Scalable Cross-Platform Software Design

Posted 2020-11-24 simplebluejava

 

Scalable Cross-Platform Software Design: Data Structures, Algorithms and Design Coursework#2
25% of the module mark: Marking Scheme on Moodle
In this task the assessment allows you to demonstrate an ability to understand, analyse and design and then
implement an engineering software task, comprising data structures and algorithms to the required
professional standard. A key skill is to demonstrate is the ability to understand and analyse the task from a
typical engineering client’s style of problem description. Specifically, building upon an object oriented
decomposition (a pre-requisite skill of the module), to enhance it with generic programming approaches and
use of the STL. You are also provided with an opportunity to show skills in succinctly and clearly reporting this
type of task.
Summary: A client approaches us to design and implement a code to help them in their business. As is often
the case, they will be describing technical material that we are unfamiliar with, however we do have an in?house expert (PS) who can advise us where necessary. Assuming we have a good familiarity with coding
techniques, they offer us a project with a strict budget of 25 hours of person-hours.
Scenario: (Your client‘s field of business)
Many engineering disciplines use CAD/CAE packages which require models of
geometrical structures.
Often, structures are modelled by a triangulated approximation to their surfaces.
Triangles offer the ability to represent small and fine features with different sized
triangles.
Similarly, we might discretise a flat 2D problem space in order to numerically solve
some equations representing a physical process of interest - maybe current flow or
temperature. We might visualise this as shown here using a colour map or using a
"height" surface. In this work we are concerned with the second type of
triangulation (also referred to as a mesh).
We can "triangulate" a particular problem with given fixed vertices in many different ways. However, we will inevitably
ask, or be asked: What is the "best quality" triangulation? Note that a triangulation of given vertices is not unique:
"any edge can be flipped".
Quasi-equilateral
triangles are
good
Sliver triangles
are bad
Not allowed
Same vertices, but different edges
to form different triangulations
The definition of a "good" triangulation obviously depends upon its intended use. For example, people solving a set
of engineering equations are likely to prioritise different criteria than the computer games/animation community.
It can be shown (but not here) that there is a particular (unique-ish) choice of triangulation that minimises the
interpolation error, the so-called Delaunay Triangulation. This is what we will consider.
A Delaunay Triangulation, DT, is defined to be the triangulation where the circumcircle of each triangle is
empty, that is it contains no vertices except its own. The circumcircle of a triangle is that circle that passes
through its vertices: there are known formulae to get the circumcentre, O, and circumradius, R, from the
vertex coordinates. See appendix #2.
R O
Left Delaunay, right non-Delaunay
Generating Delaunay Triangulations: There are a number of algorithms for connecting a given set of vertices
to produce a DT. For example, the incremental Bowyer-Watson algorithm.
Consider an existing Delaunay triangulation and we want to insert a new vertex and re-triangulate: it turns
out that this is a local operation.
Initial mesh: we want to add the red vertex Identify those triangles whose circumcircles enclose the
new vertex: just the red ones here
Delete them Each exposed edge seeds a new triangle connected to the
new vertex
The algorithm inserts the vertices one at a time. We usually start with one "large" triangle big enough to
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enclose the whole problem and when all our points have been inserted, delete the large triangle‘s vertices
leaving just what we want.
A naive implementation of the Bowyer Watson Algorithm usually fails. This is not the fault of the algorithm, but
rather its implementation on a computer with finite numerical precision: This is not in the scope of our work here.
Fortunately, we are not being asked to implement the full Bowyer Watson algorithm, rather prepare a C++
object oriented data structure with algorithms that later might form the basis for generating Delaunay
triangulations – we have a context and an understanding of possible future upgrade requirements.
Hints on the basic data structure: There are a number of design issues to consider:
What are the basic objects? Maybe vertex, triangle and triangulation? – relate to the picture
on the right? What are the basic data elements defining each? What functionality should
each have? How do these objects relate to each other? More importantly, if we are
managing collections of objects, such as triangles, where is the “collection of” entity stored
– “who owns it”?
Note to students: Please recall that this is the advanced software course and the fundamental C and
intermediate Object-oriented modules are the required pre-requisites of this module and so students are
required to follow all the good practice discussed in them. (E.g. constructors, copy constructors, use of private
data …) You will lose credit for failing to do so.
One may like to consider whether it is advisable that each triangle
somehow stores its own “copy” of each of its vertices. Would this
be memory efficient? Would it be robust? Remember, the same
(e.g. orange) vertex is used in more than one triangle. Maybe we
pre-empt the client saying that that if a user moves a particular
vertex, all triangles should follow it. So, should each triangle store its own copy of its vertex or somehow
refer/point to which vertices it uses that are held somewhere else in the data structure?
Comments on required algorithms for generating a Delaunay Triangulation:
1) In which existing triangle does the new vertex fall: fast search algorithm needed?
In this assessment, a brute force “check all triangles” approach to this requirement is acceptable, but I seek
to give extra credit to those who at least discuss better alternatives and/or implement them – watch the time
budget through!
2) From the “start” triangle found above, we need to search over its adjacent triangles and then their
neighbours etc until sure all triangles whose circumcircles enclose the vertex are found.
We are not required to implement this algorithm in our assignment, but we note it as a future need that may
impact upon our data structure design
3) We need a fast and accurate implementation of the "in circumcircle" test. See the appendix.
4) Is this parallelisable?
