The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for June, 2009

Stereo Aerial Photography and False Color

Posted by Tom Benedict on 26/06/2009

It’s always fun to see what can be done with aerial photography: large stitched panoramas, Photosynths, 3D modeling, 2D maps.  One technique that KAP lends itself to particularly well is stereo aerial photography.

With an airplane, stereo photography is typically done by pointing the camera 90 degrees to the direction of flight, and taking a succession of pictures as the airplane flies across the landscape.  Careful choice of frame rate, airspeed, and altitude yields good results.

With a kite, a similar technique can be used:  Point the camera 90 degrees to the kite line, start the shutter going, and carefully walk backwards.  The kite will quickly settle into a stable flight angle with the increased apparent wind speed from the walking, and a nice steady stream of pictures is the result.  This is an example from a recent flight over the contact between two lava flows on the Big Island of Hawaii:

Lava Stereo Pair - True and False Color

The top two pictures are the natural color images as they came off the camera.  The bottom two require more explanation:

Another technique I’ve used with aerial photography is to apply false color techniques to boost certain details, certain colors, or to change the contrast of the image so that particular features will catch the eye.  Most of my experimentation along these lines has been done using an image manipulation program called ImageJ, and a plug-in called DStretch.  ImageJ is a general purpose image manipulation program written in Java.  DStretch is an implementation of the principal component algorithm for contrast stretching that was written by Jon Harman.  It was originally written for bringing up faint details in pictographs, but it has proven to be useful with aerial imagery as well.

In order to get a sense of scale of the image, the thin yellow line in the top pair is a three-section 25′ painter’s pole thath as been collapsed to its shortest length.  My flying partner and I were using it to lower a camera into one of the holes we found in the lava in order to explore the inside.

This probably isn’t the best example of either technique, but it’s the first time I used them in combination.  At some point I’ll write a more in-depth article about building stereo pairs, and a second article about the use of ImageJ and DStretch with aerial photography.  In the meanwhile, enjoy.

Tom

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KAP at Hawaii Volcanoes National Park

Posted by Tom Benedict on 24/06/2009

For a long time, it has been a dream of mine to do KAP at Hawaii Volcanoes National Park on the Big Island of Hawaii.  I’ve wanted to do this ever since I first suspended a camera from a kite line.  But when I first asked about the possibility of doing KAP in the park, the answer was a very definite no.  I’ve been trying to teach my kids to get permission first, and not have to ask for forgiveness later, so even though it hurt at the time, I followed my own advice and remained patient.  Apparently that approach worked, because I’ve now been invited to do KAP in the park.  Not only that, but the rangers who invited me would like me to fly over the active vent in Halemaumau Crater.

Gulp!

It’s one thing to have a dream, especially one that’s been filed in the pipe dream category for so long I never thought it would happen.  It’s quite another to have an invitation, and to need to set dates and times. In the hopes of getting some feedback from someone who had done similar work, I posted all this to the KAP Forums.  The responses I got were very helpful.  Aside from the obvious safety issues, there were a couple I hadn’t thought about:

  • Because of the distance from the surface of the lava lake, heat damage isn’t likely.  Even with the kite line at zero degrees off the horizon, it’s several hundred feet down to the lava lake.  Not a problem.
  • Chemical attack is an issue, but probably not a big one.  Nylon and polyester, the two materials present in my kite sails and my kite line, are prone to attack from sulfuric and hydrochloric acid, but only at concentrated levels.  One of the folks on the KAP Forums said that the UV damage from sunlight exposure would probably outweigh the chemical damage from exposure to acid gases.  Besides, the acid gases are concentrated in the plume.  Keep the kite and line out of the plume, and all’s good.
  • The aluminum parts of the rig are anodized, and shouldn’t be prone to chemical attack.  The anodizing process uses sulfuric acid to grow the oxide layer on the aluminum, so further exposure shouldn’t cause issues.  Anodized aluminum is far more prone to chemical attack from caustics.
  • The camera electronics may be damaged from even mild acidity, so I’m planning to bag the camera.  Optical coatings may also be prone to attack, so I plan to use a sacrificial UV filter over the lens.  If it looks like the coatings are being damaged, I’ll have to re-consider my approach.  But the camera should remain unharmed.

