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# Homework 9 Graphics library

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CS 2110 Homework 9

Contents
1 Overview 2
2 Instructions 2
2.1 Setting Up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Getting Started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.3 Understanding How Pixel Arrays and 16-bit Colors Work . . . . . . . . . . . . . . . . . . . . 2
2.3.1 How does a 1-d pixel array work for displaying colors on a screen? . . . . . . . . . . . 2
2.3.2 How are the individual pixels represented as integers? . . . . . . . . . . . . . . . . . . 3
2.4 Drawing Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.5 Color Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 Sandbox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Tester . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Makefile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4 Rubric 5
5 Deliverables 5
6 Rules and Regulations 6
6.1 General Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2 Submission Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.3 Submission Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.4 Syllabus Excerpt on Academic Misconduct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.5 Is collaboration allowed? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1 Overview
For this assignment, you will write in the C language a graphics library that is capable of drawing a variety
of shapes, as well as a number of filters that can be applied on the colors of the pixels of an image.
2 Instructions
2.1 Setting Up
It is expected that you work on this assignment and run your code on a computer or VM running Ubuntu
16.04. Run the following commands to install dependencies used to build and test your code:
sudo apt update
sudo apt install check build-essential pkg-config valgrind gdb
2.2 Getting Started
Read through the geometry.h file. The comments here will explain each of the structs so
that you know what you’re doing when you start writing your functions. This file is thoroughly
commented in order to make sure you are able to understand the structs here.
Now take a look at the graphics.h file. This file contains the headers and details of each of the functions
you need to implement.
You will write all of your code in the graphics.c file. If you edit other files, the tester will
use their originals and thus your code might not compile. This file is prepopulated with all of the
functions you will have to write. If you accidentally delete any, you can still find the prototypes in the
graphics.h file. Note that for this assignment, some C standard library headers are included, and you can
feel free to use any functions included in those headers.
The file is distributed by default with the UNUSED macro in each function that keeps gcc from complaining
about unused variables. Without those, you wouldn’t be able to compile the file before fully writing all of
the functions. So as you are writing each function, be sure to remove the UNUSED macro calls from
its body first, so that the compiler can accurately warn you about unused variables.
2.3 Understanding How Pixel Arrays and 16-bit Colors Work
2.3.1 How does a 1-d pixel array work for displaying colors on a screen?
At first it might seem weird to represent a 2-d object (the screen) with a 1-d array of pixels, but it’s actually
pretty straightforward. Think about a text document. If you’re typing on one line, eventually it will wrap
around to the start of the next line, even though you were just typing one character after the next. The pixel
array for the screen works the same way: the end of each row wraps around to the start of the next row. So
if the screen is 240×160, for example, then pixels 0-239 are the first row, pixels 240-479 are the second row,
continuing for all 160 rows. This convention for storing an array is referred to as “row major” order and is
used by most languages, including C, for storing two-dimensional arrays in a one-dimensional memory.
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Figure 1: Visual Explanation of Pixel Layout
2.3.2 How are the individual pixels represented as integers?
We represent each pixel as 16 unsigned bits (2 bytes) in a BGR format. With 5 bits to represent the amount
of blue, 5 bits for the amount of green, and 5 bits for the amount of red. However, this only adds up to
15 bits, so the most significant bit is unused. The reason it’s a BGR format instead of the more commonly
known RGB format is because the 5 bits for each color are organized (from most significant bits to least
significant bits) blue, green, then red. This is also the layout of pixels on the GBA that we’ll use later on.
Figure 2: The layout of bits in a 16-bit BGR format
2.4 Drawing Geometry
You need to implement a number of functions in graphics.c that will be used to draw the shapes in geometry.h.
Prototypes for each function are provided in graphics.h in the order that we recommend that you implement
them. The documentation above each function explains in detail the expected behavior for that function.
2.5 Color Filters
The filters that you are expected to implement are very simple functions of single pixels that produce a new
pixel based on the input. For example, the Greyscale filter will produce a greyscale version of the given pixel,
the Red Only filter will only let the red channel of the pixel pass through, and the provided No Filter filter
is the identity function on the pixel, so that you can use the drawImage function without implementing the
filters first. These functions are also in the graphics.h and graphics.c files for you to work on.
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You are provided with two different pieces of software to test your work with: the sandbox and the tester.
