Lesson 14: Reflection (Easy cool effect)
The reflection effect is very easy to create in OpenGL, but at the same time it is a very cool effect. Creating reflection in OpenGL can be done simply by drawing the exact same scene twice, once in the positive and once in the negative direction. We can do this as (pseudo code):
drawScene() drawPlane() flipScreen() drawScene()
We created some functions to easily call the drawing process. As you draw more things, this code will scale, so it’s a good practice to do. In addition we used blending to create a semi-transparent plane. After all, we should be able to see the reflection 
We make use of a stencil buffer to make the scene draw correctly. This is what the program output looks like:
The output will be like this, depending on if you use texturing or not.
We end up with this code:
/* * OpenGL Textured Cube * https://talkera.org/opengl/ * http://openglsamples.sf.net * * Compilation trough: * gcc texcube.c -lglut -lGL -lGLU -lSDL * */ #include#include #include #include #include "SDL/SDL.h" /* screen width, height, and bit depth */ #define SCREEN_WIDTH 640 #define SCREEN_HEIGHT 480 #define SCREEN_BPP 16 /* Setup useful booleans */ #define TRUE 1 #define FALSE 0 /* This is our SDL surface */ SDL_Surface *surface; /* Used for rotating the cube */ GLfloat rotationXaxis; GLfloat rotationYaxis; GLfloat rotationZaxis; /* Storage For One Texture ( NEW ) */ GLuint texture[1]; /* Release resources and quit */ void Quit( int returnCode ) { SDL_Quit( ); exit( returnCode ); } /* Loads in a bitmap as a GL texture */ int LoadGLTextures( ) { int Status = FALSE; /* Create storage space for the texture */ SDL_Surface *TextureImage[1]; /* Load The Bitmap into Memory */ if ((TextureImage[0] = SDL_LoadBMP("cube.bmp"))) { Status = TRUE; glGenTextures( 1, &texture[0] ); glBindTexture( GL_TEXTURE_2D, texture[0] ); glTexImage2D( GL_TEXTURE_2D, 0, 3, TextureImage[0]->w, TextureImage[0]->h, 0, GL_BGR, GL_UNSIGNED_BYTE, TextureImage[0]->pixels ); glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR ); glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR ); } /* Free up some memory */ if ( TextureImage[0] ) SDL_FreeSurface( TextureImage[0] ); return Status; } /* function to reset our viewport after a window resize */ int resizeWindow( int width, int height ) { /* Height / width ration */ GLfloat ratio; /* Protect against a divide by zero */ if ( height == 0 ) height = 1; ratio = ( GLfloat )width / ( GLfloat )height; /* Setup our viewport. */ glViewport( 0, 0, ( GLint )width, ( GLint )height ); /* * change to the projection matrix and set * our viewing volume. */ glMatrixMode( GL_PROJECTION ); glLoadIdentity( ); /* Set our perspective */ gluPerspective( 45.0f, ratio, 0.1f, 100.0f ); /* Make sure we're chaning the model view and not the projection */ glMatrixMode( GL_MODELVIEW ); /* Reset The View */ glLoadIdentity( ); return( TRUE ); } /* function to handle key press events */ void handleKeyPress( SDL_keysym *keysym ) { switch ( keysym->sym ) { case SDLK_ESCAPE: /* ESC key was pressed */ Quit( 0 ); break; case SDLK_F1: /* F1 key was pressed * this toggles fullscreen mode */ SDL_WM_ToggleFullScreen( surface ); break; default: break; } return; } /* OpenGL initialization function */ int initGL( GLvoid ) { /* Load in the texture */ if ( !LoadGLTextures( ) ) return FALSE; /* Enable Texture Mapping ( NEW ) */ glEnable( GL_TEXTURE_2D ); /* Enable smooth shading */ glShadeModel( GL_SMOOTH ); /* Set the background black */ glClearColor( 0.0f, 0.0f, 0.0f, 0.5f ); /* Depth buffer setup */ glClearDepth( 1.0f ); /* Enables Depth Testing */ glEnable( GL_DEPTH_TEST ); /* The Type Of Depth Test To Do */ glDepthFunc( GL_LEQUAL ); /* Really Nice Perspective Calculations */ glHint( GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST ); GLfloat amb_light[] = { 0.5, 0.5, 0.5, 1.0 }; GLfloat diffuse[] = { 0.6, 0.6, 0.6, 1 }; GLfloat specular[] = { 0.7, 0.7, 0.3, 1 }; glLightModelfv( GL_LIGHT_MODEL_AMBIENT, amb_light ); glLightfv( GL_LIGHT0, GL_DIFFUSE, diffuse ); glLightfv( GL_LIGHT0, GL_SPECULAR, specular ); glEnable( GL_LIGHT0 ); glEnable( GL_COLOR_MATERIAL ); glShadeModel( GL_SMOOTH ); glLightModeli( GL_LIGHT_MODEL_TWO_SIDE, GL_FALSE ); glDepthFunc( GL_LEQUAL ); glEnable( GL_DEPTH_TEST ); glEnable(GL_LIGHTING); glEnable(GL_LIGHT0); return( TRUE ); } void drawWorld() { /* Select Our Texture */ glBindTexture( GL_TEXTURE_2D, texture[0] ); /* * Draw the cube. A cube consists of six quads, with four coordinates (glVertex3f) * per quad. * */ glBegin(GL_QUADS); /* Front Face */ glNormal3f( 0.0f, 0.0f, 1.0f); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( -1.0f, -1.0f, 1.0f ); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( 1.0f, -1.0f, 1.0f ); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( 1.0f, 1.0f, 1.0f ); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( -1.0f, 1.0f, 1.0f ); /* Back Face */ glNormal3f( 0.0f, 0.0f,-1.0f); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( -1.0f, -1.0f, -1.0f ); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( -1.0f, 1.0f, -1.0f ); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( 1.0f, 1.0f, -1.0f ); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( 1.0f, -1.0f, -1.0f ); /* Top Face */ glNormal3f( 0.0f, 1.0f, 0.0f); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( -1.0f, 1.0f, -1.0f ); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( -1.0f, 1.0f, 1.0f ); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( 1.0f, 1.0f, 1.0f ); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( 1.0f, 1.0f, -1.0f ); /* Bottom Face */ /* Top Right Of The Texture and Quad */ glNormal3f( 0.0f,-1.0f, 0.0f); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( -1.0f, -1.0f, -1.0f ); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( 1.0f, -1.0f, -1.0f ); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( 1.0f, -1.0f, 1.0f ); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( -1.0f, -1.0f, 1.0f ); /* Right face */ glNormal3f( 1.0f, 0.0f, 0.0f); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( 1.0f, -1.0f, -1.0f ); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( 1.0f, 1.0f, -1.0f ); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( 1.0f, 1.0f, 1.0f ); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( 1.0f, -1.0f, 1.0f ); /* Left Face */ glNormal3f(-1.0f, 0.0f, 0.0f); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( -1.0f, -1.0f, -1.0f ); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( -1.0f, -1.0f, 1.0f ); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( -1.0f, 1.0f, 1.0f ); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( -1.0f, 1.0f, -1.0f ); glEnd( ); } void drawFloor() { glBegin(GL_QUADS); glColor3f(1,1,1); glTexCoord2f( 0.0f, 1.0f ); glVertex3f( -12.0f, 0.0f, -12.0f ); glTexCoord2f( 1.0f, 1.0f ); glVertex3f( 12.0f, 0.0f, -12.0f ); glTexCoord2f( 1.0f, 0.0f ); glVertex3f( 12.0f, 