There were other safety issues raised about the actual kite flying.  This is one area that will take some direct experimentation since there’s no way to predict what any of these will mean:

  • The Halemaumau crater is down inside the Kilauea crater.  The crater walls of the Kilauea crater are about 330′ high upwind of Halemaumau, and are about 10,000′ away from the launch site.  This presents about a 30:1 distance:height obstruction up-wind.  This is at the edge of what is acceptable for placement of meteorological equipment.  There shouldn’t be a significant wind shadow at that point, but there may be turbulence that will have to be taken into account.
  • There are two ways that heat can be transported from the lava lake into the atmosphere.  The first is convective.  The second is radiative.  Both will be present.  Convective heating should be confined to the gas plume coming out of the vent.  So long as the kite isn’t in the plume, it should be ok.  Radiative heating may cause issues even with the kite well above the plume.  One KAPer who spent time flying a KAP rig over a volcano in Vanuatu said he experienced massive thermals that fell off as the kite slid sideways away from the vent.  In the end his kite inverted and flew into the ground, despite all his efforts to get it back under control.  This must be tested before a rig is hung from the line.

And finally there are the personal safety issues to be dealt with:

  • Find out what personal protective equipment should be used (hard hat, steel toed boots, respirator, etc.)  Most of these I use regularly at work, so it will be a matter of finding out which PPEs are necessary, and under what conditions they’ll be needed.  If it’s necessary to use a respirator, I’ll need to shave.
  • Do not clip my winder onto my person at any time.  This has come up on a number of flights I’ve done that involved flying out over cliffs.  So long as there’s a cliff, don’t clip on.  Period.  With a winder in-hand, it’s always possible to let go.  With the winder clipped on, a sudden gust and an over-sized kite could drag the kite flyer off the cliff.
  • Find out how long I get before we have to head back.  This last one will determine what approaches I take, and whether I can get all the testing done up front before the photography begins.

All in all, it looks like I’ll need at least one dry-run test without a load, and then with an 860g water bottle dummy weight to match the weight of my rig.  If there are strong thermal effects, or if I can’t successfully keep the kite, line, and rig above the plume, I’ll have to pull the plug on the flight.

On a better note, the pictures themselves should be quite good.  I was concerned that the lava lake would be vanishingly small in the picture frame: a spot of orange.  But at a 40 degree angle between kite flyer and camera, the spot should be just under a thousand pixels wide.  At a 10 degree angle, it should be just under two thousand pixels wide.  Aiming will be critical, but the results should be worth it.  The lava lake should dominate the frame.

Which brings up the last point:  How to actually photograph the place?  I’m planning a couple of approaches:

  • Open with ortho autoKAP using an intervalometer set to a five second interval.  I get roughly 400-600 images per chip.  This gives me between half an hour and 45 minutes to fly a camera over the crater and get orthogonal pictures.  During this flight all my attention will be on the kite and camera, without having to worry about a remote.
  • Next do a second autoKAP flight, this time using the prototype controller a friend and I have under development.  This is where we’ve been putting the bulk of our efforts for how we like to capture a place with a camera, so it’s the best approach I could take for this part of the photography.  With the frame rates we’ve been getting, this is about a 30 minute flight before the chip fills up.
  • Finally, if time allows I’d like to do remote controlled KAP.  I would only do this if time allowed, and if the flying conditions were such that I felt comfortable splitting my attention between the remote and the kite winder.  If in doubt, this part of the program would be cut.  But this is the one I’m most looking forward to.

But in a larger sense, what I’d like to do is demonstrate KAP for the folks at the park, and show that it can provide access to vantage points that aren’t otherwise available, that it can be done safely without significant risk to personal safety or to the safety of the park itself, and that it offers the park a very real, very available method for close distance remote sensing.  In a perfect world, my follow-up to this would be to build out a set of rigs for the park rangers, and to help them get started in the world of KAP.  I think it has a lot to offer.

