What on Earth is an Anaglyph?

November 21st, 2016 by Abbey Nastan, MISR team at JPL

The science team behind the Multi-angle Imaging SpectroRadiometer (MISR) on NASA’s Terra satellite frequently publishes special images called stereo anaglyphs. For example, you might have seen our recent series of anaglyphs celebrating the centennial of the National Park Service. But what exactly is an anaglyph, and how is one made from MISR data?

All methods of viewing images in three dimensions rely on the fact that our two eyes see things at slightly different angles; this is what gives us depth perception. As a simple demonstration, hold a finger at a short distance from your face, and close one eye at a time. You will notice that your finger appears to be in a different place with each eye. The horizontal distance between the two versions of your finger is called the parallax. Your brain interprets the amount of parallax to tell you how far away your finger is from your face—the greater the parallax, the closer your finger!

However, your brain can also be tricked into thinking that a perfectly flat picture is actually a three-dimensional object by presenting each eye with a slightly different version of the picture. The first 3D viewing technology was the stereoscope, originally invented by Sir Charles Wheatstone in 1838. The stereoscope takes two images viewed from slightly different angles and mounts them next to each other. The photos are viewed using fixed lenses that fool the brain into thinking that it is looking at one picture. Stereoscopes worked well, but their major drawback was that they could only be used by one person at a time.

In 1858, Joseph D’Almeida, a French physics professor, invented a method of showing stereoscopic images to many people at once using a lantern projector equipped with red and blue filters. The viewers wore red and blue goggles. Later, Louis Du Haron adapted this technique to allow anaglyphs to be printed and viewed on paper. In 1889, William Freise-Green created the first anaglyphic motion picture. These early 3D movies were nicknamed “plastigrams” and were very popular by the 1920s.

At the most basic level, anaglyphs work by superimposing images taken from two angles.  The two images are printed in different colors, usually red and cyan. The viewer needs glasses with lenses in the same colors. The lenses are needed to filter out the unwanted image for each eye. So, if the image for the right eye is printed in red, the image can be seen through the cyan lens placed over the right eye, but not through the red lens over the left eye, and vice versa. The brain, seeing two different pictures through each eye, interprets this as a three-dimensional scene.

The reason why the MISR instrument can be used to make anaglyphs is because it has nine cameras, each fixed to point at a different angle. Therefore, as MISR passes over a particular feature on Earth, it captures nine images spanning a range of 140 degrees (diagram above). Any two of these images can be combined to make an anaglyph. The greater the angular difference between the images, the greater the resulting 3D effect; however, if the angular difference is too great, the brain will be unable to interpret the image.

Anaglyphs made with MISR must be rotated so that the north-south direction is roughly horizontal. Though this is inconvenient — we are used to viewing the Earth with north at the top — it is necessary because Terra flies from north to south, and MISR’s cameras are aligned to take images along that track. Therefore, the angular difference between the images is in the north-south direction. Since our eyes are arranged horizontally, the angular difference between the anaglyph images must be horizontal as well.

You can see this by comparing two versions of an anaglyph of Denali, Alaska (below). In the version with north upwards (left), the 3D effect does not work. But when the image is rotated so that north is to the left, suddenly the mountains pop out.

Anaglyphs are useful for science because they allow us to intuitively understand the three-dimensional structure of things like hurricanes and smoke plumes. For example, examine the three-panel image of Typhoon Nepartak below. (All three images have been rotated so north is to the left).

In the top, single-angle image, the eye of the storm appears to be quite deep due to the shadows, but otherwise it is difficult to determine how high the clouds are. Compare this to the middle image, which shows the results from MISR’s cloud top height product; it uses a computer algorithm to compare the data from multiple cameras and determine the geometry of the clouds. Now we can tell that clouds in the central part of the storm are very high (except for the eye), while the spiral cloud bands are slightly lower and there are very low clouds between the arms. However, understanding this data set requires us to interpret the color key and have at least a rudimentary idea of how 16 kilometers compares to 4 kilometers.

Now put on your red-blue glasses* and look at the anaglyph in the third image. All of the features are immediately understood by our brains. While it takes a few minutes (or paragraphs) of explanation to introduce a first-time viewer to MISR datasets, the red-blue glasses make it possible to enjoy the same experience with a simple image. This is why anaglyphs make great tools for scientists as well as for sharing unique views of Earth’s features with the public.

