SimpleGL example

This is a simple program demonstrating how to use OpenGL ES 2.0. It will output this animated spiral, which can be moved around on the screen:

<source lang="cpp"> /* Created by exoticorn ( http://talk.maemo.org/showthread.php?t=37356 )

* edited and commented by André Bergner [endboss]
*
* libraries needed:   libx11-dev, libgles2-dev
*
* compile with:   g++  -lX11 -lEGL -lGLESv2  egl-example.cpp
*/

1. include <iostream>
2. include <cstdlib>
3. include <cstring>

using namespace std;

1. include <cmath>
2. include <sys/time.h>

1. include <X11/Xlib.h>
2. include <X11/Xatom.h>
3. include <X11/Xutil.h>

1. include <GLES2/gl2.h>
2. include <EGL/egl.h>

const char vertex_src [] = " \

  attribute vec4        position;       \
  varying mediump vec2  pos;            \
  uniform vec4          offset;         \
                                        \
  void main()                           \
  {                                     \
     gl_Position = position + offset;   \
     pos = position.xy;                 \
  }                                     \

";

const char fragment_src [] = " \

  varying mediump vec2    pos;                        \
  uniform mediump float   phase;                      \
                                                      \
  void  main()                                        \
  {                                                   \
     gl_FragColor  =  vec4( 1., 0.9, 0.7, 1.0 ) *     \
       cos( 30.*sqrt(pos.x*pos.x + 1.5*pos.y*pos.y)   \
            + atan(pos.y,pos.x) - phase );            \
  }                                                   \

"; // some more formulas to play with... // cos( 20.*(pos.x*pos.x + pos.y*pos.y) - phase ); // cos( 20.*sqrt(pos.x*pos.x + pos.y*pos.y) + atan(pos.y,pos.x) - phase ); // cos( 30.*sqrt(pos.x*pos.x + 1.5*pos.y*pos.y - 1.8*pos.x*pos.y*pos.y) // + atan(pos.y,pos.x) - phase );

void print_shader_info_log (

  GLuint  shader      // handle to the shader

) {

  GLint  length;

  glGetShaderiv ( shader , GL_INFO_LOG_LENGTH , &length );

  if ( length ) {
     char* buffer  =  new char [ length ];
     glGetShaderInfoLog ( shader , length , NULL , buffer );
     cout << "shader info: " <<  buffer << flush;
     delete [] buffer;

     GLint success;
     glGetShaderiv( shader, GL_COMPILE_STATUS, &success );
     if ( success != GL_TRUE )   exit ( 1 );
  }

}

GLuint load_shader (

  const char  *shader_source,
  GLenum       type

) {

  GLuint  shader = glCreateShader( type );

  glShaderSource  ( shader , 1 , &shader_source , NULL );
  glCompileShader ( shader );

  print_shader_info_log ( shader );

  return shader;

}

Display *x_display; Window win; EGLDisplay egl_display; EGLContext egl_context; EGLSurface egl_surface;

GLfloat

  norm_x    =  0.0,
  norm_y    =  0.0,
  offset_x  =  0.0,
  offset_y  =  0.0,
  p1_pos_x  =  0.0,
  p1_pos_y  =  0.0;

GLint

  phase_loc,
  offset_loc,
  position_loc;

bool update_pos = false;

const float vertexArray[] = {

  0.0,  0.5,  0.0,
 -0.5,  0.0,  0.0,
  0.0, -0.5,  0.0,
  0.5,  0.0,  0.0,
  0.0,  0.5,  0.0 

};

void render() {

  static float  phase = 0;
  static int    donesetup = 0;

  static XWindowAttributes gwa;

  //// draw

  if ( !donesetup ) {
     XWindowAttributes  gwa;
     XGetWindowAttributes ( x_display , win , &gwa );
     glViewport ( 0 , 0 , gwa.width , gwa.height );
     glClearColor ( 0.08 , 0.06 , 0.07 , 1.);    // background color
     donesetup = 1;
  }
  glClear ( GL_COLOR_BUFFER_BIT );

  glUniform1f ( phase_loc , phase );  // write the value of phase to the shaders phase
  phase  =  fmodf ( phase + 0.5f , 2.f * 3.141f );    // and update the local variable

