LinuxFocus article number 288 http://linuxfocus.org

by Leonardo Giordani <leo.giordani(at)libero.it> About the author: I just received my diploma from the Faculty of Telecommunication Engineering in Politecnico of Milan. Interested in programming (mostly in Assembly and C/C++). Since 1999 works almost only with Linux/Unix. Translated to English by: Leonardo Giordani <leo.giordani(at)libero.it>

Implementing a scroller in SDL graphics Abstract: This series of articles has the purpose of introducing the reader to the world of multimedia productions, also known as "demos". The whole Internet is full of informations about the matter, but few people does write such beautiful thing for Linux: my goal is to describe the theory of some graphical and sound effects and their implementation using the SDL library. Further informations can be found at www.libsdl.org: reading the code of open source demos and games are the best way to learn new solutions. www.lnxscene.org: a new site, but can be a nice place to exchange knowledge; you can find me there (sometimes) as "muaddib" Prerequisites for the understanding of the article are: Basic knowledge of C language (sintax, loops, libraries) Basic knowledge of SDL library (basic functions, initializing) --> www.libsdl.org _________________ _________________ _________________

The scroller

A scroller is a part of a demo that write a moving sentence on the screen: it is one of the basic effect that you can find in a multimedia production and adds a bit of dynamicity to the text you want show to the user. In this article we will see how to build a basic scroller that moves the text from right to left.

The basic idea of a scroller is to copy a part of of the screen one or more pixel to the left (or to some direction). Executing this operation with a good speed you obtain the illusion of a continuous movement, and that's all.

The basic theory is not very complex; let's see how to translete all this in code terms: we will refer from now on to the concept of surface, well known to those who have some basic SDL programming skills. Working with the SDL library we have always to remember that we can use the powerful

For example, imagine we defined and initialized two surfaces and two rectangles

#define WIDTH 800
#define HEIGHT 600

SDL_Surface *Src;
SDL_Surface *Dst;

Src = SDL_CreateRGBSurface(SDL_HWSURFACE, WIDTH, HEIGHT, 8,
       r_mask, g_mask, b_mask, a_mask);
Dst = SDL_CreateRGBSurface(SDL_HWSURFACE, WIDTH, HEIGHT, 8,
       r_mask, g_mask, b_mask, a_mask);

SDL_Rect BigArea = {0, 0, (WIDTH / 2), HEIGHT};
SDL_Rect SmallArea = {(WIDTH / 2) + 1, 0, (WIDTH / 2), (HEIGHT / 2)};

SDL_BlitSurface(Src, 0, Dst, 0);

SDL_BlitSurface(Src, &BigArea, Dst, &SmallArea);

Now performing the scroll of a surface should no more be a mistery: it is sufficient copying a part of the surface in a shifted rectangle on the same surface; clearly all this code have to be but in a loop, so the resulting program becomes a bit more complex, but the basic concept is simple. At every tick of the loop we use two rectangles, the second shifted respects to the first in the direction of the scrolling, and we copy the surface on itself from the first to the second rectangle.

SDL_Surface *temp;

SDL_Rect From = {1, 0, WIDTH, HEIGHT};
SDL_Rect First = {0, 0, 1, HEIGHT};
SDL_Rect Last = {WIDTH-1, 0, 1, HEIGHT};

temp = SDL_CreateRGBSurface(SDL_HWSURFACE, 1, HEIGHT, 8,
       r_mask, g_mask, b_mask, a_mask);

while(1){
  SDL_BlitSurface(Src, &First, temp, 0);
  SDL_BlitSurface(Src, &From, Src, 0);
  SDL_BlitSurface(temp, &First, Src, &Last);
  SDL_BlitSurface(Src, 0, Screen, 0);
}

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"

