This class is the parent class for Transformers that project a point from the celestial sphere
to a projection plane.
Implementation within SkyView of a projection requires:
- Creation of the Xxx projection class where Xxx is the three letter
WCS name for the class (e.g., Tan, Sin, Mer, ...) The projecter class file normally includes
* The mathematical transformation between the sphere and plane.
* The associated deprojecter as an embedded class.
* Where feasible the methods that implement straddling lines in the projection
and shadow points. An associated straddler class may be defined.
* Information as to whether the projection may be extended in the X and Y directions.
- For fixed projections, an entry in the fixedPoints hash of the Projecter class.
- For projections that are intended to be available as output projections in the Web interface
* an entry in the basic query form to allow the selection of this projection for output
associated with an image point in the image output.
Some projections can tile the projection plane with repeated
copies. This method gives the vectors along which the tiles repeat.
Note that for azimuthal like (e.g., Mercator) projections, the tiling may be possible in
the longitudinal direction, but not in the latitudinal direction.
Some projections (e.g., CAR) can tile in both directions (but note that the periodicity
for tiling is 360 degrees in both directions in the CAR projection since the latitudes
will run ... -90 -80 ...0 ... 80 90 80 ... 0 ... -80 -90 -80 ....
The period in X in radians. A value of 0 means that there is not periodicity.
public double getYTiling()
The tiling period in Y
The tiling period in radians. A value of 0 means that
there is no period.
public boolean validPosition(double pos)
Is this a valid position in the projection plane for this image. This
default is appropriate for all projections where the projection plane is infinite.
public boolean allValid()
Are all points in the projection plane valid?
public boolean straddleable()
Is it possible for a pixel to straddle the valid region.
public boolean straddle(double pnts)
Does this pixel go wrap around the standard region of the image?
E.g., does it straddle the 180 deg in a Car or Ait projection?
public double straddleComponents(double pnts)
If this is a straddling pixel, then return the straddle components
public double shadowPoint(double x,
Return a shadowpoint for the input location.
Shadowpoints are not defined for all projections.
public double tissot(double x,
Return the Tissot ellipse parameters for the projection.
This gives the axes and position angle of an ellipse that reflects
the distotortion at the given point.
If there is no distortion or rotation, then the values
are returned. If the major axis is aligned with latitude, then
the angle (the third parameter) will be 0, or pi/2 if
greater aligned with longitude. If the projection is equal-area
then the first two parameters are multiplicative inverses.
This implementation is a place holder which should be overriden
by specific projections. It returns a set of NaNs to indicate
that the calculation has not been done.
x - The X-value at which the Tissot indicatrix is to be calculated.
y - The Y-value at which the Tissot indicatrix is to be calculated.
A three element double vector [Semimajor, semiminor, position angle]
with the axes size and positional angle of the ellipse.