I have that in a Wolfenstein-style game that I'm development. There are occasional gaps at certain distances that I can't get rid of. They appear on my ATI but not my Nvidia for some reason. In my case I'm using OpenGL directly (though Allegro sets it up), so I think we can rule out something with Allegro's drawing API...

Are you using a tile sheet? Sometimes when filtering is enabled the textures bleed together on the edges. You could try turning any filtering off if you haven't already and see if it gets rid of the problem. If it does and you still want filtering, you may have to put a border around your images (but there might be a better way I don't know about too).

How are you computing where to draw the tiles? It'd be nice to see that code.

I can't think of the code for this, but this looks like it might have something to do with texture clamping to me. There is a setting where you can turn it on/off which solves this on most systems. There is also opengl code you can add that will also enable/disable this (I can't recall which it is, turning it off or on). I'll see if I can find some code I once used for this in one of my old projects, I know I entered something for it somewhere... been a while.

EDIT:
Here's some code I found

#define GL_CLAMP_TO_EDGE  0x812F

   glBindTexture (GL_TEXTURE_2D, texture[12]);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
   glBegin(GL_TRIANGLE_STRIP);

I added in the line before and after to show where I used it. After binding a texture I used the glTexParameteri() with that define to enable it.

Look at your graphic card settings though, there is a setting for setting the clamp.

Oh and while I am on the subject of "clamps", and I can't help it, I just have to post this

I just got done programming my own Level Editor and I adjusted the code so it scales to a fullscreen window mode with some code Mathew provided and I am noticing the lines between my tiles now as well.

{"name":"606626","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/d\/0\/d0be1797966324a83a7c00f04f90679e.png","w":1920,"h":1080,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/d\/0\/d0be1797966324a83a7c00f04f90679e"}

The line tiles are one solid strip, no gaps, transparent...

{"name":"606627","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/9\/a903c07c68a7652bff748269fe96baab.png","w":512,"h":32,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/9\/a903c07c68a7652bff748269fe96baab"}

When you view the full screenshot (1920x1080) you see this... (a section magnified here)

{"name":"606628","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/1\/2\/1203dfbf51913b2e156edd60a0f325b8.png","w":386,"h":340,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/1\/2\/1203dfbf51913b2e156edd60a0f325b8"}

It does look as though portions of the texture are bleeding over. The code I used to draw them with has no errors in it:

(this is the condensed version of my code)

for(int y = 0; y < MAPY; y++)
{
   for(int x = 0; x < MAPX; x++)
   {
      xpos = 12 + x * 32;
      ypos = 48 + y * 32;
      tile = level.map[y][x].tile - 1;
      al_draw_bitmap_region(lines, tile * 32, 0, 32, 32, xpos, ypos, 0);
    }
}

Edit: Oh yes, I also use the following settings...

   al_set_new_bitmap_flags(ALLEGRO_MAG_LINEAR | ALLEGRO_VIDEO_BITMAP);
   al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA);
   al_set_new_display_option(ALLEGRO_SAMPLE_BUFFERS, 1, ALLEGRO_SUGGEST);
   al_set_new_display_option(ALLEGRO_SAMPLES, 4, ALLEGRO_SUGGEST);

Edit2: Okay, I removed the above lines and it vanished so, I'll have to play around with these settings. Be a pity if I can't use them though.

Edit3: After extensive testing, it is happening specifically with either of the two options below enabled.

al_set_new_bitmap_flags(ALLEGRO_MAG_LINEAR | ALLEGRO_VIDEO_BITMAP);
al_set_new_display_option(ALLEGRO_SAMPLE_BUFFERS, 1, ALLEGRO_SUGGEST);

Yeah, in case the pixels come from the sprite-sheet, that's kovarex' issue as well. But he said he's not using filtering, in which case it has to be something else.

I'll have to try that. I used TileStudio to create them, so it will be a simple matter of loading and resaving with a border as it has that option.

kovarex, post some of your graphics you use (the actual graphic file you load from) as well as some example code (as I did) you use to display it along with any display settings you use. It would help a good deal more.

You can show images in a message here by dropping them (drag and drop) on the box above, then once uploaded, right click them and select "Save Link Location", then in your message paste the link location surrounded by [] so it would look something like:

[ http://www.allegro.cc/theme/default/icon/67890 ]

(no spaces, that isn't a valid link by the way)

That will cause the images to appear in a message and perhaps we can better help you.

Edit: I may just divide up my sprite sheet at load time, separate the tiles into an array of individual bitmaps.

Edit2: I ended up loading my sprite sheet into a temporary bitmap pointer, then copying the individual tiles over into an array of bitmaps so they can be used without the filtering artifacts. Only tiles that had non-transparent pixels sharing the border with a neighbouring tile on the same sheet needed to be copied. Here's some of the code:

   ALLEGRO_BITMAP *tbmp = NULL;

   tbmp = al_load_bitmap("Graphics/Backgrounds.png");
   if(!tbmp) {
      printf("%s(%d): Error loading backgrounds.\n", __FILE__, __LINE__);
      goto SHUTDOWN;
   }
   for(int i = 0; i < 20; i++) {
      background[i] = al_create_bitmap(TILE_SIZE, TILE_SIZE);
      al_set_target_bitmap(background[i]);
      al_draw_bitmap_region(tbmp, i * 32, 0, 32, 32, 0, 0, 0);
   }
   al_destroy_bitmap(tbmp);
   tbmp = NULL;

Works like a charm. No more artifacts.