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© 2009 NURVE NETWORKS LLC “Inside the XGS AVR 8-Bit”
Figure 16.4 – The Atari 2600 classic “Pitfall” along with some of the tiles templated.
Referring to Figure 16.4, you can see how this system works. As an artist you might start by creating some sketches
of the kinds of screen and levels you want then you might start drawing some mock ups of the imagery. Then at some
point, you begin “ripping” the imagery into a set of common rectangular tiles usually 8x8 or 16x16 pixels in size and
then template them into a “tile set” (Figure 16.4(b)) of bitmaps. Then from the set of tiled bitmaps you generate all
your actual game screens. The memory savings comes from the indirection in this system. Instead of using the actual
bitmap data, you use an index or pointer to the bitmap in the tile set, thus a tile map might consist of 40x24 tiles, each
tile is represented by a single byte index (thus a total of 256 tiles can be represented) and each tile is 8x8 pixels, thus
the final frame buffer image or bitmap image would be:
Width
Height
40*8 = 320
24*8 = 192
But, the amount of memory to represent the tile map would only be 40*24 = 960 bytes! Of course, each tile bitmap
still consumes precious memory, but you don’t need that many of them. So depending on the game and how much
data is redundant on each screen, you can get away with few bitmap tiles. This all sounds too good to be true? So,
what’s the catch? Well, the catch is that since the graphics modes are tiled. You can’t place an image just anywhere
you want, it has to be on the tile matrix aligned perfectly. Therefore, its very hard to move a character smoothly on the
screen. Moreover, its very hard to move anything on TOP of the tile mapped image without replacing the tile at any
particular location. Of course, there are tricks to do this, but they require “re-writing” the bitmap tiles themselves and
using “tile animation techniques”. A more common solution is the use of “sprites”. A sprite is a bitmap image that is
rendered on top of the tile mapped image; moreover, sprites move smoothly in most cases, support collision with
each other and can be larger or smaller than the tile map bitmaps themselves. A good example of sprite use are say
the ghosts in PacMan, the blue maze of PacMan is a tile mapped image, but the ghosts and PacMan himself are
sprites, therefore, they can move freely around the image since they are rendered on top of the image without
disturbing the tile map. Figure 16.5 shows a screen shot of PacMan for reference.
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