UpFRONT

Last episode, we initialized console graphics (at great risk to our system's health) but we didn't do much afterwards. Now it's time for something truly impressive, a feat which Microsoft has apparently not yet accomplished—a smooth scrolltext.

“What is a scrolltext?” you may ask, in particular if you haven't been around on a Commodore 64, Amiga or PC and names like Fairlight, Red Sector and Future Crew mean very little to you. Hopefully, you've at least seen the intro screen from Jet Set Willy. What it comes down to is this: scrolltexts are the most exciting form of communication ever. Words that glide across the screen, often advertising the latest release or copy party, complaining about high school teachers, or detailing unfortunate experiences at the hands of public transit, usually accompanied by music (MODs) blaring in the background with animated graphics and the ubiquitous star fields. Perhaps you've seen Microsoft's screen saver which scrolls words across the screen and noticed it flickers terribly. It is very simple to fix, so our scrolltext won't flicker; then we can taunt the Microsofties to fix their screen saver.

The routine is quite simple. We start by initializing three graphics screens: a physical screen, a virtual screen and a scroll board screen. The physical screen is the graphics context which will be displayed, the virtual screen is the graphics context we use as a whole-screen copy to the physical screen. That is, we make changes to the virtual screen, and when everything's ready, copy it to the physical screen. The third screen is the scroll board, a virtual graphics context which will be wider than the physical screen by one character (8 pixels) and will be only as tall as the font itself (again, 8 pixels). Since we're using an 8x8 font and a 320x200 graphics screen, we can fit 40 (320/8) characters on a line, making 41 characters for our scroll board. Once we've set up these three contexts, we'll use a simple loop to get the letters scrolling. After that, we can add anything we like: 3-D graphics, dancing animals, star fields or anything else. Here's the loop:

This routine is fairly simple and requires only a couple of variables: one to keep track of how many pixels in we are and one to keep track of how many letters in we are. Also, we want to make sure we don't run out of scrolltext and start scrolling bits of random memory, which would ultimately lead to a segmentation fault. While we might prefer to draw each letter individually and just keep modulating around the length of the scrolltext, we get faster drawing if we print characters as a string once instead of calling gl_writen 41 times every time we move 8 pixels. So, leave some blank space at the beginning and end of your text to ensure smooth wrapping. We could also create the whole scrolltext as one really long graphic, but that would be cheating.

Once the basic scrolltext is going, we can do all sorts of fun things. We could, for example, have a sinusoid equation for the y value of where the text is placed such that it would bounce up and down on the screen, or we could insert some graphics and possibly call mikmod or playmidi to get some music going. Compile with

gcc -Wall -O2 scrolltext.c -lvgagl\
   -lvga -o scrolltext

For information on the specifics of svgalib or vgagl, try their respective man pages. Library functions also have their own man pages. Here's the code:

#include <stdio.h>
#include <stdlib.h>
#include <vga.h>
#include <vgagl.h>
#define VGAMODE G320x200x256
#define FONTW 8 // font width
#define FONTH 8 // font height
#define TEXTL 600 // text length
int main(void)
{
  char d;
  char text[TEXTL]="                                         Megagreetings from whomever this happens to be! This is where the scrolltext words are, so fill them as you like. Fun rarely entices everyone. Keep enjoying video interfaces nevertheless................. There is space to be filled!! ";
  short int text_pos;
  unsigned char pixel_pos;
  unsigned char speed;
  GraphicsContext *physical_screen;
  GraphicsContext *virtual_screen;
  GraphicsContext *scroll_board;
  vga_init();
  vga_setmode(VGAMODE);
  gl_setcontextvga(VGAMODE);
  physical_screen = gl_allocatecontext();
  gl_getcontext(physical_screen);
  gl_setcontextvgavirtual(VGAMODE);
  virtual_screen = gl_allocatecontext();
  gl_getcontext(virtual_screen);
  gl_clearscreen(0);
  scroll_board = malloc( (WIDTH/FONTW+1) * FONTW *
     FONTH * BYTESPERPIXEL);
  gl_setcontextvirtual(WIDTH+FONTW, FONTH,
     BYTESPERPIXEL, 8, scroll_board);
  scroll_board = gl_allocatecontext();
  gl_getcontext(scroll_board);
  gl_clearscreen(0);
  gl_setwritemode(FONT_COMPRESSED);
  gl_setfont(8, 8, gl_font8x8);
  gl_setpalettecolor(1,63,63,63);
  text_pos = 0; // text offset
  pixel_pos = 0; // pixel offset
  speed = 1; // scroller speed
  gl_setcontext(virtual_screen);
  for (d=0; d==0; d=vga_getkey()) {
    pixel_pos+=speed;
    while (pixel_pos > FONTW) {
      gl_setcontext(scroll_board);
      gl_writen(0, 0, WIDTH/FONTW,
        &text[text_pos]);
      text_pos++;
      pixel_pos-=FONTW;
      if (text_pos > TEXTL - WIDTH/FONTW)
      text_pos -= (TEXTL-WIDTH/FONTW);
      gl_setcontext(virtual_screen);
    }
    gl_copyboxfromcontext(scroll_board, pixel_pos,
       0, WIDTH, FONTH, 0, HEIGHT-FONTH-1);
    gl_copyscreen(physical_screen);
    vga_waitretrace();
  }
  return 0;
}

—Jason Kroll