[SDL] Better performance needed - am I doing this right?

[Colin Tuckley]

I need to improve the drawing performance in my BASIC emulator which uses
SDL for it's output (on a Linux system).

Because I don't know what the foreground and background colours will be
until I actually come to draw the character I'm not keeping a set of memory
mapped character images and just blitting them. Instead, (partly for
historical reasons) I have a set of 8 byte pixel masks for each of the 8x8
characters.

My (extraneous logic stripped out) code looks like this:

  if (SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER) < 0) {
    fprintf(stderr, "Unable to init SDL: %s\n", SDL_GetError());
    return FALSE;
  }

  screen0 = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, 32, 0);
  if (!screen0) {
    fprintf(stderr, "Failed to open screen: %s\n", SDL_GetError());
    return FALSE;
  }

  fontbuf = SDL_CreateRGBSurface(SDL_SWSURFACE,   8,   8, 32,
             0xff000000, 0x00ff0000, 0x0000ff00, 0x000000ff);
  sdl_fontbuf = SDL_ConvertSurface(fontbuf, screen0->format, 0);
  /* copy surface to get same format as main windows */
  SDL_FreeSurface(fontbuf);

The idea being that I will draw the character in fontbuf and then blit it
into screen0. The char drawing code looks like:

void sdlchar(char ch) {
  int32 y, line;
  place_rect.x = xtext * 8;
  place_rect.y = ytext * 8;
  SDL_FillRect(sdl_fontbuf, NULL, tb_colour);
  for (y = 0; y < 8; y++) {
    line = sysfont[ch-' '][y];
    if (line != 0) {
      if (line & 0x80) *((Uint32*)sdl_fontbuf->pixels + 0 +y*8) = tf_colour;
      if (line & 0x40) *((Uint32*)sdl_fontbuf->pixels + 1 +y*8) = tf_colour;
      if (line & 0x20) *((Uint32*)sdl_fontbuf->pixels + 2 +y*8) = tf_colour;
      if (line & 0x10) *((Uint32*)sdl_fontbuf->pixels + 3 +y*8) = tf_colour;
      if (line & 0x08) *((Uint32*)sdl_fontbuf->pixels + 4 +y*8) = tf_colour;
      if (line & 0x04) *((Uint32*)sdl_fontbuf->pixels + 5 +y*8) = tf_colour;
      if (line & 0x02) *((Uint32*)sdl_fontbuf->pixels + 6 +y*8) = tf_colour;
      if (line & 0x01) *((Uint32*)sdl_fontbuf->pixels + 7 +y*8) = tf_colour;
    }
  }
  SDL_BlitSurface(sdl_fontbuf, &font_rect, screen0, &place_rect);
  if (echo) SDL_Flip(screen0);
}

The "echo" flag is there so that I only do the buffer flip when I need to
actually see the output (so it's clear when I'm outputting multiple chars
until the last one).

Comments and suggestions please.

Colin

-- 
Colin Tuckley      |  colin at tuckley.org  |  PGP/GnuPG Key Id
+44(0)1903 236872  |  +44(0)7799 143369  |     0x1B3045CE

Banging your head against the wall uses 120 calories an hour.