignoring those little floating islands you see.

And here’s now the mesh in all its glory! 28.25 million polygons :)

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Obviously, I photogrammetrized the other parts of the bone, too, and will digitally repair the entire bone. But that’s for another post.

Posted in 3D modeling, Digitizing, Dinopics, Dinosauria, Giraffatitan, How to, MfN Berlin, photogrammetry, Sauropoda, Sauropodomorpha | 2 Comments

A more detailed take on Pinus macro photogrammetry

Previously, I described a few photogrammetry tests I did with a macro lens, but I didn’t go into the technical details much. Some people have asked me how exactly I took the photos, so here’s the detailed description.

First of all, let me point out a few things that make photography with a macro lens different from normal lenses. There is, for one thing, not really a difference between focusing and zooming with a macro lens. As you focus, the lens extends or retracts and the angle of view decreases or increases. Therefore, you can’t just select your view, then focus – you may end up with significantly more or less on your photo than you intended.

This means that I often find it easier to just leave the camera alone, and move the specimen into focus (or move the entire camera), rather than fiddle with the focus/zoom. That’s easily achieved if the camera is on a tripod, and the specimen is placed on a support that itself slides easily on the table. At the office we have Formica tables and Formica cupboard boards, so I put my turntable on a board which slides well on the tabletop – I can pull or push it with one finger while sitting next to the camera on its tripod. Or, in the case of specimens that do not need to be handled very carefully (such as a modern pine cone), I simply move the specimen around as I please.

In the case of the fossil and extant pine cones I posted on recently, there was no need for a turntable. Nor for a sliding support. I simply manually rotated the specimen the way I wanted to.

Another difference when using a macro lens is the background. Normally, you should try for a undefined background of very even colour, which is either completely ignored by the program, or can be masked using a magic wand tool. Using a macro means that the entire background is automatically totally out of focus – even if you in fact used a Persian rug the program will not find any points. Makes life a bit easier :)

So, let’s have a step-by-step playback of what I did, using this detail model of one cone scale I made.

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  1. place cone on table
  2. place camera on tripod, with 50mm macro lens
  3. extend macro lens all the way
  4. shift tripod back and forth until cone is roughly in focus (checked via LCD screen, you can also peek through the viewfinder)
  5. select f-stop (high, in this case 14, the maximum the lens will do) for a good depth of field
  6. shift (carefully!) the cone until the scale is perfectly in focus (I zoomed in on the LCD for this)
  7. use the preview button on the camera (usually hidden and unlabelled under the lens attachment on the left hand side) to see what the real depth of field is like
  8. take photo by releasing the shutter via a soft touch on the touch screen (alternatively, us a remote and ideally a mirror pre-release)
  9. turn cone a tiny bit, check focus
  10. repeat 8 & 9 until target sufficiently covered; if necessary alter tripod height for additional angle.

DONE!

Posted in 3D modeling, Digitizing, photogrammetry, photography, plants | 1 Comment

Addendum to SV-POW!’s “SO close”

Mike Taylor recently sent me an email asking for a larger version of a figure I once published in a book chapter. Naturally, I promised to send it to him. But, being away from my computer, with email available only via my phone, I couldn’t send it right away. And promptly forgot.

Thus, Mike had to go ahead and publish the planned blog post without my figure. Here’s the link.

Now, I finally remembered, and through sheer persistence managed to find my original submitted files in my sorry heap of data backup. Here goes – starting with a small part of a big figure, but a part that shows nearly all that needs to be shown to make Mike’s point:

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This is a CAD model of Diplodocus I built long ago. It is rough, and takes air spaces within the body into account in a fairly rough way, but – and that’s a big issue with dinosaur models – there is enough soft tissue mass modelled onto the tail, especially its base.

I used a rather complex multibody dynamics program to do all kinds of shit with the model, but first of all I had it calculate the position of the center of mass (COM). I won’t bore you with the full story; suffice to say that I put a small white dot in the figure to show the result.

Yes, the COM is that close to the hind limbs.

Now, one model alone would be bad practice, so I ran a bunch of variations. As long as my assumptions about density and volume stayed reasonable, the COM stayed really close to the hind limbs.