3.1 Sandbox
The sandbox is provided in the assignment directory. In the sandbox.c file there is a bunch of code that sets up
a test image and provides you with an environment to test your code with. You can change the code between
the comments indicating the area you can change. Once you save the file and run the make run-sandbox
command from the command line, the resulting image will be output to the output.bmp file which you can
view using an image viewer.
To help you see cases where you might be writing outside the image bounds, the image will be drawn such
that your actual drawing will appear between two horizontal black lines. If you cannot see these lines or
if they are incomplete, it means your functions are drawing outside the bounds of the pixel array, which is
something you should correct.
The sandbox is shipped with sample images (azul, tvtester, austinbear) that you can use to test your
drawImage function, too.
The sandbox will not be used for grading, its purpose is to simply show you what image your code draws on
a hypothetical screen, in order to make your code easier to debug.
If you need to debug code you wrote in the sandbox, you can run the make run-sandbox-gdb command to
launch gdb for the sandbox.
3.2 Tester
The tester is provided in the tester directory. It contains unit tests that call your functions with certain
parameters and compare the output with the TA solution’s outputs (the source code for the tests is in
graphics suite.c.
The more important feature of the tester is that it shows you as an image file both the output
your code produced and the expected output for the tests. Your code’s results are found in the
tester/actual/ directory, while the expected images are found in the tester/expected/ directory, so that you
can compare them using any image viewer. Moreover, the tester/diff/ directory contains images that are
diffs of the expected and the actual: pixels that match on the two are marked green and pixels that don’t
match are marked red.
This is your best bet at debugging your code, and at understanding what’s expected of each
function! The tester will have the expected image and the image your code produced for all
of the test cases. Comparing those should be able to tell you what’s wrong with your code.
To run the tester, run the following command (or any of the other ones listed below in the
Makefile section:
make run-tests
Note the tests are unweighted when you run them like this, but they are weighted on Gradescope.
3.3 Makefile
We have provided a Makefile for this assignment that will build your project.
Here are the commands you should be using with this Makefile:
1. To clean your working directory (use this command instead of manually deleting the .o files): make
clean
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2. To run the sandbox: make run-sandbox
3. To run the sandbox with gdb: make run-sandbox-gdb (gdb instructions are available below, in the
HW8 doc, and on Piazza)
4. To run the tests without valgrind or gdb: make run-tests
5. To run your tests with valgrind: make run-valgrind
6. To debug a specific test with valgrind: make TEST=test name run-valgrind
7. To debug a specific test using gdb: make TEST=test name run-gdb
Then, at the (gdb) prompt:
(a) Set some breakpoints (if you need to — for stepping through your code you would, but you
wouldn’t if you just want to see where your code is segfaulting) with b tester/graphics suite.c:420,
or b graphics.c:69, or wherever you want to set a breakpoint
(b) Run the test with run
(c) If you set breakpoints: you can step line-by-line (including into function calls) with s or step over
function calls with n
(d) If your code segfaults, you can run bt to see a stack trace
To get an individual test name, you can look at the output produced by the tester. For example, the following
failed test is test list size empty:
suites/list_suite.c:906:F:test_list_size_empty:test_list_size_empty:0: Assertion failed…
^^^^^^^^^^^^^^^^^^^^
Beware that segfaulting tests will show the line number of the last test assertion made before the segfault,
not the segfaulting line number itself. This is a limitation of the testing library we use. To see what line in
your code (or in the tests) is segfaulting, follow the “To debug a specific test using gdb” instructions above.
4 Rubric
The grading will be based on the accuracy of your library in drawing the given inputs. The output of the
tester is a good estimate of your final score, however, we reserve the right to add further tests to verify edge
case behavior. As a result, it is a good idea for you to test your code in the sandbox using a variety of inputs
and inspecting the outputs manually to check if they follow the requirements.
IMPORTANT! You will still submit this assignment on Gradescope, but you will not be able to see your
outputs or the expected ones – you will just know if your code passed or failed the tests. As a result, it’s a
good idea to use the tester locally for debugging.
5 Deliverables
1. graphics.c
Be sure to check your score to see if you submitted the right files, as well as your email
frequently until the due date of the assignment for any potential updates.
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6 Rules and Regulations
6.1 General Rules
1. Starting with the assembly homeworks, any code you write must be meaningfully commented. You
should comment your code in terms of the algorithm you are implementing; we all know what each
line of code does.
