How to draw text letter by letter in glut? - c++

Question

I can't get my GLUT program to work properly.I want to draw text (a char array) letter by letter with some miliseconds interval. My program:
#include <string.h>
#include <windows.h>
#include <glut.h>
#include<iostream>
int hei = glutGet(GLUT_SCREEN_HEIGHT)/2;
int wid = glutGet(GLUT_SCREEN_WIDTH)/2;
void *font = GLUT_BITMAP_TIMES_ROMAN_24;
void *fonts[] =
{
GLUT_BITMAP_9_BY_15,
GLUT_BITMAP_TIMES_ROMAN_10,
GLUT_BITMAP_TIMES_ROMAN_24
};
void selectFont(int newfont)
{
font = fonts[newfont];
glutPostRedisplay();
}
void selectColor(float r, float g, float b)
{
glColor3f(r, g, b);
glutPostRedisplay();
}
char *msg;
void tick(void)
{
glutPostRedisplay();
}
void timer(int value) {
}
int element=0;
void output(int x, int y, char *string, bool center=true) {
int len;
len = strlen(string);
if(center){
glRasterPos2f((wid-len), y);
}
else {
glRasterPos2f(x, y);
}
for(int i=0;i<len;i++) {
glutBitmapCharacter(font, string[i]);
glPopMatrix();
glutSwapBuffers();
Sleep(500);
}
}
void outputmsg(int x, int y, bool center=true) {
int len;
len = strlen(msg);
if(center){
glRasterPos2f((wid-len), y);
}
else {
glRasterPos2f(x, y);
}
for(int i=0;i<len;i++) {
glutBitmapCharacter(font, msg[i]);
}
}
void updatemsg() {
msg="test msg letter by letter";
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
msg="|";
updatemsg();
if(msg!="|")
output(wid, hei, msg, true);
else
output(wid,hei,msg,true);
}
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, w, h, 0);
glMatrixMode(GL_MODELVIEW);
}
void keyboard(unsigned char key, int x, int y) {
switch(key){
case 27:
exit(1);
break;
}
}
int main(int argc, char **argv)
{
int i, msg_submenu, color_submenu;
glutInit(&argc, argv);
for (i = 1; i < argc; i++) {
if (!strcmp(argv[i], "-mono")) {
font = GLUT_BITMAP_9_BY_15;
}
}
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(wid*2, hei*2);
glutCreateWindow("CHAT");
glutFullScreen();
glClearColor(0.0, 0.0, 0.0, 1.0);
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutKeyboardFunc(keyboard);
glutIdleFunc(tick);
selectFont(0);
glutMainLoop();
return 0; /* ANSI C requires main to return int. */
}
It does really weird stuff like erasing the previus one char then printing then printing the next one and the erased one. I tried everything what i could find about this topic on the forums. How can i fix it?

Answer 1

What you should do is also hold the length of the string you want to display and increment it every few milliseconds:
int len;
char *msg;
void incrementLength(int value){
if(msg[len]==0)return; //end of string stop the callback
len++;
glutTimerFunc(100, incrementLength, 0);
glutPostRedisplay();
}
void updatemsg() {
msg="test msg letter by letter";
len=0;
glutTimerFunc(100, incrementLength, 0);
}
And then in outputmsg (if you actually call it) you only loop until len:
void outputmsg(int x, int y, bool center=true) {
if(center){
glRasterPos2f((wid-len), y);
}
else {
glRasterPos2f(x, y);
}
for(int i=0;i<len;i++) {
glutBitmapCharacter(font, msg[i]);
}
}

Answer 2

This code has a couple of issues:
for(int i=0;i<len;i++) {
glutBitmapCharacter(font, string[i]);
glPopMatrix();
glutSwapBuffers();
Sleep(500);
}
You are popping a matrix that you never pushed anywhere, and you are doing it len-many times.
You are swapping buffers without clearing the screen in-between... that's going to lead to undefined results (generally a garbled mess of characters from previous frames).
You really need to re-think how you've implemented this.
I would suggest you implement some elapsed time check in your display (...) function that determines which character to print, rather than doing this in a loop that calls Sleep (...).