Obtain a linear approximation to a Mapping, if appropriate astLinearApprox
’
s forward transformation within the specified
range of output coordinates. If the transformation is not sufficiently linear,
no coefficients are returned. ’
s Nin attribute. This box should specify the region over which linearity is
required. "
( 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[0] fit[3]X_in fit[4]Y_in
Y_out = fit[1] fit[5]X_in fit[6]Y_in
Z_out = fit[2] fit[7]X_in fit[8]Y_in
This function fits the Mapping’
s forward transformation. To fit the inverse
transformation, the Mapping should be inverted using astInvert before invoking this
function.
If a Mapping output is found to have a bad value (AST__BAD) at one or more of the test points used in the linearity test, then all the values in the returned fit that correspond to that output are set to AST__BAD. However, this does not affect the linearity tests on the other Mapping outputs - if they are all found to be linear then usable coefficients will be returned for them in the fit, and the function will return a non-zero value. Consequently, it may be necessary to check that the values in the returned fit are not AST__BAD before using them. If all Mapping outputs generate bad values, then zero is returned as the function value.
A value of zero will be returned if this function is invoked with the global error status set, or if it should fail for any reason.