2D tile based game, shows gaps between the tile sprites when I zoom in with the camera?

Question

I am using the D3DXSPRITE method to draw my map tiles to the screen, i just added a zoom function which zooms in when you hold the up arrow, but noticed you can now see gaps between the tiles, here's some screen shots

normal size (32x32) per tile

zoomed in (you can see white gaps between the tiles)

zoomed out (even worst!)

Here's the code snipplet which I translate and scale the world with.

D3DXMATRIX matScale, matPos;

D3DXMatrixScaling(&matScale, zoom_, zoom_, 0.0f);
D3DXMatrixTranslation(&matPos, xpos_, ypos_, 0.0f);

device_->SetTransform(D3DTS_WORLD, &(matPos * matScale));

And this is my drawing of the map, (tiles are in a vector of a vector of tiles.. and I haven't done culling yet)

LayerInfo *p_linfo = NULL;
  RECT rect = {0};
  D3DXVECTOR3 pos;
  pos.x = 0.0f;
  pos.y = 0.0f;
  pos.z = 0.0f;

  for (short y = 0; 
    y < BottomTile(); ++y)
  {
    for (short x = 0; 
          x < RightTile(); ++x)
    {
      for (int i = 0; i < TILE_LAYER_COUNT; ++i)
      {
        p_linfo = tile_grid_[y][x].Layer(i);

        if (p_linfo->Visible())
        {
          p_linfo->GetTextureRect(&rect);

          sprite_batch->Draw(
            p_engine_->GetTexture(p_linfo->texture_id), 
            &rect, NULL, &pos, 0xFFFFFFFF);
        }
      }
      pos.x += p_engine_->TileWidth();
    }
    pos.x = 0;
    pos.y += p_engine_->TileHeight();
  }

Answer 1

Welcome to the world of floating-point. Those gaps exist due to imperfections using floating-point numbers.

You might be able to improve the situation by being really careful when doing your floating-point math but those seams will be there unless you make one whole mesh out of your terrain.

It's the rasterizer that given the view and projection matrix as well as the vertex positions is slightly off. You maybe able to improve on that but I don't know how successful you'll be.

Instead of drawing different quads you can index only the visible vertexes that make up your terrain and instead use texture tiling techniques to paint different stuff on there. I believe that won't get you the ugly seam because in that case, there technically isn't one.