How to change the color of object in OpenGL(GLUT) using keyboard function

If you have problem in changing color of object in OpenGL using keyboard while running program then this article may help you.Simply we can’t change the color of object while running for this we have to create a timer thread by using

Void  glutTimerFunc(unsigned int msecs,void (*func(int value),value)  function or other techinque can also be use.

Parameter:

msecs

Number of milliseconds to pass before calling the callback.

func

The timer callback function.

value

Integer value to pass to the timer callback.

Description

glutTimerFunc registers the timer callback func to be triggered in at least msecs milliseconds. The value parameter to the timer callback will be the value of the value parameter to glutTimerFunc. Multiple timer callbacks at same or differing times may be registered simultaneously.

In my example I have use glutTimerFunc(25,update,0) .In every 25 milliseconds it calls the update function inside glutMainLoop.

At fist object(rectangle) has white color you can change it color by pressing left and right key to decrease and increase red color value respectively ,also you can change green color value similarly by pressing Up and Down key.But I haven’t use any key to change the color of blue color value.We know that color for object is define by the function 

glcolor3f(GLfloat ,GLfloat,GLfloat) which takes RBG value as parameters.

Here is the code you can copy and compile it.

#include<windows.h>

#include <GL/glut.h>

#include<iostream>

#include <stdlib.h>

#include <cmath>

using namespace std;

GLfloat red=1.0f,green=1.0f,blue=1.0f;

GLfloat red_change =0.0f,green_change=0.0f;

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

{

switch(key)

{

case 27:

exit(0);

}

}

void pressKey(int key, int x, int y) {

switch (key) {

case GLUT_KEY_LEFT : red_change =-1.0f;break;

case GLUT_KEY_RIGHT : red_change =1.0f;break;

case GLUT_KEY_UP : green_change = 1.0f;break;

case GLUT_KEY_DOWN : green_change = -1.0f;break;

}

}

void releaseKey(int key, int x, int y) {

switch (key) {

case GLUT_KEY_LEFT :

case GLUT_KEY_RIGHT : red_change = 0.0f;break;

case GLUT_KEY_UP :

case GLUT_KEY_DOWN : green_change = 0.0f;break;

}

}

void change_red(GLfloat r)

{

red=red-sin(r);

}

void change_green(GLfloat g)

{

green=green-sin(g);

}

void initRendering()

{

glEnable(GL_DEPTH_TEST);

glEnable(GL_COLOR_MATERIAL);

glClearColor(0.1f,0.3f,0.6f,1.0f);

}

void handleResize(int w,int h)

{

glViewport(0,0,w,h);

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

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

}

float _angle=30.0;

float _cameraangle=0.0;

void drawScene()

{

glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);

glMatrixMode(GL_MODELVIEW);

glLoadIdentity();

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

glTranslatef(0.0f, 0.0f, -10.0f);

glPushMatrix();

glColor3f(red,green,blue);

glTranslatef(0.0f, -1.0f, 0.0f);

glScalef(2.0f,2.0f,2.0f);

glRotatef(_angle,1.0f,3.0f,2.0f);

glBegin(GL_QUADS);

glVertex3f(-0.7f,0.0f,0.0);

glVertex3f(0.7f,0.0f,0.0);

glVertex3f(0.5f,2.0f,0.0);

glVertex3f(-0.5f,2.0f,0.0);

glEnd();

glPopMatrix();

glutSwapBuffers();

}

void update(int value)

{

_angle+=2.0f;

if(_angle>360.f)

{

_angle-=360;

}

if(red_change>0.0f)

{

if(red!=1.0f)

red=red+0.01f;

else

red=red;

}

else if(red_change<0.0f)

{

if(red!=0.0f)

red=red-0.01f;

else

red=red;

}

else

red=red;

if(green_change>0.0f)

{

if(green!=1.0f)

green=green+0.01f;

else

green=green;

}

else if(green_change<0.0f)

{

if(green!=0.0f)

green=green-0.01f;

else

green=green;

}

else

green=green;

glutPostRedisplay();

glutTimerFunc(25,update,0);

}

int main(int argc,char**argv)

{

glutInit(&argc,argv);

glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB|GLUT_DEPTH);

glutInitWindowSize(600,600);

glutCreateWindow("rotate");

initRendering();

glutIgnoreKeyRepeat(1);

glutDisplayFunc(drawScene);

glutKeyboardFunc(handleKeypress);

glutSpecialFunc(pressKey);

glutSpecialUpFunc(releaseKey);

glutReshapeFunc(handleResize);

glutTimerFunc(25,update,0);

glutMainLoop();

return 0;

}