In this assessment, I do not ask for a parallel code, just serial. However, I seek to give extra credit to those
who at least discuss, or even partially implement, the scope for parallelisation – which aspects? – watch the
time budget through
Specific Assessment Requirements:
Assume that you are provided with a triangulation stored in files as described in Appendix 3: Note to
students: Please be aware that in recent years understanding the file format has caused some students
difficulty. Do not leave it until near the deadline to clarify this in your mind – talk to me earlier.
? Design and implement a data structure and an interface for one or more C++ classes to encapsulate the
triangulation. See hints above.
? Use of the STL is likely to be appropriate and all code must run on Jenna.
Specifically, your interface must permit the following:
? File streaming input and output of triangulations in the format given in the appendix. It is expected that
the professional and conventional use of stream operators covered in the pre-requisite modules is used.
? Queries of the form: Given a point x,y at run time which triangle contains it. Note to students: Do not
waste time writing a human being screen-keyboard interface for this. The task requires that an “object”
in your data structure design has a member function that accepts suitable arguments and returns the
required result.
? Queries of the form: given a function (known at compile time) f(x,y), return the integral of f(x,y) over
the domain of the triangulation. For this we will approximate the integration in two possible ways - see
Appendix #1. Hint to students: Surely this is seeking to assess your ability to implement a generic
programming code similar to the example we consider quite early on in the module?
? A check whether a triangulated mesh provided by the user is Delaunay. Note to students – same
comment as for the point contained in triangle queries above
Required submission: Your codes and a brief report to convince the client that it works correctly. As in any
professional client paying for work scenario, evidence to support the assertion it works is critical. As
guidance, excluding listings, I would predict that 5-10 well-spaced pages will be sufficient for the report. A
high standard of in-code documentation is essential for the award of a good mark.
Note to students: I am looking to credit your selection of sensible tests to run and the associated discussion
of what each test proves and whether there is scope or need to test further. Again, be aware of the time
budget; the client can only expect to get what they pay for!
All student Work for assessment is to be uploaded via the Moodle submission link by the deadline given.
Submission by email is not accepted
Marks and feedback will be provided 15 working days after the submission deadline; each student will receive
an individual copy of the marking rubric posted on Moodle by email. If required, further individual feedback
is then available by requesting a one-to-one meeting.
Appendix #1: Linear Interpolation Over Individual Triangles
The "x-y" plane The "height" represents
the function value f(x,y)
Exact function values
given at the vertices
Between vertices, linear
interpolation
An arc drawn on the true
function surface f(x,y)
true f(x,y) value
Linearly Interpolated value
If we have a triangulation and we know the true values of the function f(x,y) at its vertices, how do we
interpolate to estimate the function at any point P in the x, y plane?
Well we consult a friendly mathematician who provides the following:
The "x-y" plane
The "height" represents
the function value f(x,y)
Exact function values
given at the vertices
Between vertices, linear
interpolation
V A1 0 V1 V2
P(x,y)
Triangle V0-V1-V2 has area A= A0+A1+A2 A2 A0 F2 F1 F0 FP
true f(x,y) value
Linearly Interpolated value
If the values of the function f(x,y) are known to be F0, F1 and F2 at the vertices V0 , V1 and V2 then the linearly
interpolated value at any point P(x,y) inside the triangle is,
A A x y F A x y F A x y F F x y P 0 0 1 1 2 2 ( , ) ( , ) ( , ) ( , ) ? ? ?
where the Ai are the areas of the sub-triangles shown in the figure - they are functions of where P is - hence
x,y. A is the total area of the triangle.
For interest the quantities ??? ??? A A x y A A x y A A x y ( , ) , ( , ) , ( , ) 0 1 2
are called the area, or barycentric coordinates
of the point P with respect to the triangle. These have many convenient properties.
How to numerical approximate the integral of the function with respect to x and y?
Again, the same friendly mathematician provides:
There are two ways we could evaluate an approximation to dx dy f (x, y) T
?? where T is a triangle:
1) Constant value approximation ( , ) ( , ) T xT yT
T
dx dy f x y ? A f O O ?? i.e. evaluate the function at the
circumcentre, O, of each triangle and weight by the triangle‘s area.
2) Linear interpolation approximation using the above and the useful, but non-obvious, fact that:
dx dy Ai(x, y) A / 3
area
Triangle
? ?? Not sure about this - consult your client!
Appendix #2: The Circumcentre of Individual Triangles R O V1 V0 V2
It can be shown that if vertex Vi has coordinates Xi, Yi that the coordinates of the circumcentre Ox , Oy and
the circumradius R are found from solutions of,
?????? ?????? ? ? ????? ????? ??????? ?????? ??? 2 2 2 2 2 1 1 0 0 22 22 21 21 20 20 22 111 x y yx R O O OO X Y X Y X Y X Y X Y X Y
(See if you can prove this - just for fun, no marks.)
This order 3 matrix equation can be solved quite effectively using basic algebraic techniques which you are
expected to identify yourselves from the wide range of resources available to an engineer.
Note to students: This is not a maths module! However, the ability to understand algorithms when presented
in the language of the client is definitely a skill to value. I am very open to any approaches to explain the
mathematics further and will discuss in class. However, reflect on what exactly do we need from these 2
appendices – just to identify the key “formulae” we need to implement.
Appendix #3: File Format
Ascii file format for reading/writing 2D triangulations. This is your client‘s legacy format - can‘t be changed.
Triangulation files should have the extension “.tri” and test examples are on Moodle.
"triangulation.tri"
no_points dimensions (2 or 3) no_attributes_per_point
i xi yi {zi} attribute 1. . . . . // no_points off
no_cells no_vertices_per_cell no_attributes_per_cell
i v1. . . . . v no_vertices_per_cell attribute1. . . . . // no_cells off
At least maintaining the same format means you can use the client’s "visualisation" software, (available on Moodle):
its use will be explained in the lectures as well and it is rather helpful to debug your software to be able to see what
is happening.

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