The next thing I did is look at another sauropod, one with a quite different overall look: Giraffatitan. Unsurprisingly, a much shorter and thinner tail and much longer and stouter forelimbs meant the COM came to rest somewhere else entirely. You’ll see in a moment…..

Then, I went ahead with the work I had set out to do originally: look at the ability of the two sauropods to rear into an upright stance. Not just rear up and come back down with a thump! right away thanks to gravity, but the ability to adopt a bipedal pose that puts the head high up for feeding on large trees. A position that must thus be held for quite a while, which in turn requires that it is inherently stable, that you can get there easily, and that you can get back down speedily, too.

Why inherently stable? Well, think of a ladder and how you behave on it. When you are high up on a ladder you will either be careful and restrict your movements, or you will fall. If you’re positioned so that the tiniest motion unbalances you, there is no way you can do useful work for a prolonged time. For a sauropod poking its head into a mass of branches, grabbing them and pulling vigorously most certainly was an activity that required it to be posed so that it did not constantly worry about toppling over.

Why easy to reach? Because if it is really hard to reach, harder to reach than a similar pose is for an elephant, then the effort to get there makes it so difficult to use that it is no good for regular behaviour. Sure, you can do the weirdest stuff, like walking on your hands. But every day?

And why is getting back down so important? That is in fact the simplest point: if a big theropod ambles by, you do not want to spent several minutes carefully letting yourself back down into a pose in which you can deal with the threat.

So, I modelled on, and here are the figures as the appear in full in the article:

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This is one way Diplodocus can easily get into an upright position. By pushing its butt backwards a bit, flexing the knees a tiny bit, the COM comes to rest right above the hind feet. Now, all it takes is a very slight rotation in the hip joints, easily achieved by the strong caudofemoral muscles of both sides acting together, and the entire animal minus the legs starts rotating. When the tail hits the ground (slowly and softly) it is time to stop. There you go – feeding height doubled, or tripled, depending on how mobile you believe the neck is in extension.

Back down is also easy: just a push forward with the forelimbs to get some momentum, and slowly relaxing the caudofemoralis muscles and the animal is back down in a few seconds, ready to fight or run away (well, amble away that is).

In between, all is well balanced, because the limbs that can counter a slight imbalance easily, the hind limbs, are attached to the body at roughly the height the center of mass is at. Motions of the COM can therefore be countered quickly, before the shit hits the fan. If there was a long lever arm between the point where the animal can influence the trunk’s position relative to the limbs and the COM, it would be a very difficult task to fiddle things into equilibrium.  Additionally, in lateral view the angle between the line connecting the edge of the support area and the COM and the vertical through the COM is larger the further down the COM is, meaning higher stability as well.

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Giraffatitan left, Diplodocus right. Guess who’s standing stably, and who’s more like a drunk on a ladder?

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Various poses I tried for Giraffatitan. The COM is shown by a tiny sphere in the middle of the body. High, high up, far away from the hip joints. This doesn’t look good for balance. Add to that the emaciated tail base and thus weak caudofemoralis muscles of Giraffatitan, and you get an animal that was really bad at rearing up and staying there. Diplodocus, on the other hand, was (I concluded) well capable of rearing up and staying there for an extended feast.

So, what does that mean for Mike’s bipedal Diplodocus? For one thing, the COM is in the right place. The other thing is that a bipedal pose would have required somewhat flexed hind limbs. Not something I guess the animal did for a long time. And there are lots of issues with walking bipedally, not least the issue of retaining steering. Obviously, one can use various tricks for keeping the body position stable, but why bother if there is an easier way? Even a paltry 10% of body weight supported by a forelimb placed well forward of the hind feet would help a  tremendous lot with going in the actual direction you want to go. So bipedal Diplodocus yes, but not regularly!

Oh, here’s the full citation for the paper; email me for a PDF.

Mallison, H. (2011). Rearing Giants – kinetic-dynamic modeling of sauropod bipedal and tripodal poses. Pp. 237-250 in Klein, N., Remes, K., Gee, C. & Sander M. (eds): Biology of the Sauropod Dinosaurs: Understanding the life of giants. Life of the Past (series ed. Farlow, J.) Indiana University Press.

 

 

Posted in 3D modeling, Biomechanics, classic CAD, Dinosaur models, Dinosauria, Diplodocus, Giraffatitan, Sauropoda, Sauropodomorpha | 11 Comments