Auto-align time-series of spherical images

From time to time the spherical Theta S camera used in my project needs maintenance, such as a reset after a power outage or lens cleaning. These manipulations guarantee consistency in the quality of the recorded images but sometimes cause misalignment from one image to the next.

Using the internal gyroscopic data of the camera one can correct the deformations due to an inclination of the camera, which deform the horizon. Sadly, the included digital compass data is mostly unreliable without an external GPS connected. With no sensor data to rely on the challenge remains in aligning these images (automatically).

[caption id=”attachment_1708” align=”aligncenter” width=”640”] Two spherical images with an offset along the x-axis.[/caption]

In the above image corrections were made to straighten the horizon. Yet, there still is a clear shift along the x-axis of the images (or a rotation along the vertical axis of the camera). Although the images are fine, they can’t be used in a time series or a movie as their relative position changes. Due to this misalignment it also becomes really hard to monitor a specific part of the image for research purposes. One solution would be to manually find and correct these shifts. But, with ~100Gb of data on file (2017-12-25) this is not a workable solution.

A standard way of dealing with misaligned images which are only translated (movement along x and y axis) is by applying phase correlation. Phase correlation is based upon aligning the phase component (hence the name) of a (dicrete) fourier transform of the image. A fourier transform translates (image) data and expresses it as a sum of sinus waves (plus their intensities / frequencies) and the relative position of these waves (or phase).  In more technical terms it translates data from the time / space domain in to the frequency / phase domain in order to among others speed up convolutional calculations or filter data based frequency (noise reduction). The use of a fourier transform in this case can be seen as a way to speed up calculating a cross-correlation between two images.

[caption id=”attachment_1701” align=”aligncenter” width=”640”] A selection of an image to use in determining lateral shifts (along the x-axis).[/caption]

In general, the phase correlation algorithm is fairly robust with respect to noise but self-similarity of vegetation corrupts the algorithm non the less. As such, I decided to use only a portion of the original spherical images to determine lateral shifts. I extracted the stems of the trees out of the image as this provides the most information with respect to lateral shifts. In a way the stems are a barcode representing the orientation of the camera. (In other use cases, where more man-made structures are included, this extra step is probably not needed.)

Using only these stem barcode sections I was able to successfully align one season of images. The result is an image time series which can be stacked for movies, analysis of the same portion of the image or used in interactive displays without any visible jumps!

[caption id=”attachment_1707” align=”aligncenter” width=”640”] The same two spherical images aligned using an offset calculated with phase correlation.[/caption]

An interactive temporal spherical display covering one year of imagery can be seen at this link:


Why scientists should learn from Aaron Swartz.

"He wanted openness, debate, rationality and critical thinking and above all refused to cut corners." -- Lawrence Lessig

Aaron Swartz helped draft the RDF Site Summary (RSS) standard at age 13 and was in many respects a prodigy. As Lawrence Lessig wrote about Aaron: “He wanted openness, debate, rationality and critical thinking and above all refused to cut corners.” Sadly, he perished by his own hand after particularly severe legal action against his person for copyright infringements. The documentary The Internet’s Own Boy: The Story of Aaron Swartz  provides a homage to his  life and work.

He left a legacy of writings which excel in clarity and brilliance I’ve rarely encountered. This is further contrasted by the age at which a lot of these blog posts or essays were written. Few people come close to the the way Aaron articulated his ideas in writing.

In a series of blog posts I’ll summarize some of his ideas with respect to technology, politics and media within the context of contemporary scientific (ecological) research. The fact that his ideas and his vision remain key to what I consider solid scientific practice reflect his genius and insight.

release late, release rarely (release early, release often)

In a blog post written on July 5, 2006 (release late, release rarely) Aaron outlines how to develop software. Yet, this essay could as well apply to scientific research, going from idea to publication.

Similarly to software (pet) projects, the subject of this blog post, science projects often have strong emotions attached to it. While these emotions are truthful the content or quality of the research might not pass muster.

"When you look at something you’re working on ... you can’t help but see past the actual thing to the ideas that inspired it... But when others look at it, all they see is a piece of junk."

In science, this basically means that you should do your homework and don’t oversell your research. In peer-review reviewers will see past these claims and, rightfully so, reject manuscripts because of it. So when you publish, release late, aim for quality not quantity.  This will raise the chance of getting your work published, while at the same time increasing the likelihood of stumbling on errors. Raising the true quality, or making it look good, often highlights inconsistencies you can’t move past in good conscious.

“Well, it looks great but I don’t really like it” is a lot better then “it’s a piece of junk”.

Releasing work late means that no one knows what you are doing and you might miss out on key feedback. So, informally, research benefits from releasing early.

"Still, you can do better. Releasing means showing it to the world. There’s nothing wrong with showing it to friends or experts or even random people in a coffee shop. The friends will give you the emotional support you would have gotten from actual users, without the stress. The experts will point out most of the errors the world would have found, without the insults. And random people will not only give you most of the complaints the public would, they’ll also tell you why the public gave up even before bothering to complain."

Releasing early, means that you get valuable feedback that might otherwise would not make it into a high quality paper (released late). This feedback does not only come from experts, but as correctly observed, from everyone within a larger (research) community.

In short, scientific communication and progress requires a split approach where manuscripts should be released as late as possible, with ideas mature and solidly supported by open code and data, which was released as early as possible.

Note: Although the argument can be made that conferences serve the purpose of “early releases” I have yet to see a conference where people present truly early work. Most of the time either published or nearly published work is presented.


A long read on the Yangambi research station, past and future

After a nice write up on my Jungle Rhythms project in The Guardian a more lengthy write up of other projects in and around the Yangambi research station, both in the past and ongoing,  is now online on Good to once more bring some attention to safeguard local historical collections and capacity building within this context in DR Congo.

Want to get published, show me your code.

All too often one is still confronted with a statement at the end of the manuscript reading: "Code is available from the authors at reasonable request".

The last few years there has been a strong focus on open data and open access journals. This is in part stimulated by a reproducibility crisis in science, often in the biomedical sciences. However, the strong focus on data and journal access alone is misplaced.

Jungle Rhythms made it into The Guardian

A cache of decaying notebooks found in a crumbling Congo research station has provided unexpected evidence with which to help solve a crucial puzzle – predicting how vegetation will respond to climate change. . . . (by Dan Grossman)

My Jungle Rhythms has made some waves as of late. The project sparked the interest of dr. Dan Grossman, a science journalist, and his nice summary of all the Jungle Rhythms work was published in The Guardian. As a result of this IFLscience picked it up as well. Especially in the comments section of The Guardian the response was really positive. I’m happy to see some global exposure of the project, and the larger context and importance of similar work. I also hope that this exposure might bring about more funding to safeguard historical collections and capacity building within this context in DR Congo.


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