i wanna make a small app that allow me to move the triangle to top/bottm left/right.

and when i press w -> rotate above and move above , s => rotate bottom and move bottom ,, d -> rotate right and moves right , a -> rotate left and move left ..

here is my code :

#include <GL/glut.h> // (or others, depending on the system in use)
float xpoint1 = 0.0f;
float xpoint2 = 50.0f;
float xpoint3 = 25.0f;
float ypoint1 = 0.0f, ypoint2 = 0.0f, ypoint3 = 20.0f;
double direction = 0.0;
void Keys(unsigned char key, int x, int y) {

    if (key == 'a') {
        if (xpoint1 != 0) {
            xpoint1 -= 1.0f;
            xpoint2 -= 1.0f;
            xpoint3 -= 1.0f;
            direction = 90.0;
        }

    }
    else if (key == 'd') {
        if (xpoint2 != 200) {
            xpoint1  = 1.0f;
            xpoint2  = 1.0f;
            xpoint3  = 1.0f;
            direction = 270.0;
        }
    }
    else if (key == 'w') {
        if (ypoint3 != 150) {
            ypoint1  = 1.0f;
            ypoint2  = 1.0f;
            ypoint3  = 1.0f;
            direction = 0.0;
        }
    }
    else if (key == 's') {
        if (ypoint3 != 0) {
            ypoint1 -= 1.0f;
            ypoint2 -= 1.0f;
            ypoint3 -= 1.0f;
            direction = 180.0;
            
        }
    }
    
    glutPostRedisplay();
}
void resizeChange(int w, int h) {

    glClearColor(1.0, 1.0, 1.0, 0.0); // Set display-window color to white.
    glMatrixMode(GL_PROJECTION); // Set projection parameters.
    glLoadIdentity();
    gluOrtho2D(0.0, 200.0, 0.0, 150.0);
    glViewport(0, 0, w, h);
    glMatrixMode(GL_MODELVIEW);
}
void lineSegment(void)
{
    glClear(GL_COLOR_BUFFER_BIT); // Clear display window.
    glColor3f(0.0, 0.4, 0.2); // Set line segment color to green.
    
    //gluLookAt(0.0, 0.0, 5.0,
    //  0.0, 0.0,0.0,
    //  0.0, 1.0, 0.0
    //);

    glLoadIdentity();

    glRotated(direction, 0.0, 0.0, 1.0);
        glBegin(GL_TRIANGLES);
            glColor3f(1.0, 0.0, 0.0);
            glVertex2f(xpoint1, ypoint1);
            glColor3f(0.0, 1.0, 0.0);
            glVertex2f(xpoint2, ypoint2);
            glColor3f(0.0, 0.0, 1.0);
            glVertex2f(xpoint3, ypoint3);
        glEnd();
        //degree  = 1.0;
    glFlush(); // Process all OpenGL routines as quickly as possible.
        //glutSwapBuffers();
}
void main(int argc, char** argv)
{
    glutInit(&argc, argv); // Initialize GLUT.
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); // Set display mode.
    glutInitWindowPosition(50, 100); // Set top-left display-window position.
    glutInitWindowSize(600, 600); // Set display-window width and height.
    glutCreateWindow("An Example OpenGL Program"); // Create display window.
    glutDisplayFunc(lineSegment); // Send graphics to display window.
    glutReshapeFunc(resizeChange);
    //glutIdleFunc(lineSegment);
    glutKeyboardFunc(Keys);
    glutMainLoop(); // Display everything and wait.
}

the problem is when i try to change direction of the triangle it disappered ?! , whats the problem ?!

CodePudding user response：

glRotated rotates around (0, 0). Actually rotate the triangle out of sight. You have to rotate the triangle first and then move it to its place in the world. Do not change the vertex coordinates but add a translation matrix with glTranslate:

double tx = 0.0, ty = 0.0;
double direction = 0.0;

void Keys(unsigned char key, int x, int y)
{
    if (key == 'a') {
        tx -= 1.0;
        direction = 90.0;
    }
    else if (key == 'd') {
        tx  = 1.0;
        direction = 270.0;
    }
    else if (key == 'w') {
        ty  = 1.0;
        direction = 0.0;
    }
    else if (key == 's') {
        ty -= 1.0;
        direction = 180.0;
    }
    glutPostRedisplay();
}

void lineSegment(void)
{
    // [...]

    glTranslated(tx, ty, 0.0);
    glRotated(direction, 0.0, 0.0, 1.0);
    glBegin(GL_TRIANGLES);
            glColor3f(1.0, 0.0, 0.0);
            glVertex2f(-25.0, 0.0);
            glColor3f(0.0, 1.0, 0.0);
            glVertex2f(25.0, 0.0);
            glColor3f(0.0, 0.0, 1.0);
            glVertex2f(0.0, 20.0);
    glEnd();

    // [...]
}