OpenGL: Error moving object using keyboard - opengl

Question

I'm new to OpenGL and I'm trying to paint a stickman and then move it using the arrow-keys from the keyboard.
My idea was to use global variables for the stickman and to change them when a certain key is pressed. Afterwards the draw-function (myDisplay()) is called again.
Unfortunately i always get the following error-message:
"Error 10 error C2371: 'myDisplay' : redefinition; different basic types "
When I replace the myDisplay()-call in the keyboard-function with glutPostRedisplay() as suggested in some tutorials I read the error message disapears and the build is successful. But the stickman doesn't move when pressing the keys.
Here's my code:
#include <GL/glut.h>
GLint x; GLint y; GLint d; //parameters for the stickman
void myKeyboard(unsigned char key, int mx, int my) {
int x1 = mx;
int y1 = 480 - my;
switch(key){
case GLUT_KEY_LEFT :
x = x-50;
myDisplay();
break;
case 'E' :
exit(-1);
break;
default:
break;
}
}
void stickman () {
glBegin(GL_LINES); //body
glVertex2i(x, y);
glVertex2i(x, y-2*d);
glVertex2i(x-d, y-d);
glVertex2i(x+d, y-d);
glVertex2i(x, y-2*d);
glVertex2i(x-d, y-3*d);
glVertex2i(x, y-2*d);
glVertex2i(x+d, y-3*d);
glEnd();
glBegin(GL_LINE_LOOP); //head
glVertex2i(x,y);
glVertex2i(x+0.5*d, y);
glVertex2i(x+0.5*d, y+0.5*d);
glVertex2i(x-0.5*d, y+0.5*d);
glVertex2i(x-0.5*d, y);
glEnd();
}
void myDisplay() {
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0, 0.0, 0.0);
stickman();
glFlush();
}
void myInit() {
glClearColor(1.0, 1.0, 1.0, 1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0.0, 640.0, 0.0, 480.0);
}
void main(int argc, char** argv) {
x = 320;
y = 350;
d = 100;
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(200, 50);
glutCreateWindow("Stickman");
glutDisplayFunc(myDisplay);
glutKeyboardFunc(myKeyboard);
glutPostRedisplay();
myInit();
glutMainLoop();
}

Answer 1

As I found out (GLUT Keyboard Tutorial) the problem wasn't the repaint of the picture, but rather that the left arrow key wasn't recognized.
I needed to write another keyboard-function for my special keys such as the arrow keys.
Now my code looks like this and it works:
void processNormalKeys (unsigned char key, int mx, int my) {
if (key == 'E')
exit(-1);
}
void processSpecialKeys (int key, int mx, int my) {
switch(key){
case GLUT_KEY_LEFT :
x = x-5;
glutPostRedisplay();
break;
case GLUT_KEY_RIGHT :
x = x+5;
glutPostRedisplay();
break;
case GLUT_KEY_UP :
y = y+5;
glutPostRedisplay();
break;
case GLUT_KEY_DOWN :
y = y-5;
glutPostRedisplay();
break;
default:
break;
}
}
I also had to adjust the main function:
glutKeyboardFunc(processNormalKeys);
glutSpecialFunc(processSpecialKeys);
But thanks for your help anyways!

Answer 2

glutPostRedisplay() is used to tell mark that the current window needs to be redisplayed as said in the documentation. That means that you need to call this function at the end of your myKeyboard function.
I will also recommend you to use double or tripple buffer to avoid undesired results. For double buffer you need to change glutInitDisplayMode( GLUT_SINGLE | GLUT_RGB ); with glutInitDisplayMode( GLUT_DOUBLE | GLUT_RGB ); and then at the end of your display function you need to add glutSwapBuffers();. You can find more info here http://www.swiftless.com/tutorials/opengl/smooth_rotation.html.