Editor’s note: If you don’t have a pair of red-blue glasses, this page lists companies that sell them. Or if you can find some red and blue plastic wrap, you can make your own. The instructions are here.

I Can See My Home from Here — Landsat Contest

June 4th, 2012 by Mike Carlowicz

From the Landsat team at NASA and the U.S. Geological Survey…

To celebrate the 40th anniversary of the Landsat Earth-observing program — which first rocketed into space on July 23, 1972 — NASA and the U.S. Geological Survey will be giving something special to members of the American public. NASA will create customized Landsat chronicles of changing local landscapes for six U.S. citizens who enter the “My American Landscape” contest.

To enter, all you have to do is send an e-mail describing the landscape changes in your home area and what you hope to learn about them from Landsat’s four decades of observations from space. The deadline for submissions is Wednesday, June 6, 2012. Contest winners will be announced live on NASA Television at a Landsat 40th anniversary press briefing on Monday, July 23. Click here for more details.

How to Enter

Answer the following questions in an email to:

HQ-LandsatContest@mail.nasa.gov

1) What types of landscape changes interest you in your area? Select one or more from this list: farms and fields; forests; cities and suburbs; lakes, rivers, and coasts; natural disasters; wildlife habitat.

2) Describe in at least 100 words the local changes you are interested in and what you hope to learn about them from a Landsat “space chronicle.”

4) The county and state where you live

Where on Earth quiz #27 – answered

February 27th, 2012 by Mike Carlowicz

For those of you who played geographical detective with the MISR image quiz, here are this month’s clues and answers:

1) Within this country lies a picturesque desert, located at the bottom right of the image. This desert was home to a group of enigmatic ancient people who were known for their skill and resourcefulness. Their capital is a UNESCO World Heritage Site. Name the desert and the given name of the people.

The Rum Desert or Wadi Rum, and the people are the Nabateans.

2) The wavy lines that cross the middle of the image are natural geologic features that often carry descriptive names of their location. What is the native word used to accurately describe these features?

The native word is the Arabic word for valley: Wadi.

3) The name of the body of water at the bottom of the image is actually a misnomer. The nearby hills that protect the landscape from weather fronts also enable a “rain shadow,” thus contributing to the surrounding aridity. What is the name of the body of water?

The body of water is the Dead Sea.

4) At the bottom left, small city developments are visible. One of those cities came was developed at the beginning of the 20th century. It now accounts for 50 percent of the industrial output of the country. Name the city.

The city is Zarqa.

5) The landscape dominating most of the image is an extension of a much larger, natural feature. This feature is home to a very limited floristic diversity and to a number of critical and endangered species—though there are no formally protected areas. Name this feature.

It is the Arabian Desert.

6) Name the country that fills most of the area in this image.

Jordan.

Nearly 250 people from around the world responded before the deadline. Individuals who answered all questions correctly are listed below in the order in which responses were received. The prize winners are indicated by an asterisk. Well done to all and thank you for participating!

1. James Mackie, Fort Collins, CO *
2. Philip Bedggood, Melbourne, Australia *
3. Roel Duijnhouwer, Oud-Beijerland, The Netherlands *
5. Kristin Poinar, Seattle, WA
6. Pam Stewart, Eugene, OR
7. Tyler Rundel, Topanga
8. John Thomas, Oakland, IA
9. Ivan P Anderson, Ditton, Kent, England
10. Jonathan Frishtick, Norwich, VT

Where on Earth, round 2

August 8th, 2011 by Mike Carlowicz

We had fun with last month’s “Where on Earth” mystery, so we thought we’d throw a new image out for your guessing pleasure.

A few hints…

+ German U-boats sank a tug nearby in 1918…

+ The ponds in the image were formed by retreating glaciers…

+ The main waterway in the image is partly man-made…

+ Down the road, a famous stretch of parkland just celebrated its 50th anniversary

We are a bit short on prizes — and federal laws keep us from offering much beyond our admiration. But after all, success is its own reward.

PS (added at 1:30 p.m.) — Well, clearly that was too easy. So let me make it harder – can you find my house in there? Aha!