0.0f, 12.0f ); glTexCoord2f( 0.0f, 0.0f ); glVertex3f( -12.0f, 0.0f, 12.0f ); glEnd(); } /* Here goes our drawing code */ int drawGLScene( GLvoid ) { /* Clear The Screen And The Depth Buffer */ glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT); glMatrixMode(GL_MODELVIEW); glLoadIdentity( ); /* Move Into The Screen 5 Units */ glTranslatef( 0.0f, -2.0f, -12.0f ); /* Draw First Scene */ glPushMatrix(); glTranslatef(0, -2, 0); glRotatef( rotationXaxis, 1.0f, 0.0f, 0.0f); /* Rotate On The X Axis */ glRotatef( rotationYaxis, 0.0f, 1.0f, 0.0f); /* Rotate On The Y Axis */ glColor3f(0.2,0.2,1); drawWorld(); glPopMatrix(); // turn on alpha blending for transparency glEnable(GL_BLEND); glDisable(GL_DEPTH_TEST); glBlendFunc(GL_SRC_ALPHA,GL_ONE); /* Draw Floor */ drawFloor(); /* Disable Blending */ glDisable(GL_BLEND); glColor3f(1,1,1); /* Enable Stencil Buffer */ glEnable(GL_STENCIL_TEST); glColorMask(0, 0, 0, 0); glDisable(GL_DEPTH_TEST); glStencilFunc(GL_ALWAYS, 1, 1); glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE); glColorMask(1, 1, 1, 1); glEnable(GL_DEPTH_TEST); glStencilFunc(GL_EQUAL, 1, 1); glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); /* Draw Scene Second time */ glPushMatrix(); glTranslatef(0, 2, 0); glScalef(1, -1, 1); glRotatef( rotationXaxis, 1.0f, 0.0f, 0.0f); /* Rotate On The X Axis */ glRotatef( rotationYaxis, 0.0f, 1.0f, 0.0f); /* Rotate On The Y Axis */ drawWorld(); glPopMatrix(); /* Draw it to the screen */ SDL_GL_SwapBuffers( ); /* Rotate Cube */ rotationXaxis += 0.3f; rotationYaxis += 0.2f; return( TRUE ); } /* https://talkera.org.cp-in-1.webhostbox.net/opengl/ */ int main( int argc, char **argv ) { /* Flags to pass to SDL_SetVideoMode */ int videoFlags; /* main loop variable */ int done = FALSE; /* used to collect events */ SDL_Event event; /* this holds some info about our display */ const SDL_VideoInfo *videoInfo; /* whether or not the window is active */ int isActive = TRUE; /* initialize SDL */ if ( SDL_Init( SDL_INIT_VIDEO ) hw_available ) videoFlags |= SDL_HWSURFACE; else videoFlags |= SDL_SWSURFACE; /* This checks if hardware blits can be done */ if ( videoInfo->blit_hw ) videoFlags |= SDL_HWACCEL; /* Sets up OpenGL double buffering */ SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 ); /* get a SDL surface */ surface = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, videoFlags ); /* Verify there is a surface */ if ( !surface ) { fprintf( stderr, "Video mode set failed: %s\n", SDL_GetError( ) ); Quit( 1 ); } /* initialize OpenGL */ initGL( ); /* resize the initial window */ resizeWindow( SCREEN_WIDTH, SCREEN_HEIGHT ); /* wait for events */ while ( !done ) { /* handle the events in the queue */ while ( SDL_PollEvent( &event ) ) { switch( event.type ) { case SDL_ACTIVEEVENT: if ( event.active.gain == 0 ) isActive = FALSE; else isActive = TRUE; break; case SDL_VIDEORESIZE: /* handle resize event */ surface = SDL_SetVideoMode( event.resize.w, event.resize.h, 16, videoFlags ); if ( !surface ) { fprintf( stderr, "Could not get a surface after resize: %s\n", SDL_GetError( ) ); Quit( 1 ); } resizeWindow( event.resize.w, event.resize.h ); break; case SDL_KEYDOWN: /* handle key presses */ handleKeyPress( &event.key.keysym ); break; case SDL_QUIT: /* handle quit requests */ done = TRUE; break; default: break; } } /* draw the scene */ if ( isActive ) drawGLScene( ); } /* clean ourselves up and exit */ Quit( 0 ); /* Should never get here */ return( 0 ); }
