// This code example is created for educational purpose
// by Thorsten Thormaehlen (contact: www.thormae.de).
// It is distributed without any warranty.

#include <QApplication>
#include <QOpenGLWidget>
#include <QOpenGLFunctions>
#include <QKeyEvent>
#include <QTimer>
#include <QMessageBox>
#include <math.h>
#include <stdio.h>

#include <GL/glu.h>

class Renderer : protected QOpenGLFunctions {

public:
  float t;
  int mode;
private:
  GLuint fboID;
  GLuint texID;
  GLuint depthID;
  int fboTexSize;
  int width;
  int height;
public:
  // constructor
  Renderer() : t(0.0), mode(1),
               fboID(0), texID(0), depthID(0),
               fboTexSize(512), width(0), height(0) {}

public:
 void init() {
    initializeOpenGLFunctions();
    glEnable(GL_DEPTH_TEST);
    glEnable(GL_DEPTH);

    // create a frame buffer object
    glGenFramebuffers(1, &fboID);

    // bind the frame buffer
    glBindFramebuffer(GL_FRAMEBUFFER, fboID);

    // Generate render texture
    glGenTextures (1, &texID);
    glBindTexture(GL_TEXTURE_2D, texID);

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    int level = 0;
    glTexImage2D(GL_TEXTURE_2D, level, GL_RGBA, fboTexSize, fboTexSize, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

    // Attach the texture to the fbo
    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texID, level);
    if(!checkFramebufferStatus()) exit(1);

    // generate a renderbuffer for the depth buffer
    glGenRenderbuffers(1, &depthID);
    glBindRenderbuffer(GL_RENDERBUFFER, depthID);
    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, fboTexSize, fboTexSize);

    // Attach the depth buffer to the fbo
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT,
                              GL_RENDERBUFFER, depthID);
    if(!checkFramebufferStatus()) exit(1);

    // unbind texture
    glBindTexture(GL_TEXTURE_2D, 0);
    //unbind fbo
    glBindFramebuffer(GL_FRAMEBUFFER, 0);
  }

  void resize(int w, int h) {
    width = w;
    height = h;
    changeViewport(w, h);
  }

  void display() {
    if(mode == 1) {
      // bind the fbo
      glBindFramebuffer(GL_FRAMEBUFFER, fboID);
      changeViewport(fboTexSize,fboTexSize);
    }
    glClearColor(0.5f, 0.5f, 0.5f, 1.0f);
    // render to attached fbo
    drawScene();

    if(mode == 1) {

      // unbind frame buffer
      glBindFramebuffer(GL_FRAMEBUFFER, 0);

      changeViewport(width, height);
      glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
      glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

      // using first render pass result as texture
      glEnable(GL_TEXTURE_2D);
      glBindTexture(GL_TEXTURE_2D, texID);

      // second render pass
      drawScene();

      glDisable(GL_TEXTURE_2D);
    }
  }

  void dispose() {
    if(texID !=0) glDeleteTextures( 1, &texID);
    if(fboID !=0) glDeleteFramebuffers(1, &fboID);
    if(depthID != 0) glDeleteRenderbuffers(1, &depthID);
  }

private:
  void changeViewport(int w, int h) {
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(30.0, (float)w/(float)h, 0.1, 50.0);
  }

  void drawScene() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    // set camera
    gluLookAt(8.0, -2.0, 5.0, 0.0, 0.0, 0.0, 0.0, 0.0, 1.0);
    // draw scene
    glRotatef(t, 0.0f, 0.0f, 1.0f);
    drawTexturedCube();
  }

  bool checkFramebufferStatus() {
    GLenum status;
    status = (GLenum) glCheckFramebufferStatus(GL_FRAMEBUFFER);
    switch(status) {
    case GL_FRAMEBUFFER_COMPLETE:
      return true;
    case GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT:
      printf("Framebuffer incomplete, incomplete attachment\n");
      return false;
    case GL_FRAMEBUFFER_UNSUPPORTED:
      printf("Unsupported framebuffer format\n");
      return false;
    case GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT:
      printf("Framebuffer incomplete, missing attachment\n");
      return false;
    case GL_FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER:
      printf("Framebuffer incomplete, missing draw buffer\n");
      return false;
    case GL_FRAMEBUFFER_INCOMPLETE_READ_BUFFER:
      printf("Framebuffer incomplete, missing read buffer\n");
      return false;
    }
    return false;
  }

