Description
I. Introduction
In this assignment, you will explore topics on geometric modeling covered in lecture. You will build Bezier
curves and surfaces using de Casteljau algorithm. For possible extra credit, you can optionally implement the vertex
normal for smooth shading.
In computer graphics, Bezier curves and surfaces are frequently used to model smooth and infinitely scalable
curves and surfaces, such as in Adobe Illustrator (a curve drawing program) and in Blender (a surface modeling
program).
A Bezier curve of degree n is defined by (n+1) control points. It is a parametric curve based on a single
parameter t, ranging between 0 and 1. Similarly a Bezier surface of degree (n,m) is defined by (n+1)x(m+1) control
points. It is a parametric surface based on two parameters u and v, both ranging between 0 and 1.
Part I: Bezier Curves with 1D de Casteljau Subdivision.
In Part I of this assignment, you will use de Casteljau algorithm to evaluate Bezier curves and surfaces for any
given set of control points and parameters, such as the Bezier curve below.
Fig. 1: The Bezier curves of four and five control points, respectively
In the image, white squares are the given control points, blue squares are the intermediate control points
evaluated at the given parameters by de Casteljau algorithm, and the red square is the final evaluated point that lies
on the Bezier curve.
In Part 1, you will implement Bezier curves. To get started, take a look at bezierCurve.h and examine the
member variables defined in the class. Specifically, you will primarily be working with the following:
• std::vector<glm::vec2> controlPoints: A std::vector of original control points that define Bezier curve,
initialized from input Bezier curve file (.bzc)
• float t: A parameter at which to evaluate the Bezier curve, ranging between 0 to 1.
Recall from lecture that de Casteljau algorithm gives us the recursive step below.
Given n (possibly intermediate) control points �!, … , �” and the parameter t, we can use linear interpolation to
compute n-1 intermediate control points at the parameter t in the next subdivision level, �!’ … �’”#!, where
�’$ = ����(�$, �$%!,�) = (1 − �)�$ + � �$%!
By applying this step recursively, we eventually arrive at a final, single point and this point, in fact, lies on the
Bezier curve at the given parameter t.
You need to implement this recursive step within BezierCurve::evaluteStep(…) in student_code.cpp.
This
function takes as input a std::vector of 2D points and outputs a std::vector of intermediate control points at the
parameter t in the next subdivision level.
Note that the parameter t is a member variable of the BezierCurve class,
which you have access to within the function. Each call to this function should performs only one step of the
algorithm, i.e., one level of subdivision.
To check if your implementation is likely correct, you can run hw4 with a Bezier curve file (.bzc) and view the
generated Bezier curve on screen. For example, you can run the following command in command line tools
(assuming that the executable file is named as hw4):
./hw4 ../curve1.bzc
For this command, it consists of two arguments where the first one is the path to your executable file, and
second one is the path to your Bezier file, and they might vary on different computers. Here, curve1.bzc is a cubic
Bezier curve, and the provided curve2.bzc is a degree-4 Bezier curve.
Feel free to create your own .bzc files and
explore other Bezier curves.
Functions to modify: (Part I)
• BezierCurve::evaluateStep()
For your write-up: (Part I)
• Briefly explain de Casteljau’s algorithm and how you implemented it in order to evaluate Bezier
curves.
• Take a look at the provided .bzc files and create your own Bezier curve with 6 control points of your
choosing. Use this Bezier curve for your screenshots below.
• Show screenshots of each step/level of the evaluation from the original control points down to the
final evaluated point. Press E to step through. Toggle C to show the completed Bezier curve as well.
• Show a screenshot of a slightly different Bezier curve by moving the original control points around
and modifying the parameter t via mouse scrolling.
Part 2. Bezier Surfaces with Separable 1D de Casteljau
In Part 2, you will adapt what you have implemented for Bezier curves to Bezier surfaces. To get started, take
a look at bezierPatch.h and examine the member variables defined in the class. Specifically, you will primarily be
working with the following:
• std::vector<std::vector<glm::vec3>> controlPoints: A 2D std::vector that has nxn grid of original
control points. controlPoints is of size n and each inner std::vector, i.e., controlpoints[i] contains n
control points at the ith-row. For example, controlPoints[0] and controlPoints[n-1] each have n control
points at the bottom row and the top row, respectively.
Recall from the lecture that separable 1D de Casteljau algorithm works as the following:
The inputs to the algorithm are (1) a nxn grid of original control points, �$&, where i and j are row and column
index, and (2) the two parameters u and v.
We first consider that each row of n control points, �$
‘, … , �$
“#!, define a Bezier curve parameterized by u. Just
as in Part 1, we can use the recursive step to evaluate the final, single point �$ on this Bezier curve at the parameter
u. We then consider that �$ for all n rows define a Bezier curve parameterized by v. (These n points lie roughly along
a column.) We can again use the recursive step from Part 1 to evaluate the final, single point P on this Bezier curve
at the parameter v. This point P, in fact, lies on the Bezier surface at the given parameter u and v.
You need to implement the following functions in student_code.cpp.
• BezierPatch::evaluateStep(…): very similar to BezierCurve::evaluateStep(…) in Part 1, this recursive
function takes as inputs a std::vector of 3D points and a parameter t. It outputs a std::vector of intermediate
control points at the parameter t in the next subdivision level.
