[
https://issues.apache.org/jira/browse/SIS-455?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Martin Desruisseaux closed SIS-455.
-----------------------------------
> Compute length of cubic Bézier curve
> ------------------------------------
>
> Key: SIS-455
> URL: https://issues.apache.org/jira/browse/SIS-455
> Project: Spatial Information Systems
> Issue Type: Improvement
> Components: Geometry, Referencing
> Reporter: Martin Desruisseaux
> Assignee: Martin Desruisseaux
> Priority: Major
> Fix For: 1.0
>
>
> We need a way to estimate the length of a cubic Bézier curve from its
> starting point at _t_=0 to an arbitrary _t_ value where _t_ ∈ [0…1].
> Conversely, we need to estimate the _t_ parameter for a given length since
> the starting point. There is no exact solution for this problem, but we may
> estimate the length using Legendre-Gauss approach documented in [A Primer on
> Bézier Curves|https://pomax.github.io/bezierinfo/#arclength] page. The
> accuracy is determined by the number [Gaussian Quadrature Weights and
> Abscissae|https://pomax.github.io/bezierinfo/legendre-gauss.html] used. For
> example with 3 terms:
> {noformat}
> w₁ = 0.8888888888888888; a₁ = 0;
> w₂ = 0.5555555555555556; a₂ = -0.7745966692414834;
> w₃ = 0.5555555555555556; a₃ = +0.7745966692414834;
> length(t) ≈ t/2 * (w₁*f(a₁*t/2 - t/2) + w₂*f(a₂*t/2 - t/2) + w₃*f(a₃*t/2 -
> t/2))
> {noformat}
> with _f(t)_ defined as {{hypot(x′(t), y′(t))}} and with _x′(t)_ and _y′(t)_
> the first derivatives of Bézier equations for _x(t)_ and _y(t)_.
> Once we have the length for a given _t_ value, we can try to find the
> converse by using an iterative approach as described in the [Moving Along a
> Curve with Specified
> Speed|https://www.geometrictools.com/Documentation/MovingAlongCurveSpecifiedSpeed.pdf]
> paper from geometric tools.
> Once we are able to estimate the _t_ parameters for a given length, we should
> delete the {{Bezier.isValid(x, y)}} method (and consequently remove its use
> and the {{TransformException}} in the {{curve}} method). Instead, given the
> geodesic distance from Bézier start point to ¼ of the distance from start
> point to end point, estimate the _t_ parameter at that position. It should be
> a value close but not identical to _t_≈¼. We can then compute the (_x_, _y_)
> coordinates of the point on that curve at that _t_ parameter value and
> compare with the expected coordinates. It should (hopefully) be a point
> closer to expected than the point computed at exactly _t_=¼, thus removing
> the need for the {{Bezier.isValid(x,y)}} hack.
> Alternatively, all the above is a complicated way to say that we want the
> shortest distance between a point on the geodesic path and a point on the
> curve which is known to be at position close to (but not exactly at) _t_≈¼
> and _t_≈¾.
--
This message was sent by Atlassian Jira
(v8.3.4#803005)
