Writing a graphics program in C++ is almost identical to writing any other type of program (see the C++ tutorial), but there are a few things you need to include in the program to load the graphics commands.

When you start writing a program including graphics, you must:

• Start the first line of the code with #include to load these commands.
• The next line of code should be#include. This provides functions specific to the X Windowing System.
• You must declare an integer variable to hold the "serial number" of your graphics window, such as int window; The various graphics commands use this number to know which window to write to.
• Create a graphics window with the line window = g2_open_X11(width, height); where width and height can be variables or numbers, and window is an integer variable.
• At the end of your program, close with window with g2_close(window);

Here is a sample program you may want to start with:

#include <iostream.h>
#include <g2++.h>
#include <g2_X11++.h>

const int HEIGHT= 200;
const int WIDTH = 200;

int main(void)
{
    int window;

    /* Open a window on screen.  Keep it's device ID */
    window = g2_open_X11(WIDTH, HEIGHT);

    /* Let's define some colors now */
    int black_ink = g2_ink(window,0,0,0);
    int blue_ink = g2_ink(window,0,0,1);
    int yellow_ink = g2_ink(window, 0.7,0.7,0);
    int white_ink = g2_ink(window,1,1,1);

    /* clear screen */
    g2_clear(window);
    g2_set_background(window, black_ink);

    /* Give it a face */
    g2_pen(window,yellow_ink);
    g2_filled_circle(window,100,100,50);

    /* and eyes */
    g2_pen(window,white_ink);
    g2_filled_ellipse(window,85,115,9,6);
    g2_filled_ellipse(window,115,115,9,6);
    g2_pen(window,blue_ink);
    g2_filled_circle(window,83,115,5);
    g2_filled_circle(window, 113,115,5);
    g2_pen(window,black_ink);
    g2_filled_circle(window,83,115,3);
    g2_filled_circle(window,113,115,3);

    /* and a nose */
    g2_pen(window,black_ink);
    g2_line(window,102,105,95,88);
    g2_line(window,95,88,102,88);

    /* and a mouth */
    g2_line(window,90,80,110,80);

    cin.get();
    g2_close(window);

    return 0;
}

For a description of many of the graphics commands you can use, look at the g2 graphics library documentation

Compiling a Graphics Program

Now, to compile the program, open a terminal window. While you are in the directory containing your program, type at the command prompt:

g++ -o program1 program1.cpp -L/usr/X11R6/lib -lm -lX11 -lgd -lg2

This tells g++, the compiler, to look in the graphics library when compiling your program in order to find the commands. Running a graphics program is exactly like running any other kind of program; just type the program name at the command prompt.