Today, Hubble released some images of a sequence of four eclipses on Saturn. Technically, we call these "transits", not eclipses, but a headline about "Saturn" and "transit" sounds to many like it refers to an interplanetary travel system in a science fiction story.

The images and a rather cool visualization can be found on the HubbleSite press release page.

While the images are definitely not science fiction, one can argue a bit about the visualization. Hubble is not a movie camera, so it could not have taken the frames necessary for a movie. But it is also not an animation, as it is based directly on astronomical observations. What is it? I prefer to call it a scientific visualization.

Let me explain ...

The Data

The source data for the Saturn transit visualization are just a few images from Hubble. These images captured selected times when one or more of the moons crossed the face of Saturn from our point of view at Earth. Three images are presented in the press release, and the total number taken is probably not more than ten.

Note that Saturn's larger moons orbit Saturn in roughly the same plane as the rings. Hence, we can see these transits only when the rings appear near to edge-on. Saturn's rings will be fully edge-on, what we call a ring plane crossing, on September 4, 2009. Unfortunately, Saturn will be about 10 degrees away from the Sun on that date and not suitable for good observations.

The visualization lasts for 30 seconds. Greg Bacon, who created the sequence, confirmed to me that he used only 6 images and that the master version has 60 frames per second (the versions posted on the web have a lower framerate). That means that the visualization created 1800 frames from 6 images. How?

Graphics

To me, the story starts about twenty years ago.

In 1991, Michael Jackson released a video called "Black or White" that helped popularize the idea of "morphing". The last minute of that video is a sequence of smooth transitions from one person's face to another and another and another. The sequence moves seamlessly between people of different sexes, races, and hairstyles. The strong multicultural message of the visuals greatly enhanced the message in the music.

To do a morph, you start with two images and identify the similar points in each. For facial transitions, one would identify the eyes, nose, mouth, etc. in each image. Then you use computer software to do two things: 1) distort each image so that the tracking points line up with the other image, and 2) create a cross-fade between the distorting images. As image 1 distorts to match image 2, it slowly fades out. In the reverse sense, image 2 starts out distorted and fades in as it returns to its normal shape. Back in the early 90s, only powerful graphics computers could handle such morphing. Today, it is a standard trick that most computers can handle with ease.

Now the morphing in Michael Jackson's music video is complicated by the fact that the people are moving during the morphs. The sequence was morphing video clips, not just images. For Hubble's images, the situation is easier, although things do move between images.

It is easy to track the locations of the moons and their shadows in each image. They will move along simple linear paths between the images. The details of the moons will not change much from image to image, but morphing between them is no problem.

Saturn should be rotating underneath these moons and shadows. In addition, changes will occur in Saturn's cloud patterns. However, if you examine the images, Saturn doesn't really change much. It did not seem worth the effort to model the planet rotation and cloud movements. Hence, Saturn in the visualization is just a series of cross-fades between the six images with, of course, the shadows filled in. You have to watch carefully to see the changes in Saturn. They are there, but they are subtle.

Visualization

Finally, we get to the question posed in the headline. Is this a movie, an animation, or a scientific visualization? We had this very discussion during the news team meeting before the release.

Calling it a movie is misleading. Hubble did not record movie frames. In fact, Hubble only took 0.3% of the frames. The other 99.7% were interpolated.

Calling it an animation is also misleading. When folks think of animations, they think of something created whole from an artist's imagination. Yes, it may be based on scientific ideas and motivated by observations, but the main creative force is artistic, not scientific.

The term I use is scientific visualization. The artist creating this sequence did not have free reign to do as he wished, he was tightly constrained by the science data. The six images showed him how to portray the planet, moons, and shadows at each point in time. The transitions between those times were created by computer, but constrained to follow the orbits as governed by Newton's law of gravity. The main artistic freedom is how the artist filled in the shadows in Saturn's atmosphere.

That is what I mean by scientific visualization. A presentation that is based upon science data and portrays it as accurately as is  reasonable. Note that I do not say as accurately as possible. Many times a totally accurate scientific visualization will be incomprehensible or boring. The purpose of a scientific visualization is to convey understanding for what is happening. Making sure the message is delivered is paramount, so long as one does not distort the science.

I like to say that scientific visualizations should lie as little as possible.