gfx: A Simple Graphics Library (V4)

This page describes gfx, a simple graphics library for CSE. This library is meant to be simple and easy to learn, so that beginning CSE students can get right into the interesting parts of programming. The gfx library only requires that the programmer understand how to invoke basic C functions with scalar arguments.

It is more than enough to explore raster and vector graphics, create animations, draw fractal shapes, and write simple video games. It doesn't do everything that you might want a graphics library to do. As you learn more about programming, more advanced libraries that you might consider using are OpenGL for precise 3-D graphics, Qt for windowed applications, and SDL for video games.


This library runs on Unix machines with the X11 Windowing System. At Notre Dame, your program will run on one of the student servers and display on your desktop or laptop.

From a Linux Laptop or Desktop

There is nothing to install, just use ssh with the X forwarding option (-Y) enabled:
ssh -Y

From a Mac Laptop or Desktop

Install the XQuartz software on your laptop. Start the XQuartz application. Then, open a Terminal and connect to one of the student machines:
ssh -Y

From a Windows Laptop or Desktop

Install the X-Windows Client and PuTTY on your laptop. Start the X Windows application. Then, open PuTTY, enable Connection > SSH > X11 > Enable X11 Forwarding, and connect to one of the student machines.

Getting Started

Download the following files and put them in your working directory:
  • gfx.c (the source file: read if you like, but don't change)
  • gfx.h (the header file: read if you like, but don't change)
  • example.c (an example program: go ahead and change this)
  • To compile a program that uses the gfx library, you need to use the following command, which compiles your program with gfx.c then adds the X11 and math libraries. (X11 is short for the X Window System used on Unix based machines.)

    gcc example.c gfx.c -o example -lX11 -lm
    Run the example program, and you should see a window pop up with a green triangle.

    Line by Line Example

    To give you the idea of how to use the library, we will point out the important parts of example.c in detail. Then, below you can read about how the individual functions work.

    Begin by including gfx.h, to make the library available to the program. Note that quotes rather than angle brackets are used in this statement. Quotes indicate the header file is in the current directory, rather than in some system-defined place.

    #include "gfx.h"
    To open a graphics window, call gfx_open once at the beginning of the program, giving the width and height of the window in pixels, and the title of the window on the screen:
    gfx_open(xsize,ysize,"Example Graphics Program");
    gfx_color sets the current drawing color to a pale green:
    Now, use the gfx_line function to draw a line from (100,100) to (200,100):
    Wait for the user to press a key (or button):
    c = gfx_wait();
    If the key is the letter 'q', break out of the loop:
    if(c=='q') break;
    That shows you enough to get started. There are a few more useful functions in the library, read the following section to see them all.

    Function Reference

    void gfx_open( int width, int height, const char *title );
    gfx_open creates a new graphics window on the screen. width is the width of the window in pixels. height is the height of the window in pixels. title is the name of the window in the title bar.
    void gfx_line( int x1, int y1, int x2, int y2 );
    gfx_line draws a line in the graphics window, using the current color, from location x1,y1 to x2,y2
    void gfx_color( int red, int green, int blue );
    gfx_color sets the current color for drawing, using the RGB color model, in which you mix red, green, and blue components in order to get the desired color. gfx_color takes three integers between 0 and 255, each representing red, green, and blue. The following are some example colors:
    RedGreenBlueColor Name
    void gfx_clear();
    gfx_clear clears the window to the current background color, which is black by default.
    void gfx_clear_color( int red, int green, int blue );
    gfx_clear_color changes the background color, using the same format as gfx_clear above. The new background color takes effect the next time gfx_clear is called.
    char gfx_wait();
    gfx_wait waits until the user presses a key or mouse button. If the user has pressed a mouse button, the integer 1, 2, or 3 will be returned, indicating the button pressed. If the user has pressed a key, that character will be returned. To determine the mouse location at that time, call gfx_xpos and gfx_ypos.
    int gfx_xpos(); int gfx_ypos();
    gfx_xpos() and gfx_ypos return the X and Y coordinates, respectively, of the mouse pointer when the last key or button was pressed.
    int gfx_event_waiting();
    gfx_wait causes the program to wait until something is pressed, which can be inconvenient in a real-time program. To avoid this, call gfx_event_waiting, which returns true if an event (key or button press) has occurred, otherwise returns false immediately. If it returns true, then you can call gfx_wait to retrieve the event without waiting.
    void gfx_flush();
    gfx_flush flushes all output to the graphics window. This forces all previous drawing commands to take effect. Output is normally flushed after a called to gfx_wait. If your program does not call gfx_wait then you need to call gfx_flush after drawing to the screen.