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bernstein

Bernstein polynomials

Syntax

Description

example

bernstein(f,n,t) with a function handle f returns the nth-order Bernstein polynomial symsum(nchoosek(n,k)*t^k*(1-t)^(n-k)*f(k/n),k,0,n), evaluated at the point t. This polynomial approximates the function f over the interval [0,1].

example

bernstein(g,n,t) with a symbolic expression or function g returns the nth-order Bernstein polynomial, evaluated at the point t. This syntax regards g as a univariate function of the variable determined by symvar(g,1).

If any argument is symbolic, bernstein converts all arguments except a function handle to symbolic, and converts a function handle's results to symbolic.

example

bernstein(g,var,n,t) with a symbolic expression or function g returns the approximating nth-order Bernstein polynomial, regarding g as a univariate function of the variable var.

Examples

Approximation of the Sine Function Specified as a Function Handle

Approximate the sine function by the 10th- and 100th-degree Bernstein polynomials:

syms t
b10 = bernstein(@(t) sin(2*pi*t), 10, t);
b100 = bernstein(@(t) sin(2*pi*t), 100, t);

Plot sin(2*pi*t) and its approximations:

pr = ezplot(sin(2*pi*t),[0,1]);
hold on
pg = ezplot(b10,[0,1]);
pb = ezplot(b100,[0,1]);

set(pr,'Color','red')
set(pg,'Color','green')
set(pb,'Color','blue')

legend('sine function','10th-degree polynomial',...
                      '100th-degree polynomial')
title('Bernstein polynomials')
hold off

Approximation of the Exponential Function Specified as a Symbolic Expression

Approximate the exponential function by the second-order Bernstein polynomial in the variable t:

syms x t;
bernstein(exp(x), 2, t)
ans =
(t - 1)^2 + t^2*exp(1) - 2*t*exp(1/2)*(t - 1)

Approximate the multivariate exponential function. When you approximate a multivariate function, bernstein regards it as a univariate function of the default variable determined by symvar. The default variable for the expression y*exp(x*y) is x:

syms x y t;
symvar(y*exp(x*y), 1)
ans =
x

bernstein treats this expression as a univariate function of x:

bernstein(y*exp(x*y), 2, t)
ans =
y*(t - 1)^2 + t^2*y*exp(y) - 2*t*y*exp(y/2)*(t - 1)

To treat y*exp(x*y) as a function of the variable y, specify the variable explicitly:

bernstein(y*exp(x*y), y, 2, t)
ans =
t^2*exp(x) - t*exp(x/2)*(t - 1)

Approximation of a Linear Ramp Specified as a Symbolic Function

Approximate function f representing a linear ramp by the fifth-order Bernstein polynomials in the variable t:

syms f(t);
f(t) = triangularPulse(1/4, 3/4, Inf, t);
p = bernstein(f, 5, t)
p =
7*t^3*(t - 1)^2 - 3*t^2*(t - 1)^3 - 5*t^4*(t - 1) + t^5

Simplify the result:

simplify(p)
ans =
-t^2*(2*t - 3)

Numerical Stability of Simplified Bernstein Polynomials

When you simplify a high-order symbolic Bernstein polynomial, the result often cannot be evaluated in a numerically stable way.

Approximate this rectangular pulse function by the 100th-degree Bernstein polynomial, and then simplify the result:

f = @(x)rectangularPulse(1/4,3/4,x);
b1 = bernstein(f, 100, sym('t'));
b2 = simplify(b1);

Convert the polynomial b1 and the simplified polynomial b2 to MATLAB® functions:

f1 = matlabFunction(b1);
f2 = matlabFunction(b2);

Compare the plot of the original rectangular pulse function, its numerically stable Bernstein representation f1, and its simplified version f2. The simplified version is not numerically stable.

t = 0:0.001:1;
plot(t, f(t), t, f1(t), t, f2(t))
hold on
legend('original function','Bernstein polynomial',...
                'simplified Bernstein polynomial')
hold off

Input Arguments

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f — Function to be approximated by a polynomialfunction handle

Function to be approximated by a polynomial, specified as a function handle. f must accept one scalar input argument and return a scalar value.

g — Function to be approximated by a polynomialsymbolic expression | symbolic function

Function to be approximated by a polynomial, specified as a symbolic expression or function.

n — Bernstein polynomial ordernonnegative integer

Bernstein polynomial order, specified as a nonnegative number.

t — Evaluation pointnumber | symbolic number | symbolic variable | symbolic expression | symbolic function

Evaluation point, specified as a number, symbolic number, variable, expression, or function. If t is a symbolic function, the evaluation point is the mathematical expression that defines t. To extract the mathematical expression defining t, bernstein uses formula(t).

var — Free variablesymbolic variable

Free variable, specified as a symbolic variable.

More About

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Bernstein Polynomials

A Bernstein polynomial is a linear combination of Bernstein basis polynomials.

A Bernstein polynomial of degree n is defined as follows:

Here,

are the Bernstein basis polynomials, and is a binomial coefficient.

The coefficients are called Bernstein coefficients or Bezier coefficients.

If f is a continuous function on the interval [0, 1] and

is the approximating Bernstein polynomial, then

uniformly in t on the interval [0, 1].

Tips

  • Symbolic polynomials returned for symbolic t are numerically stable when substituting numerical values between 0 and 1 for t.

  • If you simplify a symbolic Bernstein polynomial, the result can be unstable when substituting numerical values for the curve parameter t.

See Also

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