The BILINEAR function uses a bilinear interpolation algorithm to compute the value of a data array at each of a set of subscript values.

This routine is written in the IDL language. Its source code can be found in the file bilinear.pro in the lib subdirectory of the IDL distribution.

## Examples

Create a 3 x 3 floating point array *P*:

P = FINDGEN(3,3)

Suppose we wish to find the value of a point half way between the first and second elements of the first row of *P*. Create the subscript arrays* IX *and *JY*:

`; Define the X subscript.`

`IX = 0.5`

`; Define the Y subscript.`

`JY = 0.0 `

`; Interpolate.`

`Z = BILINEAR(P, IX, JY) `

`; Print the value at the point IX,JY within P.`

`PRINT, Z `

IDL prints:

0.500000

Suppose we wish to find the values of a 2 x 2 array of points in *P*. Create the subscript arrays* IX *and *JY*:

`;Define the X subscripts.`

IX = [[0.5, 1.9], [1.1, 2.2]]

;Define the Y subscripts and interpolate.

JY = [[0.1, 0.9], [1.2, 1.8]]

Z = BILINEAR(P, IX, JY)

;Print the array of values.

PRINT, Z

IDL prints:

0.800000 4.60000

4.70000 7.40000

## Syntax

*Result* = BILINEAR(*P, IX, JY *[, MISSING=*value*])

## Return Value

This function returns a two-dimensional interpolated array of the same type as the input array.

## Arguments

### P

A two-dimensional data array.

### IX and JY

Arrays containing the X and Y “virtual subscripts” of *P *for which to interpolate values. *IX *and *JY *can be either of the following:

- One-dimensional,
*n*-element floating-point arrays of subscripts to look up in*P.*One-dimensional arrays will be converted to two-dimensional arrays in such a way that*IX*contains*n*identical rows and*JY*contains*n*identical columns. - Two-dimensional,
*n*-element floating-point arrays that uniquely specify the X subscripts (the*IX*array) and the Y subscripts (the*JY*array) of the points to be computed from the input array*P*.

Location points outside the bounds of the array *P*. Elements of the *IX* or *IY* arguments that are either less than zero or greater than the largest subscript in the corresponding dimension of *P* are interpolated to the closest value within the bounds of the array *P*.

It is better to use two-dimensional arrays for *IX *and *JY *because the algorithm is somewhat faster. If *IX* and *JY *are specified as one-dimensional, the returned two-dimensional arrays *IX *and *JY *can be re-used on subsequent calls to take advantage of the faster 2D algorithm.

## Keywords

### MISSING

The value to return for elements outside the bounds of *P.* The bounds of *P* are 0 to *n*-1 and 0 to *m*-1 where *P* is an *n * x *m* array.

If MISSING value is set to a complex number, IDL uses only the real part.

## Version History

Original |
Introduced |

6.1 |
Added MISSING keyword |