uk.ac.starlink.table.join
Class EllipseSkyMatchEngine

java.lang.Object
  extended by uk.ac.starlink.table.join.AbstractSkyMatchEngine
      extended by uk.ac.starlink.table.join.EllipseSkyMatchEngine
All Implemented Interfaces:
MatchEngine

public class EllipseSkyMatchEngine
extends AbstractSkyMatchEngine

MatchEngine implementation for ellipses on the surface of a (celestial) sphere. The tuples it uses are five-element arrays of Number objects, as follows:

  1. alpha: right ascension coordinate of ellipse centre in radians
  2. delta: declination coordinate of ellipse centre in radians
  3. mu: primary radius of ellipse in radians
  4. nu: secondary radius of ellipse in radians
  5. zeta: position angle in radians (from north pole to primary radius, in direction of positive alpha axis)

Two tuples are considered to match if their ellipses touch or partially overlap. The match score is a normalized value; it is zero for concentric ellipses, 1 if the centre of one ellipse falls on the circumference of the other, and 2 if the ellipses just touch. Intermediate values are assumed for intermediate situations.

Other RA/Dec-like sky coordinate systems may alternatively be used for the alpha/delta coordinates.

The calculations are approximate since in some cases they rely on projecting the ellipses onto a Cartesian plane before evaluating the match, so for large ellipses the criterion will be less exact. For objects the size of most observed stars and galaxies, this approximation is not expected to be problematic.

The calculations are currently done using numerical optimisation.

Since:
30 Aug 2011

Field Summary
 
Fields inherited from interface uk.ac.starlink.table.join.MatchEngine
NO_BINS
 
Constructor Summary
EllipseSkyMatchEngine(SkyPixellator pixellator, double scale)
          Constructor.
 
Method Summary
 boolean canBoundMatch()
          Indicates that the MatchEngine.getMatchBounds(uk.ac.starlink.table.join.NdRange[], int) method can be invoked to provide some sort of useful result.
 Object[] getBins(Object[] tuple)
          Returns a set of keys for bins into which possible matches for a given tuple might fall.
 NdRange getMatchBounds(NdRange[] inRanges, int index)
          Given a range of tuple values, returns a range outside which no match to anything within that range can result.
 DescribedValue[] getMatchParameters()
          Returns a set of DescribedValue objects whose values can be modified to modify the matching criteria.
 ValueInfo getMatchScoreInfo()
          Returns a description of the value returned by the MatchEngine.matchScore(java.lang.Object[], java.lang.Object[]) method.
 double getScale()
          Returns the length scale.
 double getScoreScale()
          Returns a scale value for the match score.
 ValueInfo[] getTupleInfos()
          Returns a set of ValueInfo objects indicating what is required for the elements of each tuple.
 double matchScore(Object[] tuple1, Object[] tuple2)
          Indicates whether two tuples count as matching each other, and if so how closely.
 void setRecogniseCircles(boolean recogniseCircles)
          Determines whether short cuts should be taken in the calculations when the ellipses are actually circles.
 void setScale(double scale)
          Sets the length scale.
 String toString()
           
 
Methods inherited from class uk.ac.starlink.table.join.AbstractSkyMatchEngine
calculateSeparation, getTuningParameters
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Constructor Detail

EllipseSkyMatchEngine

public EllipseSkyMatchEngine(SkyPixellator pixellator,
                             double scale)
Constructor.

Parameters:
pixellator - handles sky pixellisation
scale - initial value for length scale, in radians
Method Detail

setScale

public void setScale(double scale)
Sets the length scale.

Overrides:
setScale in class AbstractSkyMatchEngine
Parameters:
scale - rough value of per-object errors, in radians

getScale

public double getScale()
Returns the length scale.

Overrides:
getScale in class AbstractSkyMatchEngine
Returns:
length scale value in radians

setRecogniseCircles

public void setRecogniseCircles(boolean recogniseCircles)
Determines whether short cuts should be taken in the calculations when the ellipses are actually circles. This is generally a good idea, since it is faster and improves accuracy; the default is therefore true. But you might want to turn it off for purposes of debugging or testing.

Parameters:
recogniseCircles - whether to take circle-specific short cuts

getTupleInfos

public ValueInfo[] getTupleInfos()
Description copied from interface: MatchEngine
Returns a set of ValueInfo objects indicating what is required for the elements of each tuple. The length of this array is the number of elements in the tuple. Each element should at least have a defined name and content class. The info's nullable attribute has a special meaning: if true it means that it makes sense for this element of the tuple to be always blank (for instance assigned to no column).

Returns:
array of objects describing the requirements on each element of the tuples used for matching

getMatchParameters

public DescribedValue[] getMatchParameters()
Description copied from interface: MatchEngine
Returns a set of DescribedValue objects whose values can be modified to modify the matching criteria. Typically at least one of these will be some sort of tolerance separation which determines how close tuples must be to count as a match. This match engine's behaviour can be modified by calling DescribedValue.setValue(java.lang.Object) on the returned objects.

