GL11.glPushMatrix();
GlStateManager.color(0.75f, 0.75f, 0.75f, 0.5f);
GlStateManager.enableAlpha();
GlStateManager.enableBlend();

drawCircle(width/2, height/2, 100, 0, Math.PI * 2);
public static void drawCircle(float x, float y, float radius, double angleBegin, double angleEnd){
        GL11.glBegin(GL11.GL_TRIANGLE_FAN);
        GL11.glVertex2f(x, y);
        for(double i = angleBegin; i <= angleEnd; i += Math.PI/40){
            GL11.glVertex2f(x + ((float)Math.cos(i) * radius), y + ((float)Math.sin(i) * radius));
        }
        GL11.glEnd();
    }

GL11.glPopMatrix();