贝塞尔曲线

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分类专栏： Geometry 文章标签：算法

于 2018-02-09 10:52:08 首次发布

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本文深入讲解了贝塞尔曲线的基本概念及一阶、二阶、三阶乃至n阶曲线的数学表达式。通过实例演示了如何利用参数方程计算特定点，并提供了JavaScript实现代码。

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贝塞尔曲线

贝塞尔曲线得名于法国工程师贝塞尔(Pierre Bézier)。他从1962年开始大力推广其应用。最初应用于汽车造型设计中车身曲线拟合。

##一阶贝塞尔曲线

一阶贝塞尔曲线需要两个控制点 $P_{0} , P_{1} $, 它的参数方程如下所示:
$P_{0} + t(P_{1} - P_{0}) = (1-t)P_{0} + t P_{1}, ~~ t\in[0,1]$

其中 $t$ 为参数。一阶贝塞尔曲线上各点是两个控制点 $P_{0}$ 和 $ P_{1} $之间的线性插值计算得出, 实际上就是连接两控制点的直线段。

##二阶贝塞尔曲线

二阶贝塞尔曲线需要三个控制点 $P_{0} , P_{1}, P_{2} $. 二阶贝塞尔曲线的解析表达式如下:
$(1-t)^{2} P_{0} + 2 t (1-t) P_{1} + t^{2} P_{2} ， ~~ t\in[0,1]$
$1-t)^{2} P_{0} + t (1-t) P_{1} + t (1-t) P_{1}+ t^{2} P_{2}$
$1-t)[(1-t) P_{0} + t P_{1}] + t[ (1-t) P_{1}+ t P_{2} ]$
$\begin{bmatrix} (1-t)^2 & 2(1-t)t & t^2 \end{bmatrix} \begin{bmatrix} P_{0} \\ P_{1} \\ P_{2} \end{bmatrix}$
$\begin{bmatrix} 1 & t & t^2 \end{bmatrix} \begin{bmatrix} 1 & 0 & 0\\ -2 & 2& 0 \\1& -2 & 1\end{bmatrix} \begin{bmatrix} P_{0} \\ P_{1} \\ P_{2} \end{bmatrix}$

二阶贝塞尔曲线绘制过程如图所示[1]
2nd-order-Bezier

由二阶贝塞尔曲线参数方程
$B(t) = (1-t)[(1-t) P_{0} + t P_{1}] + t[ (1-t) P_{1}+ t P_{2} ]$
可以看出，二阶贝塞尔曲线是两个一阶贝塞尔曲线的线性插值:

首先计算出 $P_{0}$ 与 $P_{1}$ 两个控制点之间的插值点 $P_{01} = (1-t) P_{0}+ t P_{1}$ ,
然后计算出 $P_{1}$ 与 $P_{2}$ 两个控制点之间的插值点 $P_{12} = (1-t) P_{1}+ t P_{2}$ ,
最后再取 $P_{01}$ 与 $P_{02}$ 两点之间的插值点$P = (1-t) P_{01} + t P_{12} $, 点$ P$ 即二阶贝塞尔曲线上的点。

当 $t = 0.5$ 时，计算过程如下图所示:
2nd-order-Bezier-t-0.5

将二阶贝塞尔曲线公式对参数 $t$ 求导，
$B'(t) = 2(1-t)(P_{1} - P_{0}) + 2t( P_{2} - P_{1})$
由上式和二阶贝塞尔曲线图可看出，贝塞尔曲线在端点$P_{0} $处切线为$ P_{0} P_{1} $, 在端点$ P_{2} $处切线为$ P_{1} P_{2} $, 贝塞尔曲线在两端点$ P_{0} , P_{2} $处切线相交于$ P_{1} $. 二阶贝塞尔曲线上每一点的导数是两个端点处曲线导数的线性插值。

二阶贝塞尔曲线公式对参数 $t$ 的二阶导数为，
$B''(t) = 2( P_{2} - 2P_{1} + P_{0})$
由上式可知，贝塞尔曲线从端点$P_{0} $处开始脱离$ P_{0} P_{1} $，并逐渐在端点$ P_{2} $ 处逼近 $ P_{1} P_{2} $ .

##三阶贝塞尔曲线

需要四个控制点 $P_{0} , P_{1}, P_{2}, P_{3} $

$(1-t)^{3} P_{0} +3 t (1-t)^{2} P_{1} +3 t^{2}(1-t) P_{2 } + t^{3} P_{3} , ~~ t\in[0,1]$

##n阶贝塞尔曲线

需要(n+1)个控制点 $P_{0} , P_{1}, P_{2}, \cdots,P_{n} $
$\sum_{i=0}^{n}C_{i}^{n} P_{i}t^{i}(1-t)^{n-i}, ~~ t\in[0,1]$
其中 $Cin=n!i!(n−i)!C_{i}^{n} = \frac{n!}{i!(n-i)!}$ .

