Get any point's Coordinates in a Curve

[Schoumi]

Hello,

I use a QwtPlotCurve with some points assign with setRawSamples. I want to get the X or Y cooridnates of a point in the curve using Y or X associated coordinate, but this point is not always a point that i use to create the curve.

Exemple:

i have two point to create the curve

pointA(0,0) and pointB(2,2) and i want to find the coordinate of the point in the curve coresponding to x = 1. in this case the point has 1,1 for coordinates.

is this is possible and how to do.
Thanks for your help.

[FelixB]

no, that is not possible. A QwtPlotCurve has a member to find the nearest point, but it can't interpolate. You have to code such a functionality by yourself, but that should be quite easy by applying some geometrical calculations.

hth,
Felix

[Schoumi]

Hum have you some link that explain how to calculate something like this.
With some points we have a line and it's simple to calculate a coordinate on the line, but with a large number of points it was not so easy or i'm wrong?

In fact my goal is to add a marker on an X coordinate and the Y coordinate move with the curve. Is it possible to do this without recalculating the Y coordinate each time? I didn't find a good way to do this without setting Y value each time. In some cases i need to find local extremum but in this cases i need to calculate this extremum but in case of interpolated curve, the extremum can not be a point use to create the curve, i'm right?

[Uwe]

Hum have you some link that explain how to calculate something like this.
With some points we have a line and it's simple to calculate a coordinate on the line, but with a large number of points it was not so easy or i'm wrong?

You are: each coordinate is between 2 curve points and this is always ( beside of splines ) a simple line. The number of curve points doesn't matter.

Uwe

[Schoumi]

hum ok, but in an interpolated curve, two points can be on each side of an extremum and obtain the extremum value could not be as easier as calculate a line equation between two point, because the result must not be on the curve, or i'm wrong again?

Thanks for your explanation

[FelixB]

Hi schuomi,

a QwtPlotCurve is not a "real" curve. It is a number of points that are connected by straight lines. So it is not possible that there is an extremum between two points.

If your curve uses interpolated data, you must access the "real" data storage to obtain the underlying values.

Felix