Limit Definition of Derivative

Oct 13, 2025 by pynkocean.com 98 views

Introduction

A cubic Bézier curve is a fundamental concept in computer-aided design (CAD) and computer graphics. It is a type of parametric curve that is widely used to model smooth curves and surfaces. In this article, we will discuss how to find the tangent of a cubic Bézier curve at its end point.

What is a Cubic Bézier Curve?

A cubic Bézier curve is a parametric curve defined by four control points: P0, P1, P2, and P3. The curve is defined by the following equation:

B(t) = (1-t)^3P0 + 3(1-t)^2tP1 + 3(1-t)t^2P2 + t^3P3

where t is a parameter that varies from 0 to 1.

Derivative of a Cubic Bézier Curve

To find the tangent of a cubic Bézier curve, we need to take the derivative of the curve with respect to t. The derivative of the curve is given by:

B'(t) = -3(1-t)^2P0 + 6(1-t)tP1 + 6(1-t)^2P2 + 3t^2P3

Tangent at End Point

However, if we want to find the tangent at one of the end points, we need to be careful. The derivative of the curve at the end points is not defined in the classical sense. This is because the derivative of the curve is a linear combination of the control points, and when t is equal to 0 or 1, the derivative is a linear combination of the control points that are not defined.

Limit Definition of Derivative

To find the tangent at the end points, we can use the limit definition of the derivative. The limit definition of the derivative is given by:

B'(t) = \lim_{h \to 0} \frac{B(t+h) - B(t)}{h}

We can use this definition to find the tangent at the end points by taking the limit as h approaches 0.

Tangent at P0

To find the tangent at P0, we need to take the limit of the derivative as t approaches 0 from the right. This is given by:

B'(0+) = \lim_{t \to 0+} \frac{B(t) - B(0)}{t}

Using the equation of the curve, we can simplify this expression to:

B'(0+) = -3P0 + 3P1

Tangent at P3

To find the tangent at P3, we need to take the limit of the derivative as t approaches 1 from the left. This is given by:

B'(1-) = \lim_{t \to 1-} \frac{B(t) - B(1)}{t-1}

Using the equation of the curve, we can simplify this expression to:

B'(1-) = 3P2 + 3P3

Conclusion

In this article, we discussed how to find the tangent of a cubic Bézier curve at its end point. We showed that the derivative of the curve at the end points is not defined in the classical sense, and we used the limit definition of the derivative to find the tangent at the end points. We also derived the expressions for the tangent at P0 and P3.

Code Implementation

Here is a Python code implementation of the tangent of a cubic Bézier curve at its end point:

import numpy as np
def cubic_bezier_curve(P0, P1, P2, P3, t):
return (1-t)3 * P0 + 3(1-t)2 * t * P1 + 3(1-t) * t2 * P2 + t**3 * P3
def derivative_cubic_bezier_curve(P0, P1, P2, P3, t):
return -3*(1-t)2 * P0 + 6(1-t) * t * P1 + 6(1-t)2 * P2 + 3*t2 * P3
def tangent_at_p0(P0, P1):
return -3 * P0 + 3 * P1
def tangent_at_p3(P2, P3):
return 3 * P2 + 3 * P3
P0 = np.array([0, 0])
P1 = np.array([1, 2])
P2 = np.array([3, 4])
P3 = np.array([5, 6])
tangent_p0 = tangent_at_p0(P0, P1)
tangent_p3 = tangent_at_p3(P2, P3)
print("Tangent at P0:", tangent_p0)
print("Tangent at P3:", tangent_p3)

Introduction

In our previous article, we discussed how to find the tangent of a cubic Bézier curve at its end point. We showed that the derivative of the curve at the end points is not defined in the classical sense, and we used the limit definition of the derivative to find the tangent at the end points. In this article, we will answer some frequently asked questions about the tangent of a cubic Bézier curve at its end point.

Q: What is the tangent of a cubic Bézier curve?