And I can’t wait to see how the pictures turn out!

Tom

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Photosynth from a Kite – Take II

Posted by Tom Benedict on 16/06/2009

Last Saturday I returned to Kiholo Bay to make another attempt at taking a set of pictures specifically for making a Photosynth of the area.  Unfortunately the wind was much lighter than I was expecting, so the first hour was spent almost entirely with my rig on the ground.  The 6′ rokkaku I use for the bulk of my light wind flying stayed up the whole time, but the launch of the camera itself was delayed.  The wind did pick up later, however, and I got several hours of flying time in before the sun and the fact that I’d missed lunch by over an hour and a half finally drove me away.

Because of the shorter time table, I didn’t get to do everything I’d planned.  The first four passes, in particular, had to go.  But in the end I think this worked well since I’m not convinced they were necessary, or would even make for a good synth in the end.

Instead, I used a prototype autoKAP controller to take almost all the pictures used in the synth.  The prototype is loosely based on some code I had written for the Atmel ATmega168 microcontroller a few years ago, but it’s being developed for a much better, much more fully-featured controller that’s currently in the works.  Unfortunately I’m not at liberty to speak further on that subject, but it’s moving along.  Soon… soon…

The idea behind it is very similar to the idea behind the Gigapan camera controller:  Orient the camera through a series of tilt and pan angles so that the resulting images have good overlap, and can be stitched together into a large panorama.  Only in the case of Photosynth the idea is to get good overlap between pictures so that the viewer can look in any orientation from that single vantage point.

But this is where the requirements for Photosynth and the requirements for a panorama diverge:  In order to make a good synth, it’s not enough to have good overlap with adjacent images.  Images of the same part of the subject also need to be taken from different vantage points in order to provide parallax.  It’s this that lets Photosynth generate its 3D representation of the scene, and find all the spatial relationships between the individual frames that go into the synth.

In the case of the Kiholo Bay Inlet I started at one end and flew the camera up to a good working height, about 200′ above the ground and almost on the other side of the pond.  Once there, the camera took a hemispherical set of images, three images high and about 12 images around.  As the camera panned around to its starting point, I walked about 50-100′ along the pond and stopped so that another hemispherical set of images could be taken.  This process was repeated until I reached the end of the pond.  About this time the camera’s chip filled up, so it was time to switch.

The second pass was made at a much lower altitude, closer to 75-100′ above the ground.  I also moved back as far as I could toward Kiholo Bay, putting the camera over the near shore.  A similar set of hemispherical images was taken at several locations along the near shore, and at the lower altitude.   And again, about the time I finished at the last position the camera’s chip filled up.

All in all I took over 1200 images this way.  After rejecting images with large percentages of water, the few blurry frames that happened when I was taking in or letting out line, or frames that couldn’t be tied directly to another frame, I was left with about 860 images.  These went into the Photosynth software.  The result can be viewed here:

Kiholo Bay Inlet – Kite Aerial Synth

One surprising outcome of this set of pictures is that it did a better job of covering the area than the set I took the previous week where I was in manual control of the camera and rig.  This is mostly due to the brute-force effectiveness that autoKAP has of taking so many many pictures.  Even as I was going through the pictures and culling out the ones that wouldn’t work well with Photosynth, I was taking notes on which photographs to revisit from the standpoint of traditional still photography.

Paddlers Heading Out

There was a lot more activity at the pond this weekend than last, so I had some opportunities to photograph different groups of people kayaking, paddling, swimming, or in my case, flying a kite.

Wainanali`i Pond Mouth

I also had my first really successful picture of the mouth of the inlet where it connects to the sea.  The clouds and sun cooperated this time, and I was able to get some pictures that did a good job of indicating the depth of the water, the width of the channel, and the reason why tidal flow really doesn’t cause a great deal of mixing in the water.

