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The SEDRIS Data Representation Model
APPENDIX A - Classes Image Anchor |
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An instance of this DRM class specifies where the given <Image> instance is located in the specified SRF.
An <Image Anchor> instance is used in one of two ways:
In this case, the <Image> instance is not tied to a particular <Feature Representation> instance or <Geometry Representation> instance. The <Location> components of the <Image Anchor> instance specify the positions of the corners of the <Image> instance in the specified SRF.
NOTE 1 If two georeferenced <Image> instances are to be placed exactly next to each other by means of <Image Anchor> components, the <Location> components of those <Image Anchor> instances are exactly the same along the common edge.
In this case, the <Image Anchor> instance defines how the associated <Image> instance is to be applied to the DRM object having the <Image Mapping Function> instance as a component. The srf_context_info of the <Image Anchor> instance shall match that of the context in which the <Image Mapping Function> instance is being applied.
<Image Anchor> instances are used to support spherical and cylindrical image projections for <Image Mapping Function> instances. By specifying anchor points that are not in the same plane, non-orthogonal projection becomes possible.
NOTE 2 When an image mapping is applied to many <Polygon> instances using a single <Image Mapping Function> instance, a continuous image results when displayed.
A producer has a geospecific texture that has been derived from overhead photography and rectified. It is used to drape over a terrain surface and is represented as an <Image> instance with an <Image Anchor> component.
If a producer has a georeferenced <Image> instance that is to be explicitly applied to one or more terrain <Polygon> instances, the mapping of the image data to the <Polygon> instances is defined by an <Image Mapping Function> component of each <Polygon> instance that has an <Image Anchor> component and an association to the appropriate <Image> instance.
No. An <Image> instance is not required to specify an <Image Anchor> component, and in fact, an <Image> instance utilizes its optional relationship with <Image Anchor> only when the texture is geospecific.
Consequently, since an SRF is specified for an <Image> instance only if an <Image Anchor> component is present, only geospecific textures require specification of an SRF.
Although all geospecific textures in an <Image Library> instance may specify the same SRF, this is not a requirement for data providers, nor can it be relied upon by consumers. Consequently, each geospecific <Image> may specify an independent SRF.
Furthermore, many data providers generate <Image> instances that are not geospecific and thus do not require the specification of an SRF. Adding SRF information as a field of <Image Library> would require all data providers to specify SRFs for all <Image Library> instances, even those to which SRFs were not applicable.
A geospecific <Image> instance is not required to specify the same SRF as that of any of the <Environment Root> instances that may be present in its native transmittal. It is perfectly possible for a transmittal to contain multiple <Environment Root> instances, each with its own SRF, or to have an <Image Library> instance and no <Environment Root> instances whatsoever.
<Image Anchor> instances only provide for a simple method of <Image> warping. It is assumed that for more complex forms of warping (e.g., "rubber sheeting", ortho-rectification) the <Image> instance will be warped by the data provider and transmitted in the final state.
When an <Image Anchor> instance is interpreted as a component of an <Image> instance or as a component of an <Image Mapping Function> instance specifying a planar projection, the <Location> components of the <Image Anchor> instance are interpreted as follows.
The first <Location> component specifies the location to which the lower left corner of the <Image> instance is mapped.
The second <Location> component specifies the location to which the upper left corner of the <Image> instance is mapped.
The third <Location> component specifies the location to which the upper right corner of the <Image> instance is mapped.
When an <Image Anchor> instance is interpreted as a component of an <Image Mapping Function> instance that specifies a non-planar projection, the <Location> components of the <Image Anchor> instance are interpreted as follows.
For a cylindrical projection, the first <Location> component specifies the centre of the cylinder. The second specifies a point at the centre of the top of the cylinder, thus indicating direction. The third <Location> component specifies the alignment of the cylinder by specifying a point on the surface of the cylinder.
For a spherical projection, the first <Location> component specifies the centre of the sphere. The second specifies the point at the north pole of the sphere, thus indicating direction. The third <Location> component specifies a point on the equator of the sphere.
The srf_context_info field specifies the SRF within which the <Image Anchor> instance is defined.
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