
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

#define GL_GLEXT_PROTOTYPES
#include <GL/glut.h>



static int Width = 400, Height = 400;
static GLuint MyFB, MyRB;


#define CheckError() assert(glGetError() == 0)

GLenum doubleBuffer;

static void Init(void)
{
   fprintf(stderr, "GL_RENDERER   = %s\n", (char *) glGetString(GL_RENDERER));
   fprintf(stderr, "GL_VERSION    = %s\n", (char *) glGetString(GL_VERSION));
   fprintf(stderr, "GL_VENDOR     = %s\n", (char *) glGetString(GL_VENDOR));


   if (!glutExtensionSupported("GL_EXT_framebuffer_object")) {
      printf("GL_EXT_framebuffer_object not found!\n");
      exit(0);
   }


   glGenFramebuffersEXT(1, &MyFB);
   glGenRenderbuffersEXT(1, &MyRB);

   {
      glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, MyFB);

      glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, MyRB);

      glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT1_EXT,
				   GL_RENDERBUFFER_EXT, MyRB);

      glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_RGB, Width, Height);

      glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
   }

   glClearColor(0.0, 0.0, 1.0, 0.0);
}



static void
Reshape( int width, int height )
{
   glViewport( 0, 0, width, height );
   glMatrixMode( GL_PROJECTION );
   glLoadIdentity();
   glOrtho(-1.0, 1.0, -1.0, 1.0, -0.5, 1000.0);
   glMatrixMode( GL_MODELVIEW );

   Width = width;
   Height = height;
   glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_RGB, Width, Height);
}


static void Key(unsigned char key, int x, int y)
{

    switch (key) {
      case 27:
	exit(1);
      default:
	return;
    }

    glutPostRedisplay();
}



static void Draw( void )
{

   /* draw to user framebuffer */
   glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, MyFB);
   glDrawBuffer(GL_COLOR_ATTACHMENT1_EXT);
   glReadBuffer(GL_COLOR_ATTACHMENT1_EXT);

   glClearColor(0.5, 0.5, 1.0, 0.0);
   glClear(GL_COLOR_BUFFER_BIT);
   CheckError();

   glBegin(GL_TRIANGLES);
   glColor3f(0,0,.7); 
   glVertex3f( 0.9, -0.9, -30.0);
   glColor3f(.8,0,0); 
   glVertex3f( 0.9,  0.9, -30.0);
   glColor3f(0,.9,0); 
   glVertex3f(-0.9,  0.0, -30.0);
   glEnd();


   {
      GLubyte *buffer = malloc(Width * Height * 4);

      /* read from user framebuffer */
      glReadPixels(0, 0, Width-60, Height-60, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
      CheckError();

      /* draw to window */
      glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
      /* Try to clear the window, but will overwrite:
       */
      glClearColor(0.8, 0.8, 0, 0.0);
      glClear(GL_COLOR_BUFFER_BIT);

      glWindowPos2iARB(30, 30);
      glDrawPixels(Width-60, Height-60, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
      
      
      free(buffer);
   }
   
   /* Bind normal framebuffer */
   glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);

   if (1) {
      glBegin(GL_TRIANGLES);
      glColor3f(0,.7,0); 
      glVertex3f( 0.5, -0.5, -30.0);
      glColor3f(0,0,.8); 
      glVertex3f( 0.5,  0.5, -30.0);
      glColor3f(.9,0,0); 
      glVertex3f(-0.5,  0.0, -30.0);
      glEnd();
   }

   if (doubleBuffer)
      glutSwapBuffers();
   else
      glFinish();

   CheckError();
}


static GLenum Args(int argc, char **argv)
{
    GLint i;

    doubleBuffer = GL_FALSE;

    for (i = 1; i < argc; i++) {
        if (strcmp(argv[i], "-sb") == 0) {
	    doubleBuffer = GL_FALSE;
	} else if (strcmp(argv[i], "-db") == 0) {
	    doubleBuffer = GL_TRUE;
	} else {
	    fprintf(stderr, "%s (Bad option).\n", argv[i]);
	    return GL_FALSE;
	}
    }
    return GL_TRUE;
}



int
main( int argc, char *argv[] )
{
    GLenum type;

    glutInit(&argc, argv);

    if (Args(argc, argv) == GL_FALSE) {
	exit(1);
    }

    glutInitWindowPosition(100, 0); glutInitWindowSize( Width, Height );

    type = GLUT_RGB;
    type |= (doubleBuffer) ? GLUT_DOUBLE : GLUT_SINGLE;
    glutInitDisplayMode(type);

    if (glutCreateWindow(argv[0]) == GL_FALSE) {
	exit(1);
    }

    Init();

    glutReshapeFunc(Reshape);
    glutKeyboardFunc(Key);
    glutDisplayFunc(Draw);
    glutMainLoop();
    return 0;
}
