Change background color after collision detection? - c++

Question

#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.

Answer 1

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 );
}