make GLUT application to rotate on keyboard press

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.