$t∈[0,1]B_{P_{0}, \cdots, P_{n}} (t) = (1-t)B_{P_{0}, \cdots, P_{n-1}} (t) + t B_{P_{1}, \cdots, P_{n}}(t) ,~~ t\in[0,1]$
如上式所示， $n$ 阶贝塞尔曲线是两个 $(n - 1)$ 阶贝塞尔曲线的线性插值.

#Code

//
//   Author: Chunfeng Yang
//   Version: 0.2.0
//
// Parameters: 
//    controlPoints  -- control points array of the Bezier curve
//             It contains the coordinates of control points
//             data type: Array
//
//    t     -- parameter t of the Biezier curve
//             data type: float
//
//    start -- the index of start control point in the strP array 
//             data type: int
//
//    end   -- the index of end point in the strP array  
//             data type: int
//
function BezierCurve( controlPoints, t, start, end )
{

  if( undefined === controlPoints )
  {
    console.error('ERROR: undefined point array ');
    return;
  }

  if( Object.prototype.toString.call( controlPoints ) !== '[object Array]' ) 
  {
    console.error('ERROR: invalided point array ');
    return;
  }

  if( undefined === t )
  { 
    console.log('Warning: t is undefined, using default value: t = 0.0 ');
    t = 0.0;
  } 
  if( undefined === start )
  {
    console.log('Warning: start is undefined, using default value: start = 0');
    start = 0;
  }
  if( undefined === end )
  {
    console.log('Warning: end is undefined, using default value: end = controlPoints.length - 1');
    end = controlPoints.length - 1;
  }

  if( parseInt(start) > parseInt(end) - 1 )
  {
    console.error('ERROR: start > end - 1 ');
    return;
  }
 
  var len = controlPoints.length;
  if( 0 === parseInt(len) )
  {
    console.error('ERROR: point array length is ZERO');
    return;
  }

  if( parseInt(start) < 0 )
  {
    console.log('Warning: start is invalided, using default value: start = 0');
    start = 0;
  }
  if( parseInt(end) < 1 )
  {
    console.log('Warning: end is invalided, using default value: end = controlPoints.length - 1');
    end = controlPoints.length - 1;
  }
  if( parseInt(start) > parseInt(len) - 1 )
  {
    console.log('Warning: start is invalided, using default value: start = 0');
    start = 0;
  }
  if( parseInt(end) > parseInt(len) - 1 )
  {
    console.log('Warning: end is invalided, using default value: end = controlPoints.length - 1');
    end = controlPoints.length - 1;
  }

     if( 1 == ( parseInt(end) - parseInt(start) ) )
     {
       var p1 = controlPoints[start];  
       var p2 = controlPoints[end];  

       var result = [];
       for( var item in p1 )  
       {
            var delta =  parseFloat( p2[item] ) -  parseFloat( p1[item] )
            result.push( parseFloat( p1[item] ) + t * parseFloat( delta ) );
        }
        return result;

      }else {
       var p1 = BezierCurve( controlPoints, t, start, parseInt(end) -1 ) ; 
       var p2 = BezierCurve( controlPoints, t, parseInt(start) + 1, end  ); 
       var result = [];
       for( var item in p1 )  
       {
            result.push(( 1-parseFloat(t)) * parseFloat( p1[item] ) + t * parseFloat(p2[item]));
        }
        return result;
     }
     
  return;
}

Testing Scenario
取一平面二阶Bezier曲线，其控制点有三个，分别为[10, 40 ], [20, 30 ]和 [30, 40]。当参数t=0.5时，曲线中点坐标应为[20,35].

var f;
var result = 0;

//
// Test 1 
//
var a = "Hello world"
result = BezierCurve( a, 0.5, 0, 2 )
if( undefined == result )
{
  console.log( "Test 1: controlPoints type is String test -- OK" );
}


//
// Test 2 
//
var a = 3.2 
result = BezierCurve( a, 0.5, 0, 2 )
if( undefined == result )
{
  console.log( "Test 2: controlPoints type is Number test -- OK" );
}


//
// Test 3 
//
var a = {"Helloworld":3}
result = BezierCurve( a, 0.5, 0, 2 )
if( undefined == result )
{
  console.log( "Test 3: controlPoints type is JSON test -- OK" );
}