Honu Swimming I

But my favorite part, by far, was the honu.  And this is where the limitations of autoKAP make it less than ideal.  Because the camera is moving on its own, it’s difficult to do intentional wildlife photography with autoKAP.  I’ve done plenty of serendipitous wildlife photography this way, but without some ability to aim the camera in a particular direction, making pictures of particular subjects is difficult. The handful of honu pictures I made that day were all done with the rig and camera under manual control.

Most animals, sea and land alike, don’t spend a lot of time looking straight up.  Once the kite is in the air and the camera has been lifted to a good working height, for all intents and purposes it’s invisible to creatures on the ground.  In this case the camera was roughly 20′ off the ground, and because of the line angle I was about 40′ from the honu.  As far as it was concerend, I wasn’t there.  I’m often asked if I use some sort of a video feedback system to see what the camera sees.  I’ve been tempted, but so far I haven’t strictly needed it.  A KAPer can develop a sense of where the camera is and what it’s pointing at, even without video feedback.  In this case, once I knew the kite and rig was flying well and wouldn’t do anything unexpected, I positioned the camera above the honu and off to one side, and started taking pictures as it foraged on the rocks.

At some point I’ll return to Kiholo Bay just to take pictures of the honu.  But this trip was all about the synth.  Having it come out as well as it did really put a smile on my face.

Tom

(Note:  I have been calling the Kiholo Bay Inlet by the wrong name: Wainanalii Pond.  This is how it is commonly labeled on maps, hence my mistake.  But during a conversation with Bobby Camara, he pointed out that the village of Wainanalii is a two hour canoe ride to the north of Kiholo Bay, and that it was destroyed by the same lava flow that created the inlet.)

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Photosynth from a Kite

Posted by Tom Benedict on 12/06/2009

Photosynth is a web-based program that lets you assemble a number of pictures of a single subject and combine them into a 3D navigable scene built from your pictures.  It doesn’t create a 3D model, but it does arrange all the still pictures so that a viewer can move around the scene and see each picture from the angle it was taken at.  People have made some truly phenomenal synths of a range of subjects.  It’s well worth a visit.

Of course I had to try it from a kite.  I pulled out the raw pictures from a couple of previous KAP outings and came up with some decent synths:

As it turns out KAP is very well-suited to making synths.  To make a well-connected synth requires a number of pictures, capturing the subject from multiple angles.  By its very nature KAP takes pictures from a variety of angles and positions.  I’ve made a number of KAP flights specifically to generate a synth, but for the most part they have been the serendipitous outcome of a flight otherwise geared toward still photography.  These are some of the better ones, taken with Photosynth in mind:

But there are times when things just plain don’t work.  Photosynth doesn’t like water because picture-to-picture, water varies, so there’s nothing really to tie one image to the next.  I’ve had some oddball results from synths that included large bodies of water.  Special techniques need to be used.

One in particular, a synth from a set of flights over the Kiholo Bay Inlet, had a number of problems with it.  Discussion with the Photosynth development team led to the conclusion that it was the water, the lack of a good pool of overlapping images, and just a poor sampling of the area in general that led to the problems.  I’m planning a second trip to Kiholo Bay to try to take a better set of pictures for making a synth there.

An added side-benefit of Photosynth is that in the process of tying all the images together, it creates a point cloud indicating which points in the images tie to points in other images.  A good set of high detail pictures can generate a very dense point cloud.  It’s possible to extract the point cloud and use it to create a rendered 3D image of the scene.

Photosynth Surface Extraction

This model was generated using the point cloud from the Waikoloa Archaeological Site in the list above.  Considering there was no GIS data, no ground control grid, no real spatial or metric information of any kind, it’s remarkably accurate.  Though it also shows some of the problems with this method.  The road surface in the cut is quite smooth, so there are patches without any points in the point cloud.  These show up as gaps in the surface.  There are also some very rough areas of terrain on the same size scale as the mesh spacing in the surface.  This caused issues as well.  All in all, though, it’s a neat technique.