  void drawTexturedCube() {

    glColor3f(1.0f,0.0f,0.0f);
    glBegin(GL_POLYGON);
    glTexCoord2f(0.00f,0.00f); glVertex3f(-1.0f, 1.0f, 1.0f);
    glTexCoord2f(1.00f,0.00f); glVertex3f(-1.0f,-1.0f, 1.0f);
    glTexCoord2f(1.00f,1.00f); glVertex3f( 1.0f,-1.0f, 1.0f);
    glTexCoord2f(0.00f,1.00f); glVertex3f( 1.0f, 1.0f, 1.0f);
    glEnd();
    glColor3f(1.0f,1.0f,0.0f);
    glBegin(GL_POLYGON);
    glTexCoord2f(0.00f,0.00f); glVertex3f(-1.0f, 1.0f,-1.0f);
    glTexCoord2f(1.00f,0.00f); glVertex3f(-1.0f,-1.0f,-1.0f);
    glTexCoord2f(1.00f,1.00f); glVertex3f(-1.0f,-1.0f, 1.0f);
    glTexCoord2f(0.00f,1.00f); glVertex3f(-1.0f, 1.0f, 1.0f);
    glEnd();
    glColor3f(0.0f,0.0f,1.0f);
    glBegin(GL_POLYGON);
    glTexCoord2f(0.00f,0.00f); glVertex3f( 1.0f, 1.0f, 1.0f);
    glTexCoord2f(1.00f,0.00f); glVertex3f( 1.0f,-1.0f, 1.0f);
    glTexCoord2f(1.00f,1.00f); glVertex3f( 1.0f,-1.0f,-1.0f);
    glTexCoord2f(0.00f,1.00f); glVertex3f( 1.0f, 1.0f,-1.0f);
    glEnd();
    glColor3f(1.0f,1.0f,1.0f);
    glBegin(GL_POLYGON);
    glTexCoord2f(0.00f,0.00f); glVertex3f(-1.0f, 1.0f,-1.0f);
    glTexCoord2f(1.00f,0.00f); glVertex3f(-1.0f, 1.0f, 1.0f);
    glTexCoord2f(1.00f,1.00f); glVertex3f( 1.0f, 1.0f, 1.0f);
    glTexCoord2f(0.00f,1.00f); glVertex3f( 1.0f, 1.0f,-1.0f);
    glEnd();
    glColor3f(0.0f,1.0f,1.0f);
    glBegin(GL_POLYGON);
    glTexCoord2f(0.00f,0.00f); glVertex3f(-1.0f, -1.0f, 1.0f);
    glTexCoord2f(1.00f,0.00f); glVertex3f(-1.0f, -1.0f,-1.0f);
    glTexCoord2f(1.00f,1.00f); glVertex3f( 1.0f, -1.0f,-1.0f);
    glTexCoord2f(0.00f,1.00f); glVertex3f( 1.0f, -1.0f, 1.0f);
    glEnd();
    glColor3f(0.0f,1.0f,0.0f);
    glBegin(GL_POLYGON);
    glTexCoord2f(0.00f,0.00f); glVertex3f( 1.0f, 1.0f,-1.0f);
    glTexCoord2f(1.00f,0.00f); glVertex3f( 1.0f,-1.0f,-1.0f);
    glTexCoord2f(1.00f,1.00f); glVertex3f(-1.0f,-1.0f,-1.0f);
    glTexCoord2f(0.00f,1.00f); glVertex3f(-1.0f, 1.0f,-1.0f);
    glEnd();
  }
};


class MyWidget : public QOpenGLWidget {

private:
  Renderer *renderer;
  QTimer *timer;

public:
  MyWidget(QWidget *parent = NULL) : QOpenGLWidget(parent) {
    this->setWindowTitle("Press 1 to toggle between first/second render pass");
    this->resize(320, 320);
    renderer = new Renderer();
    timer = new QTimer(this);
    connect(timer, SIGNAL(timeout()), this, SLOT(update()));
    timer->start(30);
  }

  ~MyWidget() {
    renderer->dispose();
    delete renderer;
    delete timer;
  }

protected:
  void initializeGL() { renderer->init(); }
  void resizeGL(int w, int h){ renderer->resize(w, h); }
  void paintGL() {
      float offset = 1.0f;
      renderer->t += offset;
      renderer->display();
  }
  void keyPressEvent(QKeyEvent* event){
    bool redraw = false;
    QString modeStr = "";
    switch(event->key()) {
    case '1':
      if(renderer->mode == 1) {
        renderer->mode = 0;
        modeStr = "first render pass result";
      } else {
        renderer->mode = 1;
        modeStr = "second render pass result";
      }
      redraw = true;
      break;
    }
    if(redraw) {
      this->setWindowTitle(modeStr);
      this->update();
    }
  }
};

int main (int argc, char* argv[]) {
    // create a QApplication object that handles initialization,
    // finalization, and the main event loop
    QApplication appl(argc, argv);
    MyWidget widget;  // create a widget
    widget.show(); //show the widget and its children
    return appl.exec(); // execute the application
}