openGL 2D moving and object by 1 unit - c++

Question

I have some questions about how to move an object by pressing a key. All I want to do is to press the up button in my keyboard and make the object move one unit.
void display(){
//Clear Window
glClear(GL_COLOR_BUFFER_BIT);
glPushMatrix();
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(-0.1, -0.2);
glVertex2f(-0.1, 0.2);
glVertex2f(0.1, 0.2);
glVertex2f(0.1, -0.2);
glEnd();
glPopMatrix();
glFlush();
}
void keyboardListener(int key)
{
if( key == GLUT_KEY_UP)
{
glTranslatef(1.0, 0.0, 0.0);
glutPostRedisplay();
}
}
Whats missing or what concept I am not understanding?

Answer 1

use this:
#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);
glColor3f(1.0, 0.0, 0.0);
glVertex2f(-0.1, -0.2);
glVertex2f(-0.1, 0.2);
glVertex2f(0.1, 0.2);
glVertex2f(0.1, -0.2);
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 = 0.1f;
void keyboardown(int key, int x, int y)
{
switch (key){
case GLUT_KEY_RIGHT:
posX+=move_unit;;
break;
case GLUT_KEY_LEFT:
posX-=move_unit;;
break;
case GLUT_KEY_UP:
posY+=move_unit;;
break;
case GLUT_KEY_DOWN:
posY-=move_unit;;
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("Practice 1");
glutDisplayFunc(display);
init();
glutSpecialFunc(keyboardown);
glutMainLoop();
}

Answer 2

The easiest way is to do something like so
float posX = 0, posY = 0, posZ = 0;
void display(){
glClear(GL_COLOR_BUFFER_BIT);
glTranslate(posX,posY,posZ);
drawPolygon();
//...
}
void keyboardListener(int key){
if(key == GLUT_KEY_RIGHT){ posX++; }
else if(key == GLUT_KEY_LEFT){ posX--; }
//..similar for up/down
glutPostRedisplay();
}

Answer 3

This code draws a circle and is moved upon left, right, up or down key pressed.
#include <cmath>
#include <stdio.h>
float posX = 0.01, posY = -0.1, posZ = 0;
void circ() {
glColor3f(0.0, 0.0, 1.0);
glBegin(GL_TRIANGLE_FAN);
for (int i = 0; i <= 300; i++) {
angle = 2 * PI * i / 300;
x = cos(angle) / 20;
y = sin(angle) / 20;
glVertex2d(x, y);
}
glEnd();
}
void display() {
glClearColor(1.0, 1.0, 1.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-1.0, 1.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glPushMatrix();
glTranslatef(posX, posY, posZ);
circ();
glPopMatrix();
glutSwapBuffers();
}
**float move_unit = 0.02f;
void keyboardown(int key, int x, int y) {
switch (key) {
case GLUT_KEY_RIGHT:
posX += move_unit;
break;
case GLUT_KEY_LEFT:
posX -= move_unit;
break;
case GLUT_KEY_UP:
posY += move_unit;
break;
case GLUT_KEY_DOWN:
posY -= move_unit;
break;
default:
break;
}
glutPostRedisplay();
}**
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(600, 500);
glutInitWindowPosition(0, 0);
glutCreateWindow("Example");
glutDisplayFunc(display);
glutSpecialFunc(keyboardown);
glutMainLoop();
}