# Getting coefficients for PathCubic?

• Hi,

I would like to compute dependent variable based on independent variable from cubic polynomial which is basically PathCubic.
I would be grateful if someone could tell me, where do I get coefficients from PathCubic ?

Big thanks,
Mateusz

• Hi! There is no public interface other than that described in the manual: PathCubic QML Element.

• Ok, so it's only to move control points with mouse but no way to know the function or get x/y values at any given point ?

I found this https://doc-snapshots.qt.io/qt5-dev/qml-qtquick-pathinterpolator.html
But unfortunately it's progress not x value this takes as an argument and even then fitting a cubic polynomial again just to get coefficients, would be wired.

I hope they fix that...

• Of course it takes a "progress" and not an x value; the curve is parameterized along its local normalized coordinate t. Also, tbh, I don't see what you want to get fixed there, it's f(t) -> (x, y). Just give the path interpolator said progress value and read its x and y properties.

• I would like to get cubic polynomial coefficients of PathCubic, if possible. That would be really helpeful and usueful.

• I would like to get cubic polynomial coefficients of PathCubic, if possible. That would be really helpeful and usueful.

I think you're missing the point. The polynomial is given as a parametric curve f(t) = {x, y}, and not as the scalar function y = y(x), just as @Wieland wrote. So, no, you can't know what y corresponds to which x unless you know the exact way the curve was parametrized.

• Ah right, its Beizer curve not simple polynomial.

In my case I want to set control points both with mouse and with SpinBox to see exact values and let user to type exact values.

Thanks