#define SCREEN_WIDTH 480
#define SCREEN_HEIGHT 200

#if SDL_BYTEORDER == SDL_BIG_ENDIAN
static const Uint32 r_mask = 0xFF000000;
static const Uint32 g_mask = 0x00FF0000;
static const Uint32 b_mask = 0x0000FF00;
static const Uint32 a_mask = 0x000000FF;
#else
static const Uint32 r_mask = 0x000000FF;
static const Uint32 g_mask = 0x0000FF00;
static const Uint32 b_mask = 0x00FF0000;
static const Uint32 a_mask = 0xFF000000;
#endif

static SDL_Surface* img_surface;
static SDL_Surface* scroll_surface;
static SDL_Surface* temp_surface;

static const SDL_VideoInfo* info = 0;
SDL_Surface* screen;

int init_video()
{
  if( SDL_Init( SDL_INIT_VIDEO) < 0 )
    {
      fprintf( stderr, "Video initialization failed: %s\n",
               SDL_GetError( ) );
      return 0;
    }

  info = SDL_GetVideoInfo( );

  if( !info ) {
    fprintf( stderr, "Video query failed: %s\n",
             SDL_GetError( ) );
    return 0;
  }

  return 1;
}

int set_video( Uint16 width, Uint16 height, int bpp, int flags)
{
  if (init_video())
    {
      if((screen = SDL_SetVideoMode(width,height,bpp,flags))==0)
        {
          fprintf( stderr, "Video mode set failed: %s\n",
                   SDL_GetError( ) );
          return 0;
        }
    }
  return 1;
}

void quit( int code )
{
  SDL_FreeSurface(scroll_surface);
  SDL_FreeSurface(img_surface);

  SDL_Quit( );

  exit( code );
}

void handle_key_down( SDL_keysym* keysym )
{
    switch( keysym->sym )
      {
      case SDLK_ESCAPE:
        quit( 0 );
        break;
      default:
        break;
      }
}

void process_events( void )
{
    SDL_Event event;

    while( SDL_PollEvent( &event ) ) {

        switch( event.type ) {
        case SDL_KEYDOWN:
            handle_key_down( &event.key.keysym );
            break;
        case SDL_QUIT:
            quit( 0 );
            break;
        }
    }
}

void init()
{
  SDL_Surface* tmp;
  Uint16 i;

  tmp = SDL_CreateRGBSurface(SDL_HWSURFACE, SCREEN_WIDTH,
       SCREEN_HEIGHT, 8, r_mask, g_mask, b_mask, a_mask);

  scroll_surface = SDL_DisplayFormat(tmp);


  SDL_FreeSurface(tmp);

  img_surface = IMG_Load("img.pcx");

  for (i = 0; i < SDL_NUMEVENTS; ++i)
    {
      if (i != SDL_KEYDOWN && i != SDL_QUIT)
        {
          SDL_EventState(i, SDL_IGNORE);
        }
    }

  SDL_ShowCursor(SDL_DISABLE);
}

int main( int argc, char* argv[] )
{
  SDL_Rect ScrollFrom = {1, 0, SCREEN_WIDTH, SCREEN_HEIGHT};
  SDL_Rect First = {0, 0, 1, SCREEN_HEIGHT};
  SDL_Rect Last = {SCREEN_WIDTH - 1, 0, 1, SCREEN_HEIGHT};

  if (!set_video(SCREEN_WIDTH, SCREEN_HEIGHT, 8,
      SDL_HWSURFACE | SDL_HWACCEL | SDL_HWPALETTE /*| SDL_FULLSCREEN*/))
   quit(1);

  SDL_WM_SetCaption("Demo", "");

  init();

  SDL_BlitSurface(img_surface, 0, scroll_surface, 0);

  while( 1 )
    {
      process_events();

      SDL_BlitSurface(scroll_surface, &First, temp_surface, 0);

      SDL_BlitSurface(scroll_surface, &ScrollFrom, scroll_surface, 0);

      SDL_BlitSurface(temp_surface, &First, scroll_surface, &Last);

      SDL_BlitSurface(scroll_surface, 0, screen, 0);

      SDL_Flip(screen);
    }

  return 0;
}

As you can see the initialization work for the library is not short, but with the advantage that it is common to the whole demo, so while the code increases, initialization will become even more a small part of the full program, and it is a reusable and portable one.

A demo

ps: You can send me comments, corrections and questions at my mail address or through the Talkback page. Plese write me in english, german or italian.

2005-01-14, generated by lfparser_pdf version 2.51