CS2110-Homework 9 Solved

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.

1         Instructions
1.1        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

1.2        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.

1.3        Understanding How Pixel Arrays and 16-bit Colors Work
1.3.1       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 240x160, 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.

 

Figure 1: Visual Explanation of Pixel Layout

1.3.2       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

1.4        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.

1.5        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.