[ https://issues.apache.org/jira/browse/MATH1471?page=com.atlassian.jira.plugin.system.issuetabpanels:alltabpanel
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Thomas Swinicki updated MATH1471:

Description:
Upon performing a bicubic interpolation with two point (x0, y0) and (x1, y1), the returned
bicubic interpolating function returned returns the same result for variations in the estimated
y value.
For example, my inputs are (20, 20) and (25, 25) with f(20, 20) = 64 and f(25, 25) = 6468.
When I get the bicubic interpolating function for this and vary the estimated x, it works
fine. For (21, 20), the function returns 730.016. When I input (20, 21), the function returns
64, which is f(20, 20). For any y value in between 20 and 25, the result is 64. This is the
case for any function for which the y estimate is different from the value on the points.
In other instances, it is varying x values that result in the same result while varying y
estimates seem to work as expected.
was:
Upon performing a bicubic interpolation with two point (x0, y0) and (x1, y1), the returned
bicubic interpolating function returned returns the same result for variations in the estimated
y value.
For example, my inputs are (20, 20) and (25, 25) with f(20, 20) = 64 and f(25, 25) = 6468.
When I get the bicubic interpolating function for this and vary the estimated x, it works
fine. For (21, 20), the function returns 730.016. When I input (20, 21), the function returns
64, which is f(20, 20). For any y value in between 20 and 25, the result is 64. This is the
case for any function for which the y estimate is different from the value on the points.
> BicubicInterpolatingFunction not interpolating correctly for non discrete y value
> 
>
> Key: MATH1471
> URL: https://issues.apache.org/jira/browse/MATH1471
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.6.1
> Environment: JDK 1.8.0_181
> Reporter: Thomas Swinicki
> Priority: Major
>
> Upon performing a bicubic interpolation with two point (x0, y0) and (x1, y1), the returned
bicubic interpolating function returned returns the same result for variations in the estimated
y value.
> For example, my inputs are (20, 20) and (25, 25) with f(20, 20) = 64 and f(25, 25) =
6468.
> When I get the bicubic interpolating function for this and vary the estimated x, it works
fine. For (21, 20), the function returns 730.016. When I input (20, 21), the function returns
64, which is f(20, 20). For any y value in between 20 and 25, the result is 64. This is the
case for any function for which the y estimate is different from the value on the points.
> In other instances, it is varying x values that result in the same result while varying
y estimates seem to work as expected.

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