OSG Ephemeris - Architecture And Implementation

This page is intended to be a discussion on the architecture of the osgEphemeris model. Its purpose is for the understanding of the internals of osgEphemeris for developers who would like to improve or contribute to it.

The osgEphemeris model is made up of the following components:

The Sky

Strictly speaking, the Sky is not part of the ephemeris. However, it is an important component to making visual scenes look realistic, and it provides the foundation on which the rest of the osgEphemeris model is based.

The SkyDome is a sphere that is made of trianges which are oriented inward. That is, it is intended to be seen from inside. When back-face polygon culling is enabled, only the inner side of the sphere is visible. The SkyDome is made with a radius of the distance of the earth to the moon in meters - a decision which might become clearer when considering the other aspects of osgEpheremis.

Purposely, the SkyDome is made of a Norhern and Southern hemisphere. Now, a ground-bound ephemeris view would seem not to need a Southern hemisphere for a sky dome. But, the intention was that the Southern hemisphere could be used for doing reflections in water, such that the sky could be rendered "upside down" in the Southern hemisphere, and water could simply be a transparent surface that sees the "upside down" sky. This was implemented successfully for the Sun, but not for the Moon and Stars. Reasons to be discussed later.

The SkyDome uses a 1D texture to control the sky color, which is updated every frame. It is only made of 256 texels so the compute and I/O time is reasonable. Since the sky is a dome, each texel of the 1D texture represents an incidence angle with respect to the horizon. The color of each texel is computed by determining the incidence length at each given angle, given a depth of atmosphere. A little "fudge" is included to account for light properties which bend around corners (dusk hours have light when the sun has already set, for example).

The 1D texture is of format RGBA, which allows an alpha component and therefore transparency. In fact, the sky becomes transparent at night and retains 50% transparency in the day.

By default, the SkyDome uses texture unit 1 for the 1D sky color texture, and texture unit 2 for the Sun (below).

The Sun

In osgEphemeris, the Sun is simple a projected texture on the SkyDome. A rotation matrix is controlled using the Sun's azimuth and altitude in the sky to determine the corresponding rotation matrix for the projection planes.

The Sun is also projected on the Southern hemisphere of the SkyDome with the opposite rotation as the projection planes of the Northern hemisphere to give that "reflection" effect of an upside down sky. This part worked successfully.

The piece that is lacking with the Sun is a halo and controlling the color of the halo. Upon reflection of many sunset pictures, it was determined that the sun's core color did not change much regardless of its horizon incidence. However, the halo that surrounds the sun goes from white/nearly transparent in the noon day position to yellow/red and quite opaque near the horizon. This certainly must have to do with the amount of atmosphere the Sun light is penetrating.

The current implementation does not include a Sun halo. Some experimentation was done with core sun color, but when a large enough texture is used to reduce pixelation anomalies, the compute and I/O time is prohibitive for updating the color of the texture.

A suggested implementation for this would be to use a shader to draw a sun in an Frame Buffer Object or Pixel Buffer and use that image as the texture for the Sun and its halo. This would allow full leverage of pipeline rendering to be able to get accurate dynamic color changes with respect to the Sun's incidence to the horizon.

Note that the sun texture as most textures are built in the build system as internal arrays of unsigned bytes. This is to eliminate the dependency on external textures for the osgEphemeris distribution. An exception to this is the Planets discussed following.

The Moon

As indicated in the last paragraph, the moon images are built from source imagery in the build system and stored in c++ files as arrays of unsigned bytes, which are then compiled into the osgEphemeris library to reduce external dependencies. The source data for the moon images include both a surface image and a normal map image.

THis means that, yes, the moon itself is implemented as a sphere in the sky, textured with the surface texture of the moon and bumpmapped with the normal map. When running the Ephemeris with MoveWithEyePoint set to False, you can zoom in very close to the moon and see the effects of the bump map.

The moon is placed in the sky, just beyond the radius of the SkyDome (5 moon radiuses beyond, actually). The reason for this that it was necessary to be able to view the moon during the day and the portions of the moon, which are not visible (say when the moon is quartered), are blue - that is, the color of the sky. The best way, it seemed, to accomplish this was to render the sky 50% transparent and show the moon in its correct phase through the "atmosphere".

The piece that is yet unsure about the moon is whether the correct side is facing the viewpoint. The moon is geometry, which is controlled by a MatrixTransform, so fixing this would be a very simple matter of adding the right rotation in to the MatrixTransform. This still needs to be investigated a bit.

The Stars

The stars are implemented with a StarField, which is really not, strictly speaking, as StarField. It is, in fact, a simple set of points arranged in a sphere, which is slightly larger than the SkyDome sphere. From the earth-bound vantage, all points in this sphere are placed correctly to represent the constellations.

The stars are implemented with an GLSL shader for a couple of reasons. 1) Better control over brightness (magnitude) and 2) transparency control. That latter was necessary to make the stars "dissapear" during the day time. Remember that the SkyDome needed to retain 50% transparency to be able to adequately render the Moon, so the stars would also be seen through the SkyDome unless we make them fully transparent. Doing this on a point by point basis had a significant impact on I/O bandwidth, but was trivial to do in a shader.

Alternatives would have been to use a global material with transparency but that would have meant dealing with lighting issues as well and would have had an impact on star brightness.

It is really important to control the brightness of the stars to give the night sky a realistic appeance. A simple set of points with all the same brightness looks like a set of points with all the same brightness, not like stars.

Note in the code, that the uniform pointSize is set to 2.4. This allowed for a reasonable representation of the stars even with antialiasing enabled. There still remains a bit of blinking as one moves the view around, but, then, "blinking" is one of the things the atmosphere does to stars anyway.

The Planets

The planets are the least studied component of the osgEphemeris. The reason is that, when drawn to scale, they simply where not visible. The exception to this would be Venus or Mars on a good night, but although they are implemented as geometrical spheres, they don't take any more screen space than the points of light making up the stars.

Unless one is looking to implement a virtual telescope using osgEphemeris, this seemed like a component not worth putting much effort into. So, the planets are left off as a default member of the osgEphemeris components.

Planet textures use the images that come in the current osgData distribution. osgEphemeris has these as the only external data dependencies.