Integer, Intent (In)	::	mx, my, px, py, lwrk, liwrk
Integer, Intent (Inout)	::	ifail
Integer, Intent (Out)	::	iwrk(liwrk)
Real (Kind=nag_wp), Intent (In)	::	x(mx), y(my), lamda(px), mu(py), c((px-4)*(py-4))
Real (Kind=nag_wp), Intent (Out)	::	ff(mx*my), wrk(lwrk)

C Header Interface

#include <nag.h>

void	e02dff_ (const Integer mx, const Integer my, const Integer px, const Integer py, const double x[], const double y[], const double lamda[], const double mu[], const double c[], double ff[], double wrk[], const Integer lwrk, Integer iwrk[], const Integer liwrk, Integer *ifail)

The routine may be called by the names e02dff or nagf_fit_dim2_spline_evalm.

3 Description

e02dff calculates values of the bicubic spline

s (x, y)

on a rectangular grid of points in the

x

y

plane, from its augmented knot sets

{λ}

and

{μ}

and from the coefficients

c_{i j}

, for

i = 1, 2, \dots, px - 4

and

j = 1, 2, \dots, py - 4

, in its B-spline representation

s (x, y) = \sum_{i j} c_{i j} M_{i} (x) N_{j} (y) .

Here

M_{i} (x)

and

N_{j} (y)

denote normalized cubic B-splines, the former defined on the knots

λ_{i}

λ_{i + 4}

and the latter on the knots

μ_{j}

μ_{j + 4}

The points in the grid are defined by coordinates

x_{q}

, for

q = 1, 2, \dots, m_{x}

, along the

x

axis, and coordinates

y_{r}

, for

r = 1, 2, \dots, m_{y}

, along the

y

axis.

This routine may be used to calculate values of a bicubic spline given in the form produced by e01daf, e02daf, e02dcf and e02ddf. It is derived from the routine B2VRE in Anthony et al. (1982).

4 References

Anthony G T, Cox M G and Hayes J G (1982) DASL – Data Approximation Subroutine Library National Physical Laboratory

Cox M G (1978) The numerical evaluation of a spline from its B-spline representation J. Inst. Math. Appl. 21 135–143

5 Arguments

1: $mx$ – Integer Input

2: $my$ – Integer Input

On entry: mx and my must specify

m_{x}

and

m_{y}

respectively, the number of points along the

x

and

y

axis that define the rectangular grid.

Constraint:

mx \geq 1

and

my \geq 1

3: $px$ – Integer Input

4: $py$ – Integer Input

On entry: px and py must specify the total number of knots associated with the variables

x

and

y

respectively. They are such that

px - 8

and

py - 8

are the corresponding numbers of interior knots.

Constraint:

px \geq 8

and

py \geq 8

5: $x (mx)$ – Real (Kind=nag_wp) array Input

6: $y (my)$ – Real (Kind=nag_wp) array Input

On entry: x and y must contain

x_{q}

, for

q = 1, 2, \dots, m_{x}

, and

y_{r}

, for

r = 1, 2, \dots, m_{y}

, respectively. These are the

x

and

y

coordinates that define the rectangular grid of points at which values of the spline are required.

Constraint:

x

and y must satisfy

lamda (4) \leq x (q) < x (q + 1) \leq lamda (px - 3), q = 1, 2, \dots, m_{x} - 1

and

mu (4) \leq y (r) < y (r + 1) \leq mu (py - 3), r = 1, 2, \dots, m_{y} - 1 .

The spline representation is not valid outside these intervals.

7: $lamda (px)$ – Real (Kind=nag_wp) array Input

8: $mu (py)$ – Real (Kind=nag_wp) array Input

On entry: lamda and mu must contain the complete sets of knots

{λ}

and

{μ}

associated with the

x

and

y

variables respectively.

Constraint: the knots in each set must be in nondecreasing order, with

lamda (px - 3) > lamda (4)

and

mu (py - 3) > mu (4)

9: $c ((px - 4) \times (py - 4))$ – Real (Kind=nag_wp) array Input

On entry:

c ((py - 4) \times (i - 1) + j)

must contain the coefficient

c_{i j}

described in Section 3, for

i = 1, 2, \dots, px - 4

and

j = 1, 2, \dots, py - 4

10: $ff (mx \times my)$ – Real (Kind=nag_wp) array Output

On exit:

ff (my \times (q - 1) + r)

contains the value of the spline at the point

(x_{q}, y_{r})

, for

q = 1, 2, \dots, m_{x}

and

r = 1, 2, \dots, m_{y}

11: $wrk (lwrk)$ – Real (Kind=nag_wp) array Workspace

12: $lwrk$ – Integer Input

On entry: the dimension of the array wrk as declared in the (sub)program from which e02dff is called.

