| Documentation |
Background node is used to specify a color backdrop that
simulates ground and sky, as well as a background texture, or panorama, that is placed
behind all geometry in the scene and in front of the ground and sky. Background nodes are
specified in the local coordinate system and are affected by the accumulated rotation of
their ancestors as described below. The backdrop is conceptually a partial sphere (the
ground) enclosed inside of a full sphere (the sky) in the local coordinate system with the
viewer placed at the center of the spheres. Both spheres have infinite radius and each is
painted with concentric circles of interpolated color perpendicular to the local Y-axis of
the sphere. The Background node is subject to the accumulated rotations of its
ancestors' transformations. Scaling and translation transformations are ignored.
The sky sphere is always slightly farther away from the viewer than the ground partial
sphere causing the ground to appear in front of the sky where they overlap.
The skyColor field specifies the color of the sky at various
angles on the sky sphere. The first value of the skyColor field specifies the color of the
sky at 0.0 radians representing the zenith (i.e., straight up from the viewer). The skyAngle
field specifies the angles from the zenith in which concentric circles of color appear. The
zenith of the sphere is implicitly defined to be 0.0 radians, the natural horizon is at PI/2
radians, and the nadir (i.e., straight down from the viewer) is at PI radians. skyAngle is
restricted to non-decreasing values in the range [0.0, PI]. There shall be one more skyColor
value than there are skyAngle values. The first color value is the color at the zenith,
which is not specified in the skyAngle field. If the last skyAngle is less than PI, then the
color band between the last skyAngle and the nadir is clamped to the last skyColor. The sky
color is linearly interpolated between the specified skyColor values.
The groundColor field specifies the color of the ground at the
various angles on the ground partial sphere. The first value of the groundColor field
specifies the color of the ground at 0.0 radians representing the nadir (i.e., straight down
from the user). The groundAngle field specifies the angles from the nadir that the
concentric circles of color appear. The nadir of the sphere is implicitly defined at 0.0
radians. groundAngle is restricted to non-decreasing values in the range [0.0, PI/2]. There
shall be one more groundColor value than there are groundAngle values. The first color value
is for the nadir which is not specified in the groundAngle field. If the last groundAngle is
less than PI/2, the region between the last groundAngle and the equator is non-existant. The
ground color is linearly interpolated between the specified groundColor values.
The backUrl, bottomUrl, frontUrl, leftUrl, rightUrl, and
topUrl fields specify a set of images that define a background panorama between the
ground/sky backdrop and the scene's geometry. The panorama consists of six images,
each of which is mapped onto a face of an infinitely large cube contained within the
backdrop spheres and centred in the local coordinate system. The images are applied
individually to each face of the cube. On the front, back, right, and left faces of the
cube, when viewed from the origin looking down the negative Z-axis with the Y-axis as the
view up direction, each image is mapped onto the corresponding face with the same
orientation as if the image were displayed normally in 2D (backUrl to back face, frontUrl to
front face, leftUrl to left face, and rightUrl to right face). On the top face of the cube,
when viewed from the origin looking along the +Y-axis with the +Z-axis as the view up
direction, the topUrl image is mapped onto the face with the same orientation as if the
image were displayed normally in 2D. On the bottom face of the box, when viewed from the
origin along the negative Y-axis with the negative Z-axis as the view up direction, the
bottomUrl image is mapped onto the face with the same orientation as if the image were
displayed normally in 2D.
Alpha values in the panorama images (i.e., two or four
component images) specify that the panorama is semi-transparent or transparent in regions,
allowing the groundColor and skyColor to be visible.
Often, the bottomUrl and topUrl images will not be specified,
to allow sky and ground to show. The other four images may depict surrounding mountains or
other distant scenery. Applications shall support the PNG and JPEG image file formats.
Support for other image formats (e.g., GIF) that can be rendered into a 2D image is
recommended (including transparency).
Ground colors, sky colors, and panoramic images do not
translate with respect to the viewer, though they do rotate with respect to the viewer. That
is, the viewer can never get any closer to the background but can turn to examine all sides
of the panorama cube, and the viewer can look up and down to see the concentric rings of
ground and sky (if visible).
Background nodes are not affected by Fog nodes. Therefore, if
a Background node is present while a Fog node is active, then the Background node will be
displayed with no fogging effects. It is the author's responsibility to set the
Background values to match the Fog values (e.g., ground colors fade to fog color with
distance and panorama images tinted with fog color). Background nodes are not affected by
light sources.
Default skyColor is black (0.0 0.0 0.0). |
