# Move a point along a curve

I am trying to implement the de Casteljau algorithm to trace a point along a BÃ©zier curve.

I already have the BÃ©zier curve in place given by the control points

pts = {{54.78, 62.24}, {26.87, 68.24}, {1.58, 63.24},
{-1, 49.18}, {1.58, 35.18}, {28.57, 35.03}, {52.41, 35.03},
{60.48, 22.58}, {52.4, 7.4}, {24.27, 2}, {0, 10}};

I have been reading up on the de Casteljau algorithm and I understand the basic implementation of it, but I don’t understand how to manipulate the point to move along the curve using the algorithm.

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1

You might want to point to the textbook you’re using so that answerers can write something you can easily relate to what you’re reading.
– J. M.♦
Feb 16 at 19:56

1

Do you really need to implement this algorithm on your own as opposed to simply using the built in BezierFunction ?
– george2079
Feb 16 at 20:03

1

Related: mathematica.stackexchange.com/questions/14736/…
– Michael E2
Feb 16 at 23:59

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1

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As suggested by george2079

pts = {{54.78, 62.24}, {26.87, 68.24}, {1.58, 63.24}, {-1, 49.18}, {1.58,
35.18}, {28.57, 35.03}, {52.41, 35.03}, {60.48, 22.58}, {52.4,
7.4}, {24.27, 2}, {0, 10}};

f = BezierFunction[pts];

llp = ListLinePlot[Table[f[t], {t, 0, 1, .02}]];

Manipulate[
Show[llp,
Epilog -> {Red, AbsolutePointSize[6],
Point[f[u]]}],
{{u, .5}, 0, 1, .01, Appearance -> “Labeled”}]

EDIT: In a comment, yode suggested the use of ParametricPlot). This has the advantage of exploiting the adaptive sampling of the built-in plotting functions and is more direct (i.e., you don’t have to generate the Table of points used with ListLinePlot).

pp = ParametricPlot[f[t], {t, 0, 1}];

Manipulate[
Show[pp,
AspectRatio -> 1/GoldenRatio,
Epilog -> {Red, AbsolutePointSize[6],
Point[f[u]]}],
{{u, .5}, 0, 1, .01, Appearance -> “Labeled”}]

f = BezierFunction[pts]; Manipulate[ Show[ParametricPlot[f[t], {t, 0, 1}], Epilog -> {Red, AbsolutePointSize[6], Point[f[u]]}], {{u, .5}, 0, 1, .01, Appearance -> “Labeled”}]
– yode
Feb 17 at 16:39

@yode – Use of ParametricPlot is a good idea; however, since like the ListLinePlot it is independent of the Manipulate control, I would pull the plot outside of the Manipulate and use Show to keep the interaction as responsive as possible. Recommend that you post this as an answer.
– Bob Hanlon
Feb 17 at 16:57

The honor should be attributed to you.I just wanna give a little advice to your answer.
– yode
Feb 17 at 18:32

@yode – edited per your recommendation.
– Bob Hanlon
Feb 17 at 19:15