How to use glutSpecialFunc correctly? - opengl

Question

I wanna make my square go up when UO is pressed.
void displayScene(void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glTranslatef(0,x,0);
glBegin(GL_QUADS);
glVertex3f(-0.4,-0.4, -5.0);
glVertex3f( 0.4,-0.4, -5.0);
glVertex3f( 0.4, 0.4, -5.0);
glVertex3f(-0.4, 0.4, -5.0);
glEnd();
//x = x + 0.1;
glutSwapBuffers();
}
I'm using gluSpecialFunc.
void ProcessSpecialKeys(unsigned char key, int x, int y)
{
if (key == GLUT_KEY_UP)
{
x = x + 0.1;
}
glutPostRedisplay();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(0,0);
glutCreateWindow("OpenGL Window");
init();
glutDisplayFunc(displayScene);
glutSpecialFunc(ProcessSpecialKeys);
//glutKeyboardFunc(ProcessKeys);
glutMainLoop();
return 0;
}
When I leave x+=0.1 in the displayScene, the square goes up when I press any key.
Am I using glutSpecialFunc incorrectly? Because I used it before and it worked normally. What am I missing?

Answer 1

glutSpecialFunc is not invoked continuously when a key is held down. The callback is called once when a key is pressed.
freeglut provides the glutSpecialUpFunc callback which is called when a key is released.
Set a state when UP is pressed and reset the state when UP is released:
int main(int argc, char** argv)
{
// [...]
glutSpecialFunc(ProcessSpecialKeys);
glutSpecialUpFunc(ReleaseSpecialKeys);
// [...]
}
int keyUpPressed = 0;
void ProcessSpecialKeys(unsigned char key, int x, int y)
{
if (key == GLUT_KEY_UP)
keyUpPressed = 1;
}
void ReleaseSpecialKeys(unsigned char key, int x, int y)
{
if (key == GLUT_KEY_UP)
keyUpPressed = 0;
}
Change the x dependent on the state of keyUpPressed. Continuously redraw the scene by calling glutPostRedisplay in displayScene
void displayScene(void)
{
if (keyUpPressed)
x += 0.1;
// [...]
glutSwapBuffers();
glutPostRedisplay();
}