That's the Active Picture object and its "Create Backdrop At" event - the entire game is written around a custom-made editor and scripting language, and it's a bit difficult to explain the details of how to use them here.
But there's an idea for another tutorial...
Thanks for the info DavidN. A tutorial on the subject would be very interesting and useful.
If you want the convenience of using the level editor combined with the flexibility of using external tilesets, my suggestion is this:
Make your background entirely out of active objects. When the level first loads, load the new frames using Animation > Load Frame, then use Animation > Add Backdrop > Obstacle/Platform (Whichever is appropriate) to convert them into level backdrops.
In order to do this with multiple files at once, use the expression editor when it asks for the frame filename and give it something like "filename" + Str$(>filenum<) + ".bmp". You cannot keep your tilesets in a single file; you will need to split them into multiple files of one frame each. (It's just the nature of the beast; the runtime loader can't load boxed or tiled images.)
So, say for example that