//Vertex alphas are basically the same
//as vertex colors. They just set the
//alpha of selected vertices instead of
//just setting the whole object to a
//selected alpha.

//So anyway, I did this with a normal quad
//but it does work with any shape you choose
//Here it is:
//I have started my quad.
//I then set the color red for the first vertex, with an alpha of only 0.2.
//I then draw that vertex.
//The next is green, with an alpha value of 1.
//Then blue with an alpha value of 0.2.
//And finally white with an alpha value of 1.

//glBegin(GL_QUADS);
//glColor4f(1,0,0,0.2);
//glVertex3f(-0.5, -0.5, 0.0);
//glColor4f(0,1,0,1);
//glVertex3f(-0.5, 0.5, 0.0);
//glColor4f(0,0,1,0.2);
//glVertex3f(0.5, 0.5, 0.0);
//glColor4f(1,1,1,1);
//glVertex3f(0.5, -0.5, 0.0);
//glEnd();

//Just remember to call each vertex after you call the color
//for this to work.

#include <GL/gl.h>
#include <GL/glut.h>

void square (void) {
	glBegin(GL_QUADS);
	glColor4f(1,0,0,0.2); //red, alpha 0.2
	glVertex3f(-0.5, -0.5, 0.0);
	glColor4f(0,1,0,1); //green, alpha 1
	glVertex3f(-0.5, 0.5, 0.0);
	glColor4f(0,0,1,0.2); //blue, alpha 0.2
	glVertex3f(0.5, 0.5, 0.0);
	glColor4f(1,1,1,1); //white, alpha 1
	glVertex3f(0.5, -0.5, 0.0);
	glEnd();
}

void display (void) {
	glClearColor (0.0,0.0,0.0,1.0);
    glClear (GL_COLOR_BUFFER_BIT);
    glLoadIdentity(); 
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
	glTranslatef(0,0,-1);
	square();
	glFlush();
}

void reshape (int w, int h) {
	glViewport (0, 0, (GLsizei)w, (GLsizei)h);
	glMatrixMode (GL_PROJECTION);
	glLoadIdentity ();
	gluPerspective (60, (GLfloat)w / (GLfloat)h, 1.0, 100.0);
	glMatrixMode (GL_MODELVIEW);
}

int main (int argc, char **argv) {
    glutInit (&argc, argv);
	glutInitDisplayMode (GLUT_SINGLE | GLUT_RGBA);
	glutInitWindowSize (500, 500);
	glutInitWindowPosition (100, 100);
    glutCreateWindow ("A basic OpenGL Window");
    glutDisplayFunc (display);
	glutReshapeFunc (reshape);
    glutMainLoop ();
    return 0;
}