Draw Reflection of Object on the floor using OpenGL Utility Toolkit (GLUT)

Drawing reflection capture the extra realism of the Object in 3D programming.For example Bouncing ball having its reflection on the floor can make the bouncing scenario more beautiful and realistic. So,OpenGL has all the features needed to render fast high-quality reflections on planar surfaces.

        Reflection technique is not so hard its pretty simple.If you want to draw reflection on floor then suppose your floor is in x-z plane and your object along positive y-axis above the plane then for reflection of object draw the copy of object along the negative y-axis vertically below the plane or floor.Your floor must look like the reflecting surface to see the object which is below the floor for this we use alpha blending which make the floor transparent by certain percent say 40% then it is specified to be 40% the color of the floor and 60% the color of the reflection or object.But only alpha blending doesn’t make the reflection real because the reflected copy of the object may leak outside or bottom of the floor so to fixed this problem we actually going to use the stencil buffer concept.With OpenGL stenciling, we can "pre-draw" the floor into the stencil buffer without updating the color or depth buffers. Then when we go to draw the reflection,  just only allow the reflection to update pixels marked as belonging to the floor's stencil value. The floor can then be any complex (but coplanar) polygon and the reflection only shows up in the floor.

Here is the sample source code, copy and compile it.I hope it will help you.

#include <windows.h>

#include <iostream>

#include <stdlib.h>

#include <GL/glut.h>

using namespace std;

const float LIGHT_POS = 7.0f; //The length of each side of the cube

const float BOX_HEIGHT = LIGHT_POS ; //The height of the box off of the ground

const float FLOOR_SIZE = 20.0f; //The length of each side of the floor

//Draws the cube

void drawCube(float angle) {

glPushMatrix();

glRotatef(-angle, 1.0f, 1.0f, 0.0f);

glBegin(GL_QUADS);

//Cross back

glColor4f(0.0f,1.0f, 0.0f, 0.9f);

glVertex3f(-0.5, 2.5, -0.5);

glVertex3f(-0.5, -3.0, -0.5);

glVertex3f(0.5, -3.0, -0.5);

glVertex3f(0.5, 2.5, -0.5);

glVertex3f(-2.0, 1.0, -0.5);

glVertex3f(-2.0, 0.0, -0.5);

glVertex3f(2.0, 0.0, -0.5);

glVertex3f(2.0, 1.0, -0.5);

//Cross front

glColor4f(1.0f, 1.0f, 0.0f, 0.9f);

glVertex3f(-0.5, 2.5, 0.5);

glVertex3f(-0.5, -3.0, 0.5);

glVertex3f(0.5, -3.0, 0.5);

glVertex3f(0.5, 2.5, 0.5);

glVertex3f(-2.0, 1.0, 0.5);

glVertex3f(-2.0, 0.0, 0.5);

glVertex3f(2.0, 0.0, 0.5);

glVertex3f(2.0, 1.0, 0.5);

//Cross top

glColor3f(1.0,0.0,0.0);

glVertex3f(0.5, 2.5, 0.5);

glVertex3f(-0.5, 2.5, 0.5);

glVertex3f(-0.5, 2.5, -0.5);

glVertex3f(0.5, 2.5, -0.5);

//Cross left top side

glColor3f(0.0,0.0,1.0);

glVertex3f(-0.5, 2.5, 0.5);

glVertex3f(-0.5, 2.5, -0.5);

glVertex3f(-0.5, 1.0, -0.5);

glVertex3f(-0.5, 1.0, 0.5);

//Cross left side  top

glVertex3f(-0.5, 1.0, 0.5);

glVertex3f(-0.5, 1.0, -0.5);

glVertex3f(-2.0, 1.0, -0.5);

glVertex3f(-2.0, 1.0, 0.5);

//Cross left side  end

glColor3f(1.0,0.0,0.0);

glVertex3f(-2.0, 1.0, 0.5);

glVertex3f(-2.0, 1.0, -0.5);

glVertex3f(-2.0, 0.0, -0.5);

glVertex3f(-2.0, 0.0, 0.5);

//Cross left side bottom

glColor3f(0.0,0.0,1.0);

glVertex3f(-2.0, 0.0, 0.5);

glVertex3f(-0.5, 0.0, 0.5);

glVertex3f(-0.5, 0.0, -0.5);

glVertex3f(-2.0, 0.0, -0.5);

//Cross left bottom side

glVertex3f(-0.5, 0.0, 0.5);

glVertex3f(-0.5, 0.0, -0.5);

glVertex3f(-0.5, -3.0, -0.5);

glVertex3f(-0.5, -3.0, 0.5);

//Cross  bottom

glColor3f(1.0,0.0,0.0);

glVertex3f(0.5, -3.0, 0.5);

glVertex3f(0.5, -3.0, -0.5);

glVertex3f(-0.5, -3.0, -0.5);

glVertex3f(-0.5, -3.0, 0.5);

//Cross right bottom side

glColor3f(0.0,0.0,1.0);

glVertex3f(0.5, 0.0, 0.5);

glVertex3f(0.5, -3.0, 0.5);

glVertex3f(0.5, -3.0, -0.5);

glVertex3f(0.5, 0.0, -0.5);

