贝塞尔曲线
贝塞尔曲线得名于法国工程师贝塞尔(Pierre Bézier)1962年开始的大力推广,最初主要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
, 它的参数方程如下所示:
其中 为参数。一阶贝塞尔曲线上各点是两个控制点 和 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
. 二阶贝塞尔曲线的解析表达式如下:
二阶贝塞尔曲线绘制过程如图所示[1]
由二阶贝塞尔曲线参数方程
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 与 两个控制点之间的插值点 ,
- 然后计算出 与 两个控制点之间的插值点 ,
- 最后再取 与 两点之间的插值点 , 点 即二阶贝塞尔曲线上的点。
当
时, 计算过程如下图所示:
将二阶贝塞尔曲线公式对参数
求导,
由上式和二阶贝塞尔曲线图可看出, 贝塞尔曲线在端点 处切线为 , 在端点 处切线为 , 贝塞尔曲线在两端点 处切线相交于 . 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
的二阶导数为,
由上式可知,贝塞尔曲线从端点 处开始脱离 , 并逐渐在端点 处逼近 .
三阶贝塞尔曲线
需要四个控制点
n阶贝塞尔曲线
需要(n+1)个控制点
其中 .
如上式所示, 阶贝塞尔曲线是两个 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/