Class Name: Image Anchor
Definition
Defines where (in the currently scoped 'world' spatial reference frame) an
Image is located. The 3
Locations specified are the locations of corners of
the Image, NOT those of the center of the
texels in the corners of the Image.
Image Anchor is used in 2 ways by the SEDRIS
DRM:
- As a component of an Image in an
Image Library.
In this case, the Image is not tied to a particular
textured object, and the Image
Anchor's Locations merely define the (currently
scoped 'world') Locations of the corners of the
Image.
Please note that if 2 geo-referenced Images are to be
placed exactly next to each other, then the Locations
specified would be exactly the same along the common edge.
- As a component of an Image Mapping
Function.
In this case, the Image Anchor defines how the
associated Image is to be applied to the
Geometry that has the
Image Mapping Function as a component.
Image Anchors are used to support spherical and
cylindrical image projections for Image
Mapping Functions. By specifying anchor points that are not in the same
plane, non-orthogonal projection becomes possible.
Note that when an image mapping is applied to many
Polygons using a single
Image Mapping Function, a "
continuous" image should be the result when displayed.
Primary Page in DRM Diagram:
Example
- A producer has a geo-specific "global" texture that has been derived
from overhead photography and rectified. It might, for example,
be used to "drape" over a terrain surface. This would typically
be represented as an Image (in an
Image Library) with an
Image Anchor component.
- If a producer has a geo-referenced Image data that
is to be explicitly applied to one or more terrain
Polygons, the mapping of the image data to the
Polygons would be defined by an
Image Mapping Function (component of
the Polygon) that has an
Image Anchor and an association to the
appropriate Image.
FAQs
- How do I transmit an Image and its associated
warping?
- Image Anchors only provide for a simple
method of Image warping. It is assumed that for more
complex forms of warping (i.e., "rubber sheeting",
ortho-rectification) the Image will be
warped by the producer and transmitted in the final state.
- Do all the global textures in a transmittal have to have the same
spatial reference frame parameters?
- No. Although all global textures in a transmittal typically have the same
parameters, this is not a requirement for producers, nor can it be
relied upon by consumers.
- The producer must generate spatial reference frame parameters for
every global texture in the Image Library.
Why not just put a "single" definition for the spatial reference
frame within the Image Library itself?
- Putting the spatial reference frame parameters in as fields of
Image Library would require all producers to
fill in spatial reference frame parameters for the
Image Library (even though many or most
producers will not be generating global textures, and spatial reference
frame parameters are not applicable in those cases).
- Is it necessary to define the spatial reference frame parameters field
for every Image in the Image
Library?
- No. Spatial reference frame parameters are defined only if an
Image Anchor component is present. The
Image Anchor component is optional within the
SEDRIS Data Model, and is utilized only for "global", or
geo-specific, textures. Thus, only global textures require assignment of
spatial reference frame parameters.
- Can the spatial reference frame parameters defined for a global
texture differ from the parameters defined in
Environment Root?
- Yes. They will "typically" be the same, but there is
not a requirement that they be the same.
- Couldn't a business rule be adopted that the spatial reference frame
applicable to global textures is the same as that defined in
Environment Root?
- No, this would not work in all cases, because it is possible (and
completely valid) for a producer to generate a SEDRIS transmittal containing
only an Image Library (and no
Environment Root object).
Constraints
None.
Composed of (one-way)
Component of (one-way)
Notes
Composed of Notes
First is lower left,
Second is upper left, third is upper right
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