  if ( update_pos ) {  // if the position of the texture has changed due to user action
     GLfloat old_offset_x  =  offset_x;
     GLfloat old_offset_y  =  offset_y;

     offset_x  =  norm_x - p1_pos_x;
     offset_y  =  norm_y - p1_pos_y;

     p1_pos_x  =  norm_x;
     p1_pos_y  =  norm_y;

     offset_x  +=  old_offset_x;
     offset_y  +=  old_offset_y;

     update_pos = false;
  }

  glUniform4f ( offset_loc  ,  offset_x , offset_y , 0.0 , 0.0 );

  glVertexAttribPointer ( position_loc, 3, GL_FLOAT, false, 0, vertexArray );
  glEnableVertexAttribArray ( position_loc );
  glDrawArrays ( GL_TRIANGLE_STRIP, 0, 5 );

  eglSwapBuffers ( egl_display, egl_surface );  // get the rendered buffer to the screen

}

////////////////////////////////////////////////////////////////////////////////////////////

int main() {

  ///////  the X11 part  //////////////////////////////////////////////////////////////////
  // in the first part the program opens a connection to the X11 window manager
  //

  x_display = XOpenDisplay ( NULL );   // open the standard display (the primary screen)
  if ( x_display == NULL ) {
     cerr << "cannot connect to X server" << endl;
     return 1;
  }

  Window root  =  DefaultRootWindow( x_display );   // get the root window (usually the whole screen)

  XSetWindowAttributes  swa;
  swa.event_mask  =  ExposureMask | PointerMotionMask | KeyPressMask;

  win  =  XCreateWindow (   // create a window with the provided parameters
             x_display, root,
             0, 0, 800, 480,   0,
             CopyFromParent, InputOutput,
             CopyFromParent, CWEventMask,
             &swa );

  XSetWindowAttributes  xattr;
  Atom  atom;
  int   one = 1;

  xattr.override_redirect = False;
  XChangeWindowAttributes ( x_display, win, CWOverrideRedirect, &xattr );

  atom = XInternAtom ( x_display, "_NET_WM_STATE_FULLSCREEN", True );
  XChangeProperty (
     x_display, win,
     XInternAtom ( x_display, "_NET_WM_STATE", True ),
     XA_ATOM,  32,  PropModeReplace,
     (unsigned char*) &atom,  1 );

  XChangeProperty (
     x_display, win,
     XInternAtom ( x_display, "_HILDON_NON_COMPOSITED_WINDOW", False ),
     XA_INTEGER,  32,  PropModeReplace,
     (unsigned char*) &one,  1);

  XWMHints hints;
  hints.input = True;
  hints.flags = InputHint;
  XSetWMHints(x_display, win, &hints);

  XMapWindow ( x_display , win );             // make the window visible on the screen
  XStoreName ( x_display , win , "GL test" ); // give the window a name

  //// get identifiers for the provided atom name strings
  Atom wm_state   = XInternAtom ( x_display, "_NET_WM_STATE", False );
  Atom fullscreen = XInternAtom ( x_display, "_NET_WM_STATE_FULLSCREEN", False );

  XEvent xev;
  memset ( &xev, 0, sizeof(xev) );

  xev.type                 = ClientMessage;
  xev.xclient.window       = win;
  xev.xclient.message_type = wm_state;
  xev.xclient.format       = 32;
  xev.xclient.data.l[0]    = 1;
  xev.xclient.data.l[1]    = fullscreen;
  XSendEvent (                // send an event mask to the X-server
     x_display,
     DefaultRootWindow ( x_display ),
     False,
     SubstructureNotifyMask,
     &xev );

  ///////  the egl part  //////////////////////////////////////////////////////////////////
  //  egl provides an interface to connect the graphics related functionality of openGL ES
  //  with the windowing interface and functionality of the native operation system (X11
  //  in our case.

  egl_display  =  eglGetDisplay( (EGLNativeDisplayType) x_display );
  if ( egl_display == EGL_NO_DISPLAY ) {
     cerr << "Got no EGL display." << endl;
     return 1;
  }

  if ( !eglInitialize( egl_display, NULL, NULL ) ) {
     cerr << "Unable to initialize EGL" << endl;
     return 1;
  }

  EGLint attr[] = {       // some attributes to set up our egl-interface
     EGL_BUFFER_SIZE, 16,
     EGL_RENDERABLE_TYPE,
     EGL_OPENGL_ES2_BIT,
     EGL_NONE
  };