• BezierPatch::evaluate1D(…): This function takes as inputs a std::vector of 3D points and a parameter t. It
outputs directly the final, single point that lies on the Bezier curve at the parameter t. This function does
not output intermediate control points. You may want to call BeizerPatch:;evaluateStep(…) inside this
function.
• BezierPatch::evaluate(…): This function takes as inputs the parameter u and v and outputs the point that
lies on the Bezier surface, defined by the nxn controlPoints, at the parameter u and v. Note that
controlPoints is a member variable of the BezierPatch class, which you have access to within this function.
Sanity Check:
To check if your implementation is likely correct, you can run hw4 with a Bezier surface file (.bez) and view the
generated Bezier surface on screen.
For example, you can run the following command in command line tools and you should see a teapot:
./hw4 ../teapot.bez
Fig. 2: The teapot made with Bezier surface
Functions to modify: Part 2.
• BezierPatch::evaluateStep(…)
• BezierPatch::evaluate1D(…)
• BezierPatch::evaluate(…)
For your write-up: Part 2
• Briefly explain how de Casteljau algorithm extends to Bezier surfaces and how you implemented it in order
to evaluate Bezier surfaces.
• Show a screenshot of bez/teapot.bez and of bez/wavy_cube.bez evaluated by your implementation.
• Press “a” to increase the mesh density and press “d” to decrease the mesh density. Show screenshots of the
teapot and the wavy_cube at different mesh densities.
Part III: Compute the surface normal for shading
In Part III of this assignment, we are going to compute the surface normal for shading. Each Bezier Patch is
represented as a list of triangles. Thus we will just need to compute the normal for each of the triangles.
Suppose
the three vertices of a triangle is p0, p1, p2 (in anti-clock order), then the normal of the triangle is
n = cross (p1-p0, p2-p0)
n = normalize(n)
Please implement this calculation is the function: compute_triangle_normal(…) in student_code.cpp.
Sanity Check:
To check your implementation is correct, you can run the following command in command line tools:
./hw4 ../teapot.bez
And press “q” to toggle the shading mode. Press “w” to remove the wireframe. You will see the something like this
picture.
Fig. 3: The shading effects with surface normal
Functions to modify: Part 3
• compute_triangle_normal()
For your write-up: Part 3
• Show screenshots of the shading mode for both teapot and wavy_cube
• Show screenshots of the shading mode for both teapot and wavy_cube at different mesh densities (press
“a” and “w” to change the mesh densities)
• Discuss the effect of shading w.r.t mesh density. Discuss any remaining problems of this shading.
Bonus Points:
There are several ways to get bonus points in this assignment.
• In Part III, we assign the same normal to all the three vertices of a triangle. This is not ideal. You can
implement a per-vertex normal. The normal at a given vertex can be computed as a weighted average of
the surface normals of the triangles surrounded the vertex. The weight can be set as the area of each
triangle. You should show the comparison with the the previous one and tell the difference in the
write-up.
• Bezier surface allows us to compute texture coordinate analytically for each vertex as well. You can
implement texture mapping for the Bezier surfaces using the provided texture images. Your result should
be like in Fig. 4.
Fig. 4: Texture mapping for Bezier surfaces
II. Grading Scheme
Your assignment will be graded by the following marking scheme:
Basic (100%)
Draw the two Bezier curves we provided. 25%
Draw a customed Bezier curve with 6 control points. 25%
Draw the teapot and wavy_cube. 25%
Correctly calculate the surface normal. 25%
Advanced (at most 20%)
Implement a per-vertex normal. 10%
Implement texture mapping for the Bezier surfaces. 10%
Total (maximum): 120%
Note: no grade will be given if the program is incomplete or fails compilation.
III. Guidelines to submit programming assignments
1) You can write your programs on Windows and macOS. The official grading platform should be macOS. If we
encounter problems when execute/compile your program, you may have to show your demo to the teaching
assistant in person.
2) For the basic task, modify the provided student_code.cpp and write all your code in this file. For the bonus task,
you can modify existing files, but it is not recommended to create other additional .cpp or .h files. Type your
full name and student ID in student_code.cpp. Missing such essential information will lead to mark deduction
(up to 10 points).
3) We have provided a CMakeLists.txt file in the skeleton code. You are free to choose to use CMake (tutorial
week 12) or setting up the project by yourself (tutorials in week 1). Both ways should work.
4) Zip the source code file (i.e., student_code.cpp), Bezier curve with 6 control points (i.e., bezier3.bzc), the
executable file and libs (e.g., Assignment4.exe & glew32.dll) for windows users, the write-up (i.e., readme.pdf)
in a .zip (see Fig. 5). If you have changed any files to get bonus, please also put them in the zipped folder.
Name the zip with your own student id (e.g., 1155012345.zip).
5) We may check your code. And we may deduct your marks if you have seriously wrong implementation.
6) Submit your assignment via eLearn Blackboard. (https://blackboard.cuhk.edu.hk).
7) Please submit your assignment before 11:59 p.m. of the due date. Also check the course late policy.
8) In case of multiple submissions, only the latest one will be considered.
9) Fail the course if you copy.
Fig. 5. Files to be submitted
a) Windows b) Mac
