make GLUT application to rotate on keyboard press - opengl

Question

guys i try to make an GLUT application that could rotate object on key pressed, but it seems not worked.
#include <stdio.h>
#include <gl/glut.h>
GLfloat rotation = 90.0;
float posX = 0, posY = 0, posZ = 0;
void reshape(int width, int heigth){
/* window ro reshape when made it bigger or smaller*/
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//clip the windows so its shortest side is 2.0
if (width < heigth) {
glOrtho(-2.0, 2.0, -2.0 * (GLfloat)heigth / (GLfloat)width, 2.0 * (GLfloat)heigth / (GLfloat)width, 2.0, 2.0);
}
else{
glOrtho(-2.0, 2.0, -2.0 * (GLfloat)width / (GLfloat)heigth, 2.0 * (GLfloat)width / (GLfloat)heigth,2.0 , 2.0);
}
// set viewport to use the entire new window
glViewport(0, 0, width, heigth);
}
void rect(){
glBegin(GL_POLYGON);
glVertex2f(-0.2, -0.2);
glColor3f(1.0, 1.0, 0.0);
glVertex2f(-0.2, 0.2);
glColor3f(1.0, 0.0, 1.0);
glVertex2f(0.2, 0.2);
glColor3f(0.0, 1.0, 1.0);
glVertex2f(1.2, -0.2);
glColor3f(1.0, 1.0, 1.0);
glEnd();
}
void display(){
//Clear Window
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glPushMatrix();
glTranslatef(posX,posY,posZ);
rect();
glPopMatrix();
glFlush();
}
void init(){
// set clear color to black
glClearColor(0.0, 0.0, 0.0, 0.0);
// set fill color to white
glColor3f(1.0, 1.0, 1.0);
//set up standard orthogonal view with clipping
//box as cube of side 2 centered at origin
//This is the default view and these statements could be removed
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-1.0, 1.0, -1.0, 1.0);
}
float move_unit = 10;
int deg = 0;
void keyboardown(int key, int x, int y)
{
switch (key){
case GLUT_KEY_RIGHT:
glRotatef((deg+=move_unit), posX, posY, posZ);;
glutPostRedisplay();
break;
case GLUT_KEY_LEFT:
glRotatef(deg-=move_unit, posX, posY, posZ);;
break;
case GLUT_KEY_UP:
glRotatef(deg-=move_unit, posX, posY, posZ);;
break;
case GLUT_KEY_DOWN:
glRotatef(deg+=move_unit, posX, posY, posZ);;
break;
default:
break;
}
glutPostRedisplay();
}
int main(int argc, char** argv){
//initialize mode and open a windows in upper left corner of screen
//Windows tittle is name of program
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(500,500);
glutInitWindowPosition(0, 0);
glutCreateWindow("Move Test");
glutDisplayFunc(display);
init();
glutSpecialFunc(keyboardown);
glutMainLoop();
}
is there is something i did it wrong?
before, i tried to use the GLUT_KEY_ for moving 2d object and it worked, but when i change the command to glrotatef, it doesn't work.
have any suggestion?

Answer 1

The problem here is, that you override the matrix before it is used. In keyboardown the matrix is set, but at the begin of display the glLoadIdentity(); function is called, which resets the matrix and removes the rotation.
To solve this, you can, e.g., store the rotation angle in a variable. In keyboardown you increase/decrease the angle. When rendering in the display function, you reset the matrix as already done and then add the rotation by calling glRotatef with the previously stored angle.