Drawing Unfilled Rectangle Shape in openGL

Use GL_LINE_LOOP (instead of GL_POLYGON) to draw a connected series of line segments on the perimeter of your polygon rather than filling the polygon.

Alternatively, you could use glPolygonMode (GL_FRONT_AND_BACK, GL_LINE)... remember to set it back to the default glPolygonMode (GL_FRONT_AND_BACK, GL_FILL) to resume usual (filled) rendering though.

You need to set the fill mode for your rendering.

You can use the glPolygonMode(...); method.

Try the following:

void draweRect(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(0.0,0.0,1.0);
    glLineWidth(30);

    glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);

    glBegin(GL_POLYGON);
        glVertex2i(50,90);
        glVertex2i(100,90);
        glVertex2i(100,150);
        glVertex2i(50,150);
    glEnd();

    glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

    glFlush();   
}

This one worked for me..

glBegin(GL_LINE_LOOP);
    glVertex2f(x1,y1);
    glVertex2f(x2,y1);
    glVertex2f(x2,y2);
    glVertex2f(x1,y2);
glEnd();