Why isn't glTranslate doing anything in a double buffered GLUT window? - opengl

Question

I'm trying to make it so that when I press W, A, S, or D, it moves a bunch of lines around on the screen. I read in all the lines from a file and display them, and that works fine.
So I have a keyboard function that has a switch statement that increments an X and Y variable which I use in glTranslate inside of my display function, but my lines don't move. Could anyone help me out with this?
#include <GL/glut.h>
#include <stdlib.h>
#include <fstream>
#include <iostream>
int height = 640, width = 640;
int X = 0, Y = 0;
void drawPolyLineFile(char * fileName) {
std::fstream inStream;
inStream.open(fileName, std::ios::in);
if (inStream.fail()) {
std::cerr<< "Error opening file";
return;
}
glClear(GL_COLOR_BUFFER_BIT);
GLint numpolys, numLines, x, y;
inStream >> numpolys;
for ( int j =0; j < numpolys; j++) {
inStream >> numLines;
glBegin(GL_LINE_STRIP);
for (int i = 0; i < numLines; i++) {
inStream >> x >> y;
glVertex2i(x, y);
}
glEnd();
}
//glutSwapBuffers();
inStream.close();
}
void display(void)
{
/* clear all pixels */
glClear (GL_COLOR_BUFFER_BIT);
glTranslatef(X, Y, 0);
drawPolyLineFile("dino.dat");
/* don't wait!
* start processing buffered OpenGL routines
*/
glutSwapBuffers();
}
void init (void)
{
/* select clearing color */
glClearColor (0.0, 0.0, 0.0, 0.0);
/* initialize viewing values */
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 640.0, 0.0, 640.0, -1.0, 1.0);
}
void keyboard(unsigned char key, int x, int y) {
float speed = 5.0f;
switch ( key ) {
case 'a':
X -= speed;
std::cerr<< X << std::endl;
break;
case 'd':
X += speed;
std::cerr<< X << std::endl;
break;
case 's':
Y -= speed;
std::cerr<< Y << std::endl;
break;
case 'w':
Y += speed;
std::cerr<< Y << std::endl;
break;
default:
break;
}
}
/*
* Declare initial window size, position, and display mode
* (single buffer and RGBA). Open window with "hello"
* in its title bar. Call initialization routines.
* Register callback function to display graphics.
* Enter main loop and process events.
*/
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize (640, 640);
glutInitWindowPosition (100, 100);
glutCreateWindow ("hello");
init ();
glutKeyboardFunc(keyboard);
glutDisplayFunc(display);
glutMainLoop();
return 0; /* ANSI C requires main to return int. */
}

Answer 1

I haven't read too carefully, but you're most likely missing a
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
before the call to glTranslate. glTranslate composes a translation. From your code it seems like you're expecting it to set a translation.
You'll probably also want to avoid reading from disk at every frame. Load your model into a data structure at startup and render from that instead.

Answer 2

You were missing a glutPostRedisplay in your keyboard handler. Without that no redraw of the window would have been initiated. Settings display as idle func does the trick as well, but works differently: The window is constantly redrawn, almost all CPU time used up for drawing. Your code also had some other fallacies. I fixed that.
#include <GL/glut.h>
#include <stdlib.h>
#include <fstream>
#include <iostream>
#include <stdexcept>
int X = 0, Y = 0;
void drawPolyLineFile(char * fileName) throw(std::runtime_error)
{
std::fstream inStream;
inStream.open(fileName, std::ios::in);
if (inStream.fail()) {
throw std::runtime_error("Error opening file")
}
GLint numpolys, numLines, x, y;
inStream >> numpolys;
for ( int j =0; j < numpolys; j++) {
inStream >> numLines;
glBegin(GL_LINE_STRIP);
for (int i = 0; i < numLines; i++) {
inStream >> x >> y;
glVertex2i(x, y);
}
glEnd();
}
inStream.close();
}
void display(void)
{
int window_width, window_height;
window_width = glutGet(WINDOW_WIDTH);
window_height = glutGet(WINDOW_HEIGHT);
glViewport(0, 0, window_width, window_height);
/* clear all pixels */
glClearColor (0.0, 0.0, 0.0, 0.0);
glClear (GL_COLOR_BUFFER_BIT);
/* always set all projection parameters a new
* each rendering pass
*/
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, window_width, 0.0, window_height, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(X, Y, 0);
drawPolyLineFile("dino.dat");
/*
* SwapBuffers will flush the OpenGL queue indeed,
* but more importantly it brings the content from
* the back buffer to the front
*/
glutSwapBuffers();
}
void keyboard(unsigned char key, int x, int y) {
float speed = 5.0f;
switch ( key ) {
case 'a':
X -= speed;
std::cerr<< X << std::endl;
break;
case 'd':
X += speed;
std::cerr<< X << std::endl;
break;
case 's':
Y -= speed;
std::cerr<< Y << std::endl;
break;
case 'w':
Y += speed;
std::cerr<< Y << std::endl;
break;
default:
break;
}
glutPostRedisplay();
}
/*
* Declare initial window size, position, and display mode
* (single buffer and RGBA). Open window with "hello"
* in its title bar. Call initialization routines.
* Register callback function to display graphics.
* Enter main loop and process events.
*/
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize (640, 640);
glutInitWindowPosition (100, 100);
glutCreateWindow ("hello");
glutKeyboardFunc(keyboard);
glutDisplayFunc(display);
try {
glutMainLoop();
} catch (std::runtime_error &err) {
std::cerr << err.what();
}
return 0;
}