Transcript
I
ntroducing the dome master u n d e r s ta n d i n g d o m e p ro j e c t i o n & c r e at i n g f u l l d o m e i m ag e s
. introduction . ∙∙defining the dome master........... 1 ∙∙creating dome master images...... 2
Written by
kevin cain
tales of the maya skies a chabot space and science center production. major funding provided by the national science foundation maya skies supplemental materials created by insight - www.mayaskies.net
introducing the dome master Full dome theaters range in size and technical implementation, but they all feature the kind of hemispheric screen familiar to planetarium visitors. Full dome theater theaters share something else: the format for the source images they project in the dome. These images are tondos, circular 2D pictures commonly referred to as dome masters. While technical configurations for dome displays change from system to system, to ensure compatibility across different systems they all conveniently use dome masters as input. Here’s one, showing a view of the jungle above Chichen Itza, made during production for Tales of the Maya Skies: To orient yourself, imagine that the image at right is being projected in a dome theater. The center point of the dome master will fall on the zenith point of the dome, directly overhead. The circumference of the circle image will define the horizon, or ‘spring line’, of the dome. (To be more accurate, the circumference of a full dome image defines the spring line for a 180 degree dome theater with zero degrees of tilt. We will explore other common dome configurations below.) The lower center third of a dome master roughly equates to a dome’s ‘sweet spot’ – the region that the audience can most easily see when front-facing.
Dome Master view of Chichen Itza.
Right away, you’ll notice a few important things about the dome master above. First, while the content of the dome master is circular, the image containing it is square. When projected, only the circular area is processed and displayed; the remainder of the image is usually left black and contains notes about the images. Secondly, you might suspect that there is some distortion in the above image. This hunch is right, but the distortion present is intentionally computed. Full dome theaters work by projecting twodimensional dome master images onto the threedimensional hemispheric surface of the venue, not unlike the way a globe is made by mapping 2D images onto a 3D sphere. In both cases, projecting a two-dimensional map onto a three-dimensional surface requires either a distorted map or a distorted projection. Dome masters, then, appear distorted but are formatted to minimize the distortion seen by viewers in the dome.
The familiar Mercator projection used for globes.
The usual mechanism used to map images on a globe is the familiar Mercator projection shown above. For dome masters the projection type used is azimuthal equidistant projection, shown below. Azimuthal equidistant projection creates a circular image in which equally spaced concentric lines represent equal regions of the scene being imaged. This is an significant feature.
A dome master (left), with its projection in dome (right).
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It is important that images projected in the dome do not appear to change scale in either of the two image axes. Azimuthal equidistant projection ensures that equal swaths of a physical dome are mapped to equal areas in a dome master. In the images above, note the even spacing of concentric circles. Most common 180 degree fisheye lenses aim for these same image properties. Even with azimuthal equidistant projection, when projecting dome masters in a physical dome, the ‘content’ will only appear un-distorted to a viewer seated at the exact midpoint of the dome. At that ideal spot, straight lines will appear perfectly straight and circles will appear perfectly circular. In the real world, viewers will seat themselves throughout a venue’s seating, resulting in varying levels of distortion from their points of view. To these viewers, straight lines will bow and circular orbits will appear as ellipses. These effects scale with size of the projected content. As a result, important action is typically confined to small regions of the dome, such as the ‘sweet spot’, where local dome curvature is close enough to a flat screen to minimize distortion.
Orthographic renders suitable for stitching into a dome master image.
When creating dome masters in computer graphics production, there are two common approaches. The simpler of the two approaches is to render an image through a synthetic 180 degree fisheye lens. The images generated are directly usable as dome masters with no extra steps. Unfortunately, support for 180 degree fisheye rendering is not offered in all CGI rendering systems, so a second common approach can be used in these cases. In this case, several orthographic views of a given scene are rendered, rather than a single fisheye image. Each orthographic render in the set is made from a synthetic camera looking out from a given camera location along the three axes. To cover the dome, five ortho renders are required: left, front, right, back, and top. Adding a bottom ortho view is not needed for hemispherical domes. The illustration above shows five orthographic renders arranged in an ‘unwrapped box’ configuration. Note that all orthographic views except the ‘top’ channel are half-height (0.5 aspect ratio), while the top ortho view is square (1.0 aspect ratio). While full-height images can also be used as input for full dome stitchers, half of the pixels in these images will be wasted, as they lie outside the 180 visible dome region. Therefore, it is possible to save render time and disk space with ‘halfheight’ images for all channels but the top orthographic view. Once all orthographic images are generated, they can then be processed into a dome master using existing software, such as DomeXF for Adobe AfterEffects, or Pineappleware QuickStitcher. Full dome theaters are built with a wide range of dome coverages and tilts. While domes are never more than 180 degrees, they are often less. Since a dome master contains 180 degrees of view, a 170 degree dome will discard 10 degrees of the view contained in the dome master. Which 10 degrees will be ignored depends on whether the dome master is tilted during the slicing process. In an effort to coordinate the horizon present in the dome master images with the spring line of the physical dome, it is common to tilt the dome master as dome slices are produced. As we noted above, the circumference of the dome image maps to the spring line of a 180 degree dome with zero degrees of tilt. Once the dome is tilted, the spring line will no longer match the circumference of the dome master. 2
Orthographic images, projected in a dome.
