6.5 Background
The Background node is used to specify a colour 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.
Background nodes are bindable nodes as described in 4.6.10, Bindable
children nodes. There exists a Background stack, in which the top-most
Background on the stack is the currently active Background. To move a Background
to the top of the stack, a TRUE value is sent to the set_bind eventIn.
Once active, the Background is then bound to the browsers view. A FALSE
value sent to set_bind removes the Background from the stack and
unbinds it from the browser's view. More detail on the bind stack is described
in 4.6.10, Bindable children nodes.
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 centre of the spheres. Both spheres have infinite radius and
each is painted with concentric circles of interpolated colour 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 colour of the sky at various
angles on the sky sphere. The first value of the skyColor field specifies
the colour 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 colour appear. The zenith of the
sphere is implicitly defined to be 0.0 radians, the natural horizon is at
/2 radians, and the nadir (i.e.,
straight down from the viewer) is at radians. skyAngle is restricted
to non-decreasing values in the range [0.0, ]. There shall be one more skyColor
value than there are skyAngle values. The first colour value is the
colour at the zenith, which is not specified in the skyAngle field.
If the last skyAngle is less than pi, then the colour band
between the last skyAngle and the nadir is clamped to the last skyColor.
The sky colour is linearly interpolated between the specified skyColor
values.
The groundColor field specifies the colour of the ground at the
various angles on the ground partial sphere. The first value of the groundColor
field specifies the colour 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 colour
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, /2]. There shall be one more groundColor
value than there are groundAngle values. The first colour value is
for the nadir which is not specified in the groundAngle field. If
the last groundAngle is less than /2, the region between the last groundAngle
and the equator is non-existant. The ground colour 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.
Figure 6.1 illustrates the Background node backdrop
and background textures.
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.
See 4.6.11, Texture maps, for a general
description of texture maps.
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. Browsers shall support the JPEG (see
2.[JPEG]) and PNG (see 2.[PNG])
image file formats, and in addition, may support any other image format
(e.g., CGM) that can be rendered into a 2D image. Support for the GIF (see
E.[GIF]) format is recommended (including
transparency) . More detail on the url fields can be found in 4.5, VRML and the World Wide Web.
Figure 6.1 -- Background node
Panorama images may be one component (greyscale), two component (greyscale
plus alpha), three component (full RGB colour), or four-component (full
RGB colour plus alpha).
Ground colours, sky colours, 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 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 active (i.e., bound) 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 colours fade to fog colour with distance and panorama
images tinted with fog colour). Background nodes are not affected by light
sources.
version 1.3.9.1.