  EGLConfig  ecfg;
  EGLint     num_config;
  if ( !eglChooseConfig( egl_display, attr, &ecfg, 1, &num_config ) ) {
     cerr << "Failed to choose config (eglError: " << eglGetError() << ")" << endl;
     return 1;
  }

  if ( num_config != 1 ) {
     cerr << "Didn't get exactly one config, but " << num_config << endl;
     return 1;
  }

  egl_surface = eglCreateWindowSurface ( egl_display, ecfg, win, NULL );
  if ( egl_surface == EGL_NO_SURFACE ) {
     cerr << "Unable to create EGL surface (eglError: " << eglGetError() << ")" << endl;
     return 1;
  }

  //// egl-contexts collect all state descriptions needed required for operation
  EGLint ctxattr[] = {
     EGL_CONTEXT_CLIENT_VERSION, 2,
     EGL_NONE
  };
  egl_context = eglCreateContext ( egl_display, ecfg, EGL_NO_CONTEXT, ctxattr );
  if ( egl_context == EGL_NO_CONTEXT ) {
     cerr << "Unable to create EGL context (eglError: " << eglGetError() << ")" << endl;
     return 1;
  }

  //// associate the egl-context with the egl-surface
  eglMakeCurrent( egl_display, egl_surface, egl_surface, egl_context );

  ///////  the openGL part  ///////////////////////////////////////////////////////////////

  GLuint vertexShader   = load_shader ( vertex_src , GL_VERTEX_SHADER  );     // load vertex shader
  GLuint fragmentShader = load_shader ( fragment_src , GL_FRAGMENT_SHADER );  // load fragment shader

  GLuint shaderProgram  = glCreateProgram ();                 // create program object
  glAttachShader ( shaderProgram, vertexShader );             // and attach both...
  glAttachShader ( shaderProgram, fragmentShader );           // ... shaders to it

  glLinkProgram ( shaderProgram );    // link the program
  glUseProgram  ( shaderProgram );    // and select it for usage

  //// now get the locations (kind of handle) of the shaders variables
  position_loc  = glGetAttribLocation  ( shaderProgram , "position" );
  phase_loc     = glGetUniformLocation ( shaderProgram , "phase"    );
  offset_loc    = glGetUniformLocation ( shaderProgram , "offset"   );
  if ( position_loc < 0  ||  phase_loc < 0  ||  offset_loc < 0 ) {
     cerr << "Unable to get uniform location" << endl;
     return 1;
  }

  const float
     window_width  = 800.0,
     window_height = 480.0;

  //// this is needed for time measuring  -->  frames per second
  struct  timezone  tz;
  timeval  t1, t2;
  gettimeofday ( &t1 , &tz );
  int  num_frames = 0;

  bool quit = false;
  while ( !quit ) {    // the main loop

     while ( XPending ( x_display ) ) {   // check for events from the x-server

        XEvent  xev;
        XNextEvent( x_display, &xev );

        if ( xev.type == MotionNotify ) {  // if mouse has moved

// cout << "move to: << xev.xmotion.x << "," << xev.xmotion.y << endl;

           GLfloat window_y  =  (window_height - xev.xmotion.y) - window_height / 2.0;
           norm_y            =  window_y / (window_height / 2.0);
           GLfloat window_x  =  xev.xmotion.x - window_width / 2.0;
           norm_x            =  window_x / (window_width / 2.0);
           update_pos = true;
        }

        if ( xev.type == KeyPress )   quit = true;
     }

     render();   // now we finally put something on the screen

     if ( ++num_frames % 100 == 0 ) {
        gettimeofday( &t2, &tz );
        float dt  =  t2.tv_sec - t1.tv_sec + (t2.tv_usec - t1.tv_usec) * 1e-6;
        cout << "fps: " << num_frames / dt << endl;
        num_frames = 0;
        t1 = t2;
     }

// usleep( 1000*10 );

  }

  ////  cleaning up...
  eglDestroyContext ( egl_display, egl_context );
  eglDestroySurface ( egl_display, egl_surface );
  eglTerminate      ( egl_display );
  XDestroyWindow    ( x_display, win );
  XCloseDisplay     ( x_display );

  return 0;

} </source>