Constraint:

lwrk \geq \min (4 \times mx + px, 4 \times my + py)

13: $iwrk (liwrk)$ – Integer array Workspace

14: $liwrk$ – Integer Input

On entry: the dimension of the array iwrk as declared in the (sub)program from which e02dff is called.

Constraints:

if $4 \times mx + px > 4 \times my + py$ , $liwrk \geq my + py - 4$ ;
otherwise $liwrk \geq mx + px - 4$ .

15: $ifail$ – Integer Input/Output

On entry: ifail must be set to

0

−1

1

to set behaviour on detection of an error; these values have no effect when no error is detected.

A value of

0

causes the printing of an error message and program execution will be halted; otherwise program execution continues. A value of

−1

means that an error message is printed while a value of

1

means that it is not.

If halting is not appropriate, the value

−1

1

is recommended. If message printing is undesirable, then the value

1

is recommended. Otherwise, the value

0

is recommended. When the value $- 1$ or $1$ is used it is essential to test the value of ifail on exit.

On exit:

ifail = 0

unless the routine detects an error or a warning has been flagged (see Section 6).

6 Error Indicators and Warnings

If on entry

ifail = 0

−1

, explanatory error messages are output on the current error message unit (as defined by x04aaf).

Errors or warnings detected by the routine:

$ifail = 1$: On entry, $mx = ⟨ value ⟩$ .
Constraint: $mx \geq 1$ .

On entry, $my = ⟨ value ⟩$ .
Constraint: $my \geq 1$ .

On entry, $px = ⟨ value ⟩$ .
Constraint: $px \geq 8$ .

On entry, $py = ⟨ value ⟩$ .
Constraint: $py \geq 8$ .

$ifail = 2$: On entry, $liwrk = ⟨ value ⟩$ .
Constraint: $liwrk \geq ⟨ value ⟩$ .

On entry, $lwrk = ⟨ value ⟩$ .
Constraint: $lwrk \geq ⟨ value ⟩$ .

$ifail = 3$: On entry, the knots in lamda are not in nondecreasing order.

On entry, the knots in mu are not in nondecreasing order.

$ifail = 4$: On entry, $lamda (4) \leq x (1) < \dots < x (mx) \leq lamda (px - 3)$ is violated.

On entry, $mu (4) \leq y (1) < \dots < y (my) \leq mu (py - 3)$ is violated.

$ifail = - 99$: An unexpected error has been triggered by this routine. Please contact NAG.
See Section 7 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 399$: Your licence key may have expired or may not have been installed correctly.
See Section 8 in the Introduction to the NAG Library FL Interface for further information.

$ifail = - 999$: Dynamic memory allocation failed.
See Section 9 in the Introduction to the NAG Library FL Interface for further information.

7 Accuracy

The method used to evaluate the B-splines is numerically stable, in the sense that each computed value of

s (x_{r}, y_{r})

can be regarded as the value that would have been obtained in exact arithmetic from slightly perturbed B-spline coefficients. See Cox (1978) for details.

8 Parallelism and Performance

e02dff is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.

Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9 Further Comments

Computation time is approximately proportional to

m_{x} m_{y} + 4 (m_{x} + m_{y})

10 Example

This example reads in knot sets

lamda (1), \dots, lamda (px)

and

mu (1), \dots, mu (py)

, and a set of bicubic spline coefficients

c_{i j}

. Following these are values for

m_{x}

and the

x

coordinates

x_{q}

, for

q = 1, 2, \dots, m_{x}

, and values for

m_{y}

and the

y

coordinates

y_{r}

, for

r = 1, 2, \dots, m_{y}

, defining the grid of points on which the spline is to be evaluated.

e02df: FL CL CPP AD

NAG FL Interfacee02dff (dim2_​spline_​evalm)

▸▿ Contents

1 Purpose

2 Specification

3 Description

4 References

5 Arguments

6 Error Indicators and Warnings

7 Accuracy

8 Parallelism and Performance

9 Further Comments

10 Example

10.1 Program Text

10.2 Program Data

10.3 Program Results

NAG FL Interface
e02dff (dim2_spline_evalm)