Make some shapes in OpenGL to be moved with arrow keys - it zooms out instead of moving

Question

Well, I have the following C code on a GLUT Project (I use CodeBlocks). It draw some 2D shapes (something like Robot :p ).

I want to make the whole drawing shapes to be moved with the keyboard arrow keys. I have wrote the following, but for some reason when I push the arrow keys, it seems like it zoom in/out. It does not move.

#include <GL/glut.h>

GLuint head_x1=5, head_y1=30, head_x2=15, head_y2=30, head_x3=15,head_y3=40, head_x4=5,head_y4=40;

// shape
GLuint listID;
void MrRobot(GLsizei displayListID)
{
    glNewList(displayListID,GL_COMPILE);

    //Save current colour state
    glPushAttrib(GL_CURRENT_BIT);

    // body
    glColor3f(0.5,0.5,0.5);
    glBegin(GL_POLYGON);
    glVertex2f(0,10);
    glVertex2f(20,10);
    glVertex2f(20,30);
    glVertex2f(0,30);
    glEnd();

    // head
    glColor3f(0,0,1);
    glBegin(GL_POLYGON);
    glVertex2f(head_x1,head_y1);
    glVertex2f(head_x2,head_y2);
    glVertex2f(head_x3,head_y3);
    glVertex2f(head_x4,head_y4);
    glEnd();

    // legs
    glColor3f(1,0,0);
    glBegin(GL_TRIANGLE_FAN);
    glVertex2f(10,10);
    glVertex2f(20,0);
    glVertex2f(10,-5);
    glVertex2f(0,0);
    glEnd();

    // right hand
    glColor3f(0,1,0);
    glBegin(GL_TRIANGLES);
    glVertex2f(20,30);
    glVertex2f(30,27.5);
    glVertex2f(20,25);
    glEnd();

    // left hand
    glColor3f(0,1,0);
    glBegin(GL_TRIANGLES);
    glVertex2f(-10,27.5);
    glVertex2f(0,30);
    glVertex2f(0,25);
    glEnd();

    //Recall saved colour state
    glPopAttrib();

    glEndList();
}

void display()
{
    glClearColor(0,0,0,0);

    glClear(GL_COLOR_BUFFER_BIT);

    glColor3f(1,0,0);

    //Defining a modelview transform matrix
    glScalef(0.3,0.3,0.3);

    //Execute the display list (the modelview matrix will be applied)
    glCallList(listID);

    glFlush();
}

void keyboard(unsigned char key,int x, int y)
{
    printf("\nKeyboard event detected. \nCharacter key: %c\nMouse pointer position: x=%d y=%d",key,x,y);

    if (key==GLUT_KEY_UP)
    {
        head_y1++;
        head_y2++;
        head_y3++;
        head_y4++;
    }
    if (key==GLUT_KEY_DOWN)
    {
        head_y1--;
        head_y2--;
        head_y3--;
        head_y4--;
    }
    if (key==GLUT_KEY_LEFT)
    {
        head_x1--;
        head_x2--;
        head_x3--;
        head_x4--;
    }
    if (key==GLUT_KEY_RIGHT)
    {
        head_x1++;
        head_x2++;
        head_x3++;
        head_x4++;
    }

    glutPostRedisplay();
}

int main(int argc, char** argv)
{
    glutInit(&argc,argv);
    glutInitWindowPosition(50,50);
    glutInitWindowSize(800,600);
    glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
    glutCreateWindow("Mr Robot");

    glMatrixMode(GL_PROJECTION);
    gluOrtho2D(-5,35,-5,30);

    listID=glGenLists(1);

    MrRobot(listID);

    glutDisplayFunc(display);
    glutSpecialFunc(keyboard);
    glutMainLoop();

    return 0;
}

Maybe is something with the axis, but have no idea what I should change.

Any help?

Answer 1

Well there are a lot of problems. You seem to be writing OpenGL a little bit differently.

Anyways here is the problem.

• The reason its scaling is because everytime you call display you are calling glScalef(0.3, 0.3, 0.3); without loading the identity matrix.
• Also when you press a key the increment is happening but you havent even called MrRobot() in the display function. You have only called it once in the main function
• Also just before you call display you must call glMatrixMode(GL_MODELVIEW); or else all transformations you do will affect the GL_PROJECTION matrix