Returns:
array of described values which influence the match

getMatchScoreInfo

public ValueInfo getMatchScoreInfo()
Description copied from interface: MatchEngine
Returns a description of the value returned by the MatchEngine.matchScore(java.lang.Object[], java.lang.Object[]) method. The content class should be numeric (though need not be Double), and the name, description and units should be descriptive of whatever the physical significance of the value is. If the result of matchScore is not interesting (for instance, if it's always either 0 or -1), null may be returned.

Returns:
metadata for the match score results

matchScore

public double matchScore(Object[] tuple1,
                         Object[] tuple2)
Description copied from interface: MatchEngine
Indicates whether two tuples count as matching each other, and if so how closely. If tuple1 and tuple2 are considered as a matching pair, then a non-negative value should be returned indicating how close the match is - the higher the number the worse the match, and a return value of zero indicates a 'perfect' match. If the two tuples do not consitute a matching pair, then a negative number (conventionally -1.0) should be returned. This return value can be thought of as (and will often correspond physically with) the distance in some real or notional space between the points represented by the two submitted tuples.

If there's no reason to do otherwise, the range 0..1 is recommended for successul matches. However, if the result has some sort of physical meaning (such as a distance in real space) that may be used instead.

Parameters:
tuple1 - one tuple
tuple2 - the other tuple
Returns:
'distance' between tuple1 and tuple2; 0 is a perfect match, larger values indicate worse matches, negative values indicate no match

getScoreScale

public double getScoreScale()
Description copied from interface: MatchEngine
Returns a scale value for the match score. The intention is that the result of matchScore/MatchEngine.getScoreScale() is of order unity, and is thus comparable between different match engines.

As a general rule, the result should be the maximum value ever returned from the matchScore method, corresponding to the least good successful match. For binary MatchEngine implementations (all matches are either score=0 or failures) a value of 1 is recommended. If nothing reliable can be said about the scale, NaN may be returned.

Returns:
scale of successful match scores, a positive finite number or NaN

getBins

public Object[] getBins(Object[] tuple)
Description copied from interface: MatchEngine
Returns a set of keys for bins into which possible matches for a given tuple might fall. The returned objects can be anything, but should have their equals and hashCode methods implemented properly for comparison.

Parameters:
tuple - tuple
Returns:
set of bin keys which might be returned by invoking this method on other tuples which count as matches for the submitted tuple

canBoundMatch

public boolean canBoundMatch()
Description copied from interface: MatchEngine
Indicates that the MatchEngine.getMatchBounds(uk.ac.starlink.table.join.NdRange[], int) method can be invoked to provide some sort of useful result.

Returns:
true iff getMatchBounds may provide useful information

getMatchBounds

public NdRange getMatchBounds(NdRange[] inRanges,
                              int index)
Description copied from interface: MatchEngine
Given a range of tuple values, returns a range outside which no match to anything within that range can result. If the tuples on which this engine works represent some kind of space, the input values and output values specify a hyper-rectangular region of this space. In the common case in which the match criteria are based on proximity in this space up to a certain error, this method should return a rectangle which is like the input one but broadened in each direction by an amount corresponding to the error.

Both the input and output rectangles are specified by tuples representing its opposite corners; equivalently, they are the minimum and maximum values of each tuple element. In either the input or output min/max tuples, any element may be null to indicate that no information is available on the bounds of that tuple element (coordinate).

An array of n-dimensional ranges is given, though only one of them (specified by the index value) forms the basis for the output range. The other ranges in the input array may in some cases be needed as context in order to do the calculation. If the match error is fixed, only the single input n-d range is needed to work out the single output range. However, if the errors are obtained by looking at the tuples themselves (match errors are per-row) then in general the broadening has to be done using the maximum error of any of the tables involved in the match, not just the one to be broadened. For a long time, I didn't realise this, so versions of this software up to STIL v3.0-14 (Oct 2015) were not correctly broadening these ranges, leading to potentially missed associations near the edge of bounded regions.

This method can be used by match algorithms which know in advance the range of coordinates they will match against and wish to reduce workload by not attempting matches which are bound to fail.

For example, a 1-d Cartesian match engine with an isotropic match error 0.5 would turn input values of ((0,200),(10,210)) into output values ((-0.5,199.5),(10.5,210.5)).

This method will only be called if MatchEngine.canBoundMatch() returns true. Thus engines that cannot provide any useful information along these lines (for instance because none of its tuple elements is Comparable) do not need to implement it in a meaningful way.

Parameters:
inRanges - array of input ranges for the tables on which the match will take place; each element bounds the values for each tuple element in its corresponding table in a possible match (to put it another way - each element gives the coordinates of the opposite corners of a tuple-space rectangle covered by one input table)
index - which element of the inRanges array for which the broadened output value is required
Returns:
output range, effectively inRanges[index] broadened by errors
See Also:
MatchEngine.canBoundMatch()

toString

public String toString()
Specified by:
toString in class AbstractSkyMatchEngine

Copyright © 2004 CLRC: Central Laboratory of the Research Councils. All rights reserved.