Programming Glut (lesson 2) Drawing 2D objects

[t3jem 0]

This is the second of the six from Xisto, again, I hope you enjoy.

This is the second lesson in my series of tutorials on how to use GLUT to create graphics. In this tutorial I am going to be teaching you how to create different types of polygons. I am going to be adding on to last tutorial's code and will leave the notes in to help you remember what all the function are. I will also be noting the new functions that we will be using and how to use them.

#include<glut.h>//Include the GLUT functions

This time we are going to not only set the background color, but set the area that we are viewing as well.
gluOrtho2D(); sets left, right, bottom, and top borders of the viewing area, in this case we are viewing 5 units to the left, right, down, and up.

void init()	 {		 glClearColor(0,0,0,0);//Define our background color		 gluOrtho2D(-5,5,-5,5);//(NEW)  Define our viewing area	 }

We are now going to draw our polygons using the function glBegin and glEnd. All the verteces defined between these two functions will be drawn using the rules given in the glBegin function. The rules that can be used can be found at the end of this tutorial.
glColor3f(); changes the colors of objects, any verteces following this function will be changed to the color defined. The colors are defined just as they are in glClearColor();
glVertex2f(); defines a 2D vertex as an (x, y) vertex. We will use this function to define our polygons.

In the next function we will create three polygons, a triangle, square, and pentagon. Each polygon will have it's own color.

void display()	 {		 glClear(GL_COLOR_BUFFER_BIT);//Clear the screen	 		 glColor3f(1,0,0);//Change the object color to red	 		 glBegin(GL_TRIANGLES);//Start drawing a triangle		 glVertex2f(3,-4);//draw our first coordinate		 glVertex2f(3.5,-3);//Our second coordinate		 glVertex2f(4,-4);//Our last coordinate		 glEnd();//Stop drawing triangles	 	 		 glColor3f(0,1,0);//Change the object colors to green	 		 glBegin(GL_QUADS);//Start drawing quads		 glVertex2f(-4,-4);//first coordinate		 glVertex2f(-4,-2);//second coordinate	 		 glColor3f(0,0,1);//Change the color to blue halfway through to create a neat color effect	 		 glVertex2f(-2,-2);//third coordinate (now blue)		 glVertex2f(-2,-4);//last coordinate		 glEnd();//Stop drawing quads	 		 glColor3f(1,0,0);//Change color to red		 glBegin(GL_POLYGON);//Start drawing a polygon		 glVertex2f(-2,2);//first vertex		 glColor3f(0,1,0);//Change color to green		 glVertex2f(-1,3);//second vertex		 glColor3f(0,0,1);//Change color to blue		 glVertex2f(0,2);//third vertex		 glColor3f(1,0,1);//Change color to purple		 glVertex2f(-0.5,0);//fourth vertex		 glColor3f(1,1,0);//Change color to yellow		 glVertex2f(-1.5,0);//last vertex		 glEnd();//Stop drawing our polygon	 	 		 glFlush();//Draw everything to the screen		 glutPostRedisplay();//Start drawing again	 }

Now we finish the code off with our main function again.

void main(int argc, char ** argv)	 {		 glutInit(&argc, argv);//Initialize GLUT		 glutInitWindowSize(800,600);//define the window size		 glutInitWindowPosition(10,50);//Position the window		 glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);//Define the drawing mode		 glutCreateWindow("Lesson 2");//Create our window		 init();//initialize our variables		 glutDisplayFunc(display);//tell Glut what our display function is		 glutMainLoop();//Keep the program running	 }

As you can see there are many different modes for the function "glBegin()". The modes that can be used and what they are used for are listed below.

GL_POINTS This is used to just draw points, instead of polygons or lines. You only need to define one vertex to see a point
GL_LINES -- This mode lets you draw lines. You need to define 2 verteces at a time.
GL_LINE_LOOP -- This mode creates a loop of lines. You define the verteces much like GL_POLYGON and when you finish it will link the last and first two verteces together with a line.
GL_LINE_STRIP -- This mode does the same as GL_LINE_LOOP except it does not connect the first and second lines
GL_TRIANGLES -- This mode creates triangles and takes 3 verteces at a time
GL_TRIANGLE_STRIP -- This mode creates a strip of triangles. The second triangle uses 2 verteces of the first, and you just define the last vertex of the next triangle
GL_TRIANGLE_FAN -- This function creates a fan of triangles. All the triangles use one same point and 1 point from the triangle before them, you define the last vertex
GL_QUADS -- This creates quadrilaterals and take 4 verteces at a time
GL_QUAD_STRIP -- This mode creates a strip of quadrilaterals. After the first quad, it uses 2 verteces from the preveious quad then you define the other 2
GL_POLYGON -- This mode lets you create a polygon will n verteces


I hope you enjoyed the tutorial and found it helpful. Many more are on their way. Again go ahead and fiddle with the settings and modes, you can learn alot more from playing with the code then just reading it.


Editted on December 18, 2006 to make it more readable