//
// Test 4 
//
var a = {"Helloworld":3}
result = BezierCurve( f, 0.5, 0, 2 )
if( undefined == result )
{
  console.log( "Test 4: undefined controlPoints test -- OK" );
}

//
// Test 5 
//
var a = [[10, 40 ], [20, 30 ], [30, 40], [40, 30]];
result = BezierCurve( a, f, 0, 2 )
if( undefined !== result )
{
  if( ( 10 == parseInt(result[0]) ) & ( 40 == parseInt(result[1]) ) )
  {
    console.log( "Test 5: undefined t test -- OK" );
  }
}


//
// Test 6 
//
var a = [[10, 40 ], [20, 30 ], [30, 40], [40, 30]];
result = BezierCurve( a, 0, f, 2 )
if( undefined !== result )
{
  if( ( 10 == parseInt(result[0]) ) & ( 40 == parseInt(result[1]) ) )
  {
    console.log( "Test 6: undefined start test -- OK" );
  }
}


//
// Test 7 
//
var a = [[10, 40 ], [20, 30 ], [30, 40], [40, 30]];
result = BezierCurve( a, 0, 0, f )
if( undefined !== result )
{
  if( ( 10 == parseInt(result[0]) ) & ( 40 == parseInt(result[1]) ) )
  {
    console.log( "Test 7: undefined end test -- OK" );
  }
}


//
// Test 8 
//
var a = [ ];
result = BezierCurve( a, 0, 0, 2 )
if( undefined == result )
{
  console.log( "Test 8: point array length is ZERO test -- OK" );
}


//
// Test 9 
//
var a = [[10, 40 ], [20, 30 ], [30, 40], [40, 30]];
result = BezierCurve( a, 0, 2, 2 )
if( undefined == result )
{
  console.log( "Test 9: start > end - 1 test -- OK" );
}


//
// Test 10 
//
var a = [[10, 40 ], [20, 30 ], [30, 40], [40, 30]];
result = BezierCurve( a, 0, 9, 10 )
if( undefined !== result )
{
  if( ( 10 == parseInt(result[0]) ) & ( 40 == parseInt(result[1]) ) )
  {
    console.log( "Test 10: invalided end test -- OK" );
  }
}


//
// Test 11 
//
var a = [[10, 40 ], [20, 30 ], [30, 40], [40, 30]];
result = BezierCurve( a, 0.5, 0, 2 )
if( undefined !== result )
{
  if( ( 20 == parseInt(result[0]) ) & ( 35 == parseInt(result[1]) ) )
  {
    console.log( "Test 11:  calculation test -- OK" );
  }
}
console.log(result);

Results:

ERROR: invalided point array
Test 1: controlPoints type is String test -- OK
ERROR: invalided point array
Test 2: controlPoints type is Number test -- OK
ERROR: invalided point array
Test 3: controlPoints type is JSON test -- OK
ERROR: undefined point array
Test 4: undefined controlPoints test -- OK
Warning: t is undefined, using default value: t = 0.0
Test 5: undefined t test -- OK
Warning: start is undefined, using default value: start = 0
Test 6: undefined start test -- OK
Warning: end is undefined, using default value: end = controlPoints.length - 1
Test 7: undefined end test -- OK
ERROR: point array length is ZERO
Test 8: point array length is ZERO test -- OK
ERROR: start > end - 1
Test 9: start > end - 1 test -- OK
Warning: start is invalided, using default value: start = 0
Warning: end is invalided, using default value: end = controlPoints.length - 1
Test 10: invalided end test -- OK
Test 11:  calculation test -- OK
[ 20, 35 ]

[1] https://en.wikipedia.org/wiki/B%C3%A9zier_curve
[2] http://pages.mtu.edu/~shene/COURSES/cs3621/NOTES/
[3] https://www.zhihu.com/question/29565629
[4] https://www.scratchapixel.com/lessons/advanced-rendering/bezier-curve-rendering-utah-teapot/bezier-curve
[5] Bezier.js https://github.com/Pomax/bezierjs
[6] http://web.cs.wpi.edu/~matt/courses/cs563/talks/surface/bez_surf.html
[7] https://www.scratchapixel.com/lessons/advanced-rendering/bezier-curve-rendering-utah-teapot
[8] http://paulbourke.net/geometry/bezier/
[9] https://pomax.github.io/bezierinfo/
[10] https://www.particleincell.com/2012/bezier-splines/
[11] https://www.particleincell.com/2013/cubic-line-intersection/
[12] https://pomax.github.io/bezierinfo/