//Cross right side bottom

glVertex3f(0.5, 0.0, 0.5);

glVertex3f(0.5, 0.0, -0.5);

glVertex3f(2.0, 0.0, -0.5);

glVertex3f(2.0, 0.0, 0.5);

//Cross right side end

glColor3f(1.0,0.0,0.0);

glVertex3f(2.0, 1.0, 0.5);

glVertex3f(2.0, 0.0, 0.5);

glVertex3f(2.0, 0.0, -0.5);

glVertex3f(2.0, 1.0, -0.5);

//Cross right side top

glColor3f(0.0,0.0,1.0);

glVertex3f(0.5, 1.0, 0.5);

glVertex3f(2.0, 1.0, 0.5);

glVertex3f(2.0, 1.0, -0.5);

glVertex3f(0.5, 1.0, -0.5);

//Cross right side top

glVertex3f(0.5, 1.0, 0.5);

glVertex3f(0.5, 1.0, -0.5);

glVertex3f(0.5, 2.5, -0.5);

glVertex3f(0.5, 2.5, 0.5);

glEnd();

glPopMatrix();

}

//Draws the floor

void drawFloor() {

glBegin(GL_QUADS);

glNormal3f(0, 1, 0);

glVertex3f(-FLOOR_SIZE / 2, 0, FLOOR_SIZE / 2);

glVertex3f(-FLOOR_SIZE / 2, 0, -FLOOR_SIZE / 2);

glVertex3f(FLOOR_SIZE / 2, 0, -FLOOR_SIZE / 2);

glVertex3f(FLOOR_SIZE / 2, 0, FLOOR_SIZE / 2);

glEnd();

}

float _angle = 0;

void handleKeypress(unsigned char key, int x, int y) {

switch (key) {

case 27: //Escape key

exit(0);

}

}

void initRendering() {

glClearColor(1.0,1.0,1.0,1.0);

glEnable(GL_DEPTH_TEST);

glEnable(GL_LIGHTING);

glEnable(GL_LIGHT0);

glEnable(GL_NORMALIZE);

glEnable(GL_COLOR_MATERIAL);

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

}

void handleResize(int w, int h) {

glViewport(0, 0, w, h);

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

gluPerspective(45.0, (float)w / (float)h, 1.0, 200.0);

}

void drawScene() {

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

glMatrixMode(GL_MODELVIEW);

glLoadIdentity();

glTranslatef(0.0f, 0.0f, -40.0f);

glRotatef(30, 1, 0, 0);

GLfloat ambientLight[] = {0.3f, 0.3f, 0.3f, 1.0f};

glLightModelfv(GL_LIGHT_MODEL_AMBIENT, ambientLight);

GLfloat lightColor[] = {0.7f, 0.7f, 0.7f, 1.0f};

GLfloat lightPos[] = {-2 * LIGHT_POS , LIGHT_POS , 4 * LIGHT_POS , 1.0f};

glLightfv(GL_LIGHT0, GL_DIFFUSE, lightColor);

glLightfv(GL_LIGHT0, GL_POSITION, lightPos);

glPushMatrix();

glTranslatef(0, BOX_HEIGHT, 0);

drawCube(_angle);

glPopMatrix();

glEnable(GL_STENCIL_TEST); //Enable using the stencil buffer

glColorMask(0, 0, 0, 0); //Disable drawing colors to the screen

glDisable(GL_DEPTH_TEST); //Disable depth testing

glStencilFunc(GL_ALWAYS, 1, 1); //Make the stencil test always pass

//Make pixels in the stencil buffer be set to 1 when the stencil test passes

glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

//Set all of the pixels covered by the floor to be 1 in the stencil buffer

drawFloor();

glColorMask(1, 1, 1, 1); //Enable drawing colors to the screen

glEnable(GL_DEPTH_TEST); //Enable depth testing

//Make the stencil test pass only when the pixel is 1 in the stencil buffer

glStencilFunc(GL_EQUAL, 1, 1);

glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); //Make the stencil buffer not change

//Draw the cube, reflected vertically, at all pixels where the stencil

//buffer is 1

glPushMatrix();

glScalef(1, -1, 1);

glTranslatef(0, BOX_HEIGHT, 0);

drawCube(_angle);

glPopMatrix();

glDisable(GL_STENCIL_TEST); //Disable using the stencil buffer

//Blend the floor onto the screen

glEnable(GL_BLEND);

glColor4f(0, 0, 0.5, 0.5f);

drawFloor();

glDisable(GL_BLEND);

glutSwapBuffers();

}

void update(int value) {

_angle += 1.0f;

if (_angle > 360) {

_angle -= 360;

}

glutPostRedisplay();

glutTimerFunc(25, update, 0);

}

int main(int argc, char** argv) {

glutInit(&argc, argv);

glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH | GLUT_STENCIL);

glutInitWindowSize(600, 600);

glutCreateWindow("www.codeincodeblock.blogspot.com");

initRendering();

glutDisplayFunc(drawScene);

glutKeyboardFunc(handleKeypress);

glutReshapeFunc(handleResize);

glutTimerFunc(25, update, 0);

glutMainLoop();

return 0;

}