How to make action on keyboard input be instant - c++

Question

I am writing in C++ and using OpenGL in Windows.
I've created a cube and I want it to be rotated around the y axis ( using glRotate3f(), not gluLookat() ) by pressing '4' or '6' numpad keys.
The problem is that when I press any key, there is a slight rotation and then stops for a while and then it starts rotating continuously.
Is there any way to achieve this by using glutkeyboardfunc?
If not, how can it be achieved?

Answer 1

Maintain your own key state map and trigger redraws on a timer:
// g++ main.cpp -o main -lglut -lGL -lGLU && ./main
#include <GL/glut.h>
#include <map>
bool paused = false;
std::map< unsigned char, bool > state;
void keyboard_down( unsigned char key, int x, int y )
{
if( key == 'p' )
paused = !paused;
state[ key ] = true;
}
void keyboard_up( unsigned char key, int x, int y )
{
state[ key ] = false;
}
void display()
{
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
double w = glutGet( GLUT_WINDOW_WIDTH );
double h = glutGet( GLUT_WINDOW_HEIGHT );
double ar = w / h;
glOrtho( -2 * ar, 2 * ar, -2, 2, -1, 1);
glMatrixMode( GL_MODELVIEW );
glLoadIdentity();
static float angle = 0;
if( state[ '6' ] ) angle -= 3;
if( state[ '4' ] ) angle += 3;
if( !paused )
angle += 0.5;
glPushMatrix();
glRotatef( angle, 0, 0, 1 );
glColor3ub( 255, 0, 0 );
glBegin( GL_QUADS );
glVertex2i( -1, -1 );
glVertex2i( 1, -1 );
glVertex2i( 1, 1 );
glVertex2i( -1, 1 );
glEnd();
glPopMatrix();
glutSwapBuffers();
}
void timer( int extra )
{
glutPostRedisplay();
glutTimerFunc( 16, timer, 0 );
}
int main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );
glutInitWindowSize( 640, 480 );
glutCreateWindow( "Keyboard" );
glutDisplayFunc( display );
glutKeyboardFunc( keyboard_down );
glutKeyboardUpFunc( keyboard_up );
glutTimerFunc( 0, timer, 0 );
glutIgnoreKeyRepeat( GL_TRUE );
glutMainLoop();
return 0;
}