Change background color after collision detection?

#include <stdio.h> // this library is for standard input and output
#include "glut.h"// this library is for glut the OpenGL Utility Toolkit
#include <math.h>

float squareX = 162.0f;
float squareY = 0.0f;
float squareZ = 0.0f;

// background color
void drawBackground() {
    float width = 400;
    float height = 100;
    // color up
    glBegin(GL_POLYGON);
    glColor3f(0.0, 0.0, 1.0);
    glVertex2f(width, height);
    glVertex2f(width, width);
    glVertex2f(0, width);
    glVertex2f(0, height);
    glVertex2f(width, height);
    glEnd();
    // color down
    glBegin(GL_POLYGON);
    glColor3f(0.0, 1.0, 0.0);
    glVertex2f(width, height);
    glVertex2f(width, 0);
    glVertex2f(0, 0);
    glVertex2f(0, height);
    glVertex2f(width, height);
    glEnd();
}

void drawShape(void) {
    float width = 58.0f;
    float height = 40.0f;
    glTranslatef(squareX, squareY, squareZ);
    glBegin(GL_POLYGON);
    glColor3f(1.0, 0.0, 0.0);
    glVertex2f(0, 0);
    glVertex2f(width, 0);
    glVertex2f(width, height);
    glVertex2f(0, height);
    glVertex2f(0, 0);
    glEnd();
}

// called when the window is resized
void handleResize(int w, int h) {
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0.0f, (float)w, 0.0f, (float)h, -1.0f, 1.0f);
}

void drawScene() {
    glClear(GL_COLOR_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    drawBackground();
    glPushMatrix();
    drawShape();
    glPushMatrix();
    glFlush();
    glutSwapBuffers();
    glutPostRedisplay();
}

int state = 1;

// make the square go up
void update(int value) {
    // 1 : move up
    if (state == 1) {
        squareY += 1.0f;
        if (squareY > 400.0) {
            state = 2;
            squareX = 0.0f;
            squareY = 180.0f;
        }
    }
    // 2 : move right
    else if (state == 2) {
        squareX += 1.0f;
        if (squareX > 400.0) {
            state = 3;
            squareX = 180.0f;
            squareY = 400.0f;
        }
    }
    // 3 : move down
    else if (state == 3) {
        squareY -= 1.0f;
        if (squareY < 0.0) {
            state = 0;
        }
    }
    glutPostRedisplay();
    glutTimerFunc(25, update, 0);
}

int main(int argc, char** argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize(400, 400);
    glutCreateWindow("Moving Square");
    glutDisplayFunc(drawScene);
    glutReshapeFunc(handleResize);
    glutTimerFunc(25, update, 0);
    glutMainLoop();
    return(0);
}

The code above moves a square from bottom to top, left to right, and top to bottom. The background color is blue, but how do I change the background color after each collision detection? The color order I am trying to achieve after each collision detection is blue, red, and green. I've added a comment to where the background is.

Solution

A switch() based on state with appropriate glClearColor()/glColor3f() calls would work:

void drawScene()
{
    switch( state )
    {
    case 0: glClearColor( 0.0, 0.0, 0.0, 1.0 ); break;
    case 1: glClearColor( 0.0, 1.0, 0.0, 1.0 ); break;
    case 2: glClearColor( 0.2, 0.0, 0.0, 1.0 ); break;
    case 3: glClearColor( 0.0, 0.0, 1.0, 1.0 ); break;
    }
    glClear( GL_COLOR_BUFFER_BIT );
    ...

Demo:

All together:

#include <cstdio>
#include <GL/glut.h>
#include <cmath>

float squareX = 162.0f;
float squareY = 0.0f;
float squareZ = 0.0f;

void drawShape( void )
{
    float width = 58.0f;
    float height = 40.0f;
    glTranslatef( squareX, squareY, squareZ );
    glBegin( GL_POLYGON );
    glColor3f( 1.0, 0.0, 0.0 );
    glVertex2f( 0, 0 );
    glVertex2f( width, 0 );
    glVertex2f( width, height );
    glVertex2f( 0, height );
    glVertex2f( 0, 0 );
    glEnd();
}

// called when the window is resized
void handleResize( int w, int h )
{
    glViewport( 0, 0, w, h );
    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    glOrtho( 0.0f, (float)w, 0.0f, (float)h, -1.0f, 1.0f );
}

int state = 1;

void drawScene()
{
    switch( state )
    {
    case 0: glClearColor( 0.0, 0.0, 0.0, 1.0 ); break;
    case 1: glClearColor( 0.0, 1.0, 0.0, 1.0 ); break;
    case 2: glClearColor( 0.2, 0.0, 0.0, 1.0 ); break;
    case 3: glClearColor( 0.0, 0.0, 1.0, 1.0 ); break;
    }
    glClear( GL_COLOR_BUFFER_BIT );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();

    glPushMatrix();
    drawShape();
    glPushMatrix();
    glFlush();
    glutSwapBuffers();
    glutPostRedisplay();
}

// make the square go up
void update( int value )
{
    // 1 : move up
    if( state == 1 )
    {
        squareY += 1.0f;
        if( squareY > 400.0 )
        {
            state = 2;
            squareX = 0.0f;
            squareY = 180.0f;
        }
    }
    // 2 : move right
    else if( state == 2 )
    {
        squareX += 1.0f;
        if( squareX > 400.0 )
        {
            state = 3;
            squareX = 180.0f;
            squareY = 400.0f;
        }
    }
    // 3 : move down
    else if( state == 3 )
    {
        squareY -= 1.0f;
        if( squareY < 0.0 )
        {
            state = 0;
        }
    }
    glutPostRedisplay();
    glutTimerFunc( 25, update, 0 );
}

int main( int argc, char** argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );
    glutInitWindowSize( 400, 400 );
    glutCreateWindow( "Moving Square" );
    glutDisplayFunc( drawScene );
    glutReshapeFunc( handleResize );
    glutTimerFunc( 25, update, 0 );
    glutMainLoop();
    return( 0 );
}