2. Although you may ask TAs for clarification, you are ultimately responsible for what you submit. This
means that (in the case of demos) you should come prepared to explain to the TA how any piece of
code you submitted works, even if you copied it from the book or read about it on the internet.
4. Please start assignments early, and ask for help early. Do not email us the night the assignment is due
with questions.
5. If you find any problems with the assignment it would be greatly appreciated if you reported them to
the author (which can be found at the top of the assignment). Announcements will be posted if the
assignment changes.
6.2 Submission Conventions
1. All files you submit for assignments in this course should have your name at the top of the file as
a comment for any source code file, and somewhere in the file, near the top, for other files unless
otherwise noted.
2. When preparing your submission you may either submit the files individually to Canvas/Gradescope
or you may submit an archive (zip or tar.gz only please) of the files. You can create an archive by right
raw files or an archive) are exactly equivalent, so choose whichever is most convenient for you.
3. Do not submit compiled files, that is .class files for Java code and .o files for C code. Only submit the
files we ask for in the assignment.
4. Do not submit links to files. The autograder does not understand it, and we will not manually grade
assignments submitted this way as it is easy to change the files after the submission period ends.
6.3 Submission Guidelines
1. You are responsible for turning in assignments on time. This includes allowing for unforeseen circumstances. If you have an emergency let us know IN ADVANCE of the due time supplying documentation (i.e. note from the dean, doctor’s note, etc). Extensions will only be granted to those who contact
2. You are also responsible for ensuring that what you turned in is what you meant to turn in. After
submitting you should be sure to download your submission into a brand new folder and test if it
works. No excuses if you submit the wrong files, what you turn in is what we grade. In addition, your
assignment must be turned in via Canvas/Gradescope. Under no circumstances whatsoever we will
accept any email submission of an assignment. Note: if you were granted an extension you will still
turn in the assignment over Canvas/Gradescope.
3. There is a 6-hour grace period added to all assignments. You may submit your assignment without
penalty up until 11:55PM, or with 25% penalty up until 5:55AM. So what you should take from this is
not to start assignments on the last day and plan to submit right at 11:54AM. You alone are responsible
for submitting your homework before the grace period begins or ends; neither Canvas/Gradescope, nor
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your flaky internet are to blame if you are unable to submit because you banked on your computer
working up until 11:54PM. The penalty for submitting during the grace period (25%) or after (no
credit) is non-negotiable.
6.4 Syllabus Excerpt on Academic Misconduct
Academic misconduct is taken very seriously in this class. Quizzes, timed labs and the final examination are
individual work.
Homework assignments are collaborative, In addition many if not all homework assignments will be evaluated
via demo or code review. During this evaluation, you will be expected to be able to explain every aspect of
your submission. Homework assignments will also be examined using computer programs to find evidence
of unauthorized collaboration.
What is unauthorized collaboration? Each individual programming assignment should be coded by you.
You may work with others, but each student should be turning in their own version of the assignment.
Submissions that are essentially identical will receive a zero and will be sent to the Dean of Students’ Office
of Academic Integrity. Submissions that are copies that have been superficially modified to conceal that
they are copies are also considered unauthorized collaboration.
You are expressly forbidden to supply a copy of your homework to another student via electronic means. This includes simply e-mailing it to them so they can look at it. If you supply
an electronic copy of your homework to another student and they are charged with copying,
you will also be charged. This includes storing your code on any site which would allow other
parties to obtain your code such as but not limited to public repositories (Github), pastebin,
etc. If you would like to use version control, use github.gatech.edu
6.5 Is collaboration allowed?
Collaboration is allowed on a high level, meaning that you may discuss design points and concepts relevant
to the homework with your peers, share algorithms and pseudo-code, as well as help each other debug code.
What you shouldn’t be doing, however, is pair programming where you collaborate with each other on a
single instance of the code. Furthermore, sending an electronic copy of your homework to another student for
them to look at and figure out what is wrong with their code is not an acceptable way to help them, because
it is frequently the case that the recipient will simply modify the code and submit it as their own. Consider
instead using a screen-sharing collaboration app, such as http://webex.gatech.edu/, to help someone with
debugging if you’re not in the same room.
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Figure 3: Collaboration rules, explained colorfully
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