OpenGL animation for scrolling text horizontally doesnt scroll

Question

I am trying to scroll a text as a banner. I used openGL with glut to make this work. The whole code works if I use a figure like a square. The square scrolls over the screen.

Now I want to do this with text. Every time this program started. The text came at its starting position, but when the timer starts, it vanished. This is probably because the screen was cleared every clocktick, but the screen doesn't build up again.

Someone who can help me with this translation animation and text?

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <iostream>

#ifdef WIN32
#include <windows.h>
#endif
#include <GL/gl.h>
#include <GL/glut.h>
using namespace std;

static int font_index = 0;
int state = 1;


void print_bitmap_string(/*void* font,*/ const char* s)
{

    while (*s) {
        glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, *s);
        s++;
    }

}

void my_reshape(int w, int h)
{
    GLdouble size;
    GLdouble aspect;

    /* Use the whole window. */
    glViewport(0, 0, w, h);

    /* We are going to do some 2-D orthographic drawing. */
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    size = (GLdouble)((w >= h) ? w : h) / 2.0;
    if (w <= h) {
        aspect = (GLdouble)h / (GLdouble)w;
        glOrtho(-size, size, -size * aspect, size * aspect, -100000.0, 100000.0);
    }
    else {
        aspect = (GLdouble)w / (GLdouble)h;
        glOrtho(-size * aspect, size * aspect, -size, size, -100000.0, 100000.0);
    }

    /* Make the world and window coordinates coincide so that 1.0 in */
    /* model space equals one pixel in window space.                 */
    glScaled(aspect, aspect, 1.0);

    /* Now determine where to draw things. */
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

}

float yild;
float ystep;
float x_pos = -200;
float y_pos = 70;

void draw()
{
    const char* bitmap_font_names[7] = { "Hello train" };

    /* Draw the strings, according to the current mode and font. */
    glTranslatef(0.5, -100, 0);
    //set the text color
    glColor4f(0.0f, 255.0f, 140.0f, 1.0f);

    ystep = 100.0;
    yild = 20.0;
    glRasterPos2f(x_pos, y_pos + 1.25 * yild);
    print_bitmap_string(bitmap_font_names[0]);
}


void display(void)
{
    //change background color
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT);
    draw();
    glColor3f(0.0, 1.0, 0.0);
    glBegin(GL_POLYGON);


    glVertex2f(x_pos + 0.5f, 0.0f);
    glVertex2f(x_pos+1.0f, 0.5f);
    glVertex2f(x_pos+0.5f, 0.5f);

    glEnd();
    glutSwapBuffers();
}


void timer(int) {
    glutPostRedisplay();
    glutTimerFunc(1000 , timer, 0);
    switch (state) {
    case 1:
        if (x_pos > -295) {
            x_pos -= 1;
        }
        else {
            state = -1;
        }
        break;
    case -1:
        x_pos = 180;
        state = 1;
        break;
    }
    
    cout << x_pos << endl;
}

int main(int argc, char** argv)
{

    glutInitWindowSize(500, 150);
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);
    glutCreateWindow("Train Display");
    glutDisplayFunc(display);
    glutReshapeFunc(my_reshape);
    glutTimerFunc(1000, timer, 0);
    glutMainLoop();

    return 0;
}

Answer 1

glTranslate does not just set a translation matrix, but multiply the current matrix by a translation matrix. You need to load the identity matrix with glLoadIdentity before glTranslatef or save and restore the current matrix with glPushMatrix/glPopMatrix:

void draw()
{
    const char* bitmap_font_names[7] = { "Hello train" };

    glPushMatrix();

    /* Draw the strings, according to the current mode and font. */
    glTranslatef(0.5, -100, 0);
    //set the text color
    glColor4f(0.0f, 255.0f, 140.0f, 1.0f);

    ystep = 100.0;
    yild = 20.0;
    glRasterPos2f(x_pos, y_pos + 1.25 * yild);
    print_bitmap_string(bitmap_font_names[0]);

    glPopMatrix();
}