But the flight at the Kiholo Bay Inlet has been the most instructive of all.  Because of the sparse data set, it really pushed the Photosynth algorithm, and indicated some new approaches that would help with making synths from a kite.  When I go back, the plan is to:

  • Get the camera airborne over the water, and walk the length of the pond while pointing it at the far shore.  Take pictures every five seconds to get good overlap between frames.
  • Turn the camera 180 degrees and walk back the other way, taking pictures of the near shore.  Again, take pictures every five seconds to get good overlap between frames.
  • Turn the camera 90 degrees to face down the length of the pond, and tilt it down until the horizon is just out of view.  Walk the length of the pond taking pictures every five seconds.
  • Turn the camera 180 degrees to face back the other way down the length of the pond, and walk back taking pictures every five seconds.

At this point there should be enough images with enough overlap to make very good references for both shores, and enough images to tie the two shores together into a single frame of reference.  From here on out images can be taken to place them inside this frame of reference:

  • Get the camera to a good high altitude over the far shore, 50-100m or so, and begin taking hemispherical panorama sets.  Start at one end of the pond and work toward the far end, moving maybe 10-20m between sets.  This should provide a view in every direction from any point above the pond, and the high altitude should let the straight-down ortho shots tie into the frame of reference we generated above.
  • Make a second pass at a lower altitude with the camera over the near shore.  Similar spacing between hemispherical sets.
  • Finally, switch back to manual control and take detail sets of some of the features at the site (resting turtles, the turtle observation station, various rock features, key features along the shoreline, etc.)

At the end of the day, the pictures need to be culled to remove:

  • Blurries – Photosynth doesn’t work well with them, and they’re no fun to look at.
  • Bad exposures – For the same reasons as above.
  • Frames with more than 50% water in them – This was one of the key issues with my first attempt at this site.  Photosynth doesn’t deal well with water.

Finally, put all the images into Photosynth and see what comes out!

It’s a long process, and the number of images involved can be quite large.  Each set of images for the hemispehrical panoramas is 40 to 48 frames.  Ten spots along the pond can generate almost 500 images.  The two passes will crank that number up to about a thousand.  Throw in the initial framework images, and the total will probably exceed 1500 images.  This is more than I did at Green Sand Beach, by far my largest synth to date, but it should make for a very complete synth of the Kiholo Bay Inlet.

I should have a chance to try this over the weekend, if the weather holds.  Time will tell.

Tom

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Kiholo Bay Inlet

Posted by Tom Benedict on 07/06/2009

Worldwide KAP Week is over for 2009, but KAP is a year-round activity for me.  The past six months we’ve been plagued with volcanic gasses coming out of the Halemaumau Crater on Kilauea, but a few days ago the tradewinds came back, blew the volcanic plume out to sea, and for the weekend, at least, our sweeping panoramic views were back.  It was a tough call whether to do ground-based photography or to fly a camera, but in the end the kites won.

Unfortunately because we’d had such uncharacteristic wind for such an uncharacteristically long time, the wind models I rely on to decide where to fly were reporting bogus information.  The models predicted 6-8 knot kona winds at my house, whereas I could look outside and clearly see it was blowing 10-12 knot tradewinds.  When technology fails, the eyeball prevails.  I packed my gear and my son and drove down to the coast.

In addition to editing the book for Worldwide KAP Week 2009 and starting work for a book of my own, I’ve also started making some KAP posters.  But in order to print them at the size I would like, I’ve been faced with the resolution limit of my photography.  I’ve been getting my posters printed using the El-Co Color Labs Internet Special.  They use a Durst Theta printer, and print on Fuji papers using Fuji chemistry.  A quick word about digital printing:  There are two basic ways to make a digital print.  One is to use a transfered medium, like a dye sublimation process, or an inkjet process.  The other is to take a photographic paper and to expose it to light.  Each has its own advantages, but I like the second for the bulk of the photo work I do.  The Durst Theta printers expose photographic paper to light, and then develop the paper the same way a film-based photo lab would.  The result truly is a photographic print.

