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A Library for Handling
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Description:
This function tests the forward coordinate transformation implemented by a
Mapping over a given
range of input coordinates. If the transformation is found to be linear to a specified level of
accuracy, then an array of fit coefficients is returned. These may be used to implement a
linear approximation to the Mapping
’
s forward transformation within the specified range
of output coordinates. If the transformation is not sufficiently linear, no coefficients are
returned.
Invocation
RESULT = AST_LINEARAPPROX( THIS, LBND, UBND, TOL, FIT,
STATUS )
Arguments
THIS = INTEGER (Given)
Pointer to the Mapping.
LBND(
) =
DOUBLE PRECISION (Given)
An array containing the lower bounds of a box defined within the
input coordinate system of the Mapping. The number of elements in this array should equal the value
of the Mapping
’
s
Nin attribute. This box should specify the region over which linearity is required.
UBND(
) = DOUBLE PRECISION (Given)
An array containing the upper bounds of the box specifying
the region over which linearity is required.
TOL = DOUBLE PRECISION (Given)
The maximum permitted deviation from linearity, expressed as a positive Cartesian
displacement in the output coordinate space of the Mapping. If a linear fit to the forward
transformation of the Mapping deviates from the true transformation by more than this
amount at any point which is tested, then no fit coefficients will be returned.
FIT(
) =
DOUBLE PRECISION (Returned)
An array in which to return the co-efficients of the linear
approximation to the specified transformation. This array should have at least
"
( Nin
1 )
Nout"
,
elements. The first Nout elements hold the constant offsets for the transformation outputs. The
remaining elements hold the gradients. So if the Mapping has 2 inputs and 3 outputs the linear
approximation to the forward transformation is:
X_out = fit(1)
fit(4)X_in
fit(5)Y_in
Y_out = fit(2)
fit(6)X_in
fit(7)Y_in
Z_out = fit(3)
fit(8)X_in
fit(9)Y_in
STATUS = INTEGER (Given and Returned)
The global status.
Returned Value
AST_LINEARAPPROX = LOGICAL
If the forward transformation is sufficiently linear, .TRUE is
returned. Otherwise .FALSE. is returned and the fit co-efficients are set to AST__BAD.
Notes:
-
This function fits the Mapping’
s forward transformation. To fit the inverse transformation, the
Mapping should be inverted using AST_INVERT before invoking this function.
-
A value of .FALSE. will be returned if this function is invoked with the global error status set, or if it
should fail for any reason.
Copyright (C) 2014 Science & Technology Facilities Council
- ←Prev
- AST
A Library for Handling
World
Coordinate Systems
in Astronomy - Next→
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