A: The tangent of a cubic Bézier curve is a vector that represents the direction of the curve at a given point. It is a fundamental concept in computer-aided design (CAD) and computer graphics.

Q: How do I find the tangent of a cubic Bézier curve at its end point?

A: To find the tangent of a cubic Bézier curve at its end point, you need to take the limit of the derivative of the curve as the parameter t approaches 0 or 1. This is given by:

B'(0+) = \lim_{t \to 0+} \frac{B(t) - B(0)}{t}

B'(1-) = \lim_{t \to 1-} \frac{B(t) - B(1)}{t-1}

Q: What is the formula for the tangent of a cubic Bézier curve at P0?

A: The formula for the tangent of a cubic Bézier curve at P0 is:

B'(0+) = -3P0 + 3P1

Q: What is the formula for the tangent of a cubic Bézier curve at P3?

A: The formula for the tangent of a cubic Bézier curve at P3 is:

B'(1-) = 3P2 + 3P3

Q: Can I use the derivative of the curve to find the tangent at the end points?

A: No, you cannot use the derivative of the curve to find the tangent at the end points. The derivative of the curve is not defined at the end points, and you need to use the limit definition of the derivative to find the tangent at the end points.

Q: What is the significance of the tangent of a cubic Bézier curve at its end point?

A: The tangent of a cubic Bézier curve at its end point is significant because it represents the direction of the curve at the end points. It is used in various applications such as computer-aided design (CAD), computer graphics, and animation.

Q: Can I use the tangent of a cubic Bézier curve at its end point to find the normal vector?

A: Yes, you can use the tangent of a cubic Bézier curve at its end point to find the normal vector. The normal vector is a vector that is perpendicular to the tangent vector.

Q: How do I find the normal vector of a cubic Bézier curve at its end point?

A: To find the normal vector of a cubic Bézier curve at its end point, you need to take the cross product of the tangent vector and the unit vector in the direction of the tangent vector.

Conclusion

In this article, we answered some frequently asked questions about the tangent of a cubic Bézier curve at its end point. We showed that the tangent of a cubic Bézier curve at its end point is a fundamental concept in computer-aided design (CAD) and computer graphics, and it is used to represent the direction of the curve at the end points.

Code Implementation

Here is a Python code implementation of the tangent of a cubic Bézier curve at its end point:

import numpy as np
def cubic_bezier_curve(P0, P1, P2, P3, t):
return (1-t)3 * P0 + 3(1-t)2 * t * P1 + 3(1-t) * t2 * P2 + t**3 * P3
def derivative_cubic_bezier_curve(P0, P1, P2, P3, t):
return -3*(1-t)2 * P0 + 6(1-t) * t * P1 + 6(1-t)2 * P2 + 3*t2 * P3
def tangent_at_p0(P0, P1):
return -3 * P0 + 3 * P1
def tangent_at_p3(P2, P3):
return 3 * P2 + 3 * P3
def normal_vector_at_p0(tangent_vector):
return np.cross(tangent_vector, np.array([0, 1]))
def normal_vector_at_p3(tangent_vector):
return np.cross(tangent_vector, np.array([0, -1]))
P0 = np.array([0, 0])
P1 = np.array([1, 2])
P2 = np.array([3, 4])
P3 = np.array([5, 6])
tangent_p0 = tangent_at_p0(P0, P1)
tangent_p3 = tangent_at_p3(P2, P3)
normal_vector_p0 = normal_vector_at_p0(tangent_p0)
normal_vector_p3 = normal_vector_at_p3(tangent_p3)
print("Tangent at P0:", tangent_p0)
print("Tangent at P3:", tangent_p3)
print("Normal vector at P0:", normal_vector_p0)
print("Normal vector at P3:", normal_vector_p3)

This code defines the cubic Bézier curve and its derivative, and then uses the limit definition of the derivative to find the tangent at the end points. It also defines the normal vector at the end points using the cross product of the tangent vector and the unit vector in the direction of the tangent vector. The example usage shows how to use the code to find the tangent and normal vector at the end points.