But resolution is the key.  The Durst Theta can print at up to 254 dpi.  At 24×30″ that’s an image 6096×7620 pixels, or a 44 megapixel image.  I’ve made prints at resolutions as low as 150 dpi on a Durst Theta printer, but I prefer not to go below 200 dpi.  For this set of posters 200 dpi is my lowest resolution.  This is still a huge image, and the bulk of the pictures I have on file simply aren’t up to the task.

This has driven me to re-visit sites where I’ve been able to make good photos, but without the resolution necessary to print.  So I was overjoyed when driving down the coast to find that the wind at Kiholo Bay was a nice 7.4 knot on-shore.  Perfect.

Kiholo Bay has a fantastic reef, and is an outstanding place to do snorkeling and SCUBA.  There’s a good dirt road that will take you all the way to the water’s edge, so there’s no great hardship to get there.  It’s a popular spot, but not so overcrowded as, say, Hapuna Beach.  But if you’re willing to go a little further and work a little harder, Kiholo Bay has a feature that’s not to be found anywhere else in the Islands: the inlet at Kiholo Bay.

Wainanali`i Pond

There are two approaches to the inlet.  One is around the water’s edge from the park at Kiholo Bay.  The other is to park off the side of the highway, a little more than half a mile away.  The walk across the lava is not bad, but it’s not a traditional trail.  The “trail” is a series of paint splotches on the rocks to tell you vaguely where to go.  But the ground is rough lava.  It’s more than possible to get hurt, and this time I did.  I twisted my ankle.

A Study in Salinity

What makes the inlet special is that it’s a relatively deep patch of water with a very shallow mouth connecting it to the sea.  The level of the pond rises and falls with the tides, but tidal flow causes very little mixing of the water inside the pond.  As a result it is highly stratified, with seawater salinity levels at the surface, and high salinity levels further down.  The salinity at the bottom of the pond can easily be double that at the surface.  Select euryhaline organisms inhabit the higher salinity levels, turning the water its characteristic opaque aquamarine blue, whereas the surface water is all crystal clear.

The shallow mouth and opaque water in the inlet make it the perfect habitat for honu, or Pacific green sea turtles.  The pond is relatively free of predators, so the honu are able to forage in some measure of peace and security.

Flying Solo

The honu swim in the high salinity waters at the bottom of the pond, moving like phantoms through mist.  Every once in a while one will come to the surface for air, slipping into the clear waters above and then descending once more into the obscuring depths.

The arm of land that cuts the inlet off from the sea is mostly composed of rounded rocks rather than the hard lava flow that makes up the opposite shore.  This provides a good spot for honu to come up out of the water to rest and sun themselves.  Honu are protected by law, so people are not supposed to approach or touch them. A 20′ minimum distance is required, but I try to maintain at least 40′ between me and a honu on land.  With the low traffic of humans at the inlet, and the lack of predators, this provides a wonderful spot to study them.

A Study in Turtles

Hawaii Preparatory Academy (HPA) maintains a remote observation station at the Kiholo Bay Inlet.  The station has had some upgrades since my last visit, and now sports three PV panels, a fixed mount infrared camera, a fixed mount visible camera, a pan tilt zoom camera, and an RF link back to Waimea.  To learn more about this installation and the research program, you can visit:  HPA/NOAA Sea Turtle Research and Conservation Program.

But for me, the Kiholo Bay Inlet is simply a beautiful place that I have the privilege of being able to see and photograph.  It’s not the easiest place to get to, and there’s no fresh water or shade to speak of.  But Kiholo Bay isn’t here for us, it’s here for the honu.  I just feel fortunate I get to visit from time to time.

Idyllic

Tom

(Note:  I have been calling the Kiholo Bay Inlet by the wrong name: Wainanalii Pond.  This is how it is commonly labeled on maps, hence my mistake.  But during a conversation with Bobby Camara, he pointed out that the village of Wainanalii is a two hour canoe ride to the north of Kiholo Bay, and that it was destroyed by the same lava flow that created the inlet.)

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