Sierpinski Curve drawing program in Qt/OpenGL

#include "curvedisplay.h"
#include <GL/glu.h>
#include <iostream>
using namespace std;

CurveDisplay::CurveDisplay(QWidget *parent) :
    QGLWidget(parent)
{
    sierpinski_level = 0;
    dist0 = 2.5;
    x = 0.0;
    y = 0.0;
    display_grid = true;
}

void CurveDisplay::initializeGL(){
    glShadeModel(GL_SMOOTH);
    glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
    glClearDepth(1.0f);
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);
    glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);

}

void CurveDisplay::resizeGL(int width, int height){
    double xMin = 0, xMax = 10, yMin = 0, yMax = 10;
    glViewport(0, 0, (GLint)width, (GLint)height);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(-1,1,-1,1);
    if (width > height){
        height = height?height:1;
        double newWidth = (xMax - xMin) * width / height;
        double difWidth = newWidth - (xMax - xMin);
        xMin = 0.0 + difWidth / 2.0;
        xMax = 10 + difWidth / 2.0;
    } else {
        width = width?width:1;
        double newHeight = (yMax - yMin) * width / height;
        double difHeight = newHeight - (yMax - yMin);
        yMin = 0.0 + difHeight / 2.0;
        yMax = 10 + difHeight / 2.0;
    }
    gluOrtho2D(xMin, xMax, yMin, yMax);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


void CurveDisplay::paintGL(){
    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();

    //draw grid
    if (display_grid) drawSomething();

    //draw sierpinski
    paint();

}

void CurveDisplay::drawSomething()
{
    glLineWidth(1);
    glColor3f(0.9,0.9,0.9);
    for (int i = 1; i<21; i++){
        //draw vertical lines
        glBegin(GL_LINE_LOOP);
        glVertex2f((GLfloat)i/2, (GLfloat)0);
        glVertex2f((GLfloat)i/2, (GLfloat)10);
        glEnd();

        //draw horizontal lines
        glBegin(GL_LINE_LOOP);
        glVertex2f((GLfloat)0,(GLfloat)i/2);
        glVertex2f((GLfloat)10,(GLfloat)i/2);
        glEnd();
    }
}

void CurveDisplay::goToXY(double x_new, double y_new)
{
    glTranslated(x_new, y_new, 0.0);
}

void CurveDisplay::drawLine(double x1, double y1, double x2, double y2)
{
    glBegin(GL_LINES);
    glVertex2f((GLfloat)x1, (GLfloat)y1); // origin of the line
    glVertex2f((GLfloat)x2, (GLfloat)y2); // ending point of the line
    glEnd();
}

void CurveDisplay::lineRel(double deltaX, double deltaY) {
    drawLine(x, y, x+deltaX, y+deltaY);
    x += deltaX;
    y += deltaY;
}

void CurveDisplay::paint() {
    glColor3f(0,0,0);
    dist = dist0;
    for (int i=sierpinski_level; i > 0; i--) dist /= 2;
    goToXY(2*dist, dist);
    sierpA(sierpinski_level);        // start recursion
    lineRel(+dist, +dist);
    sierpB(sierpinski_level);        // start recursion
    lineRel(-dist, +dist);
    sierpC(sierpinski_level);        // start recursion
    lineRel(-dist, -dist);
    sierpD(sierpinski_level);        // start recursion
    lineRel(+dist, -dist);
}

void CurveDisplay::sierpA (int level) {
    if (level > 0) {
        sierpA(level-1);
        lineRel(+dist, +dist);
        sierpB(level-1);
        lineRel(+2*dist, 0);
        sierpD(level-1);
        lineRel(+dist, -dist);
        sierpA(level-1);
    }
}

void CurveDisplay::sierpB (int level) {
    if (level > 0) {
        sierpB(level-1);
        lineRel(-dist, +dist);
        sierpC(level-1);
        lineRel(0, +2*dist);
        sierpA(level-1);
        lineRel(+dist, +dist);
        sierpB(level-1);
    }
}

void CurveDisplay::sierpC (int level) {
    if (level > 0) {
        sierpC(level-1);
        lineRel(-dist, -dist);
        sierpD(level-1);
        lineRel(-2*dist, 0);
        sierpB(level-1);
        lineRel(-dist, +dist);
        sierpC(level-1);
    }
}

void CurveDisplay::sierpD (int level) {
    if (level > 0) {
        sierpD(level-1);
        lineRel(+dist, -dist);
        sierpA(level-1);
        lineRel(0, -2*dist);
        sierpC(level-1);
        lineRel(-dist, -dist);
        sierpD(level-1);
    }
}

void CurveDisplay::displayGrid(int choice)
{
    if (choice == 0) display_grid = false;
    else display_grid = true;
    updateGL();
}

void CurveDisplay::changeLevel(int new_level){
    sierpinski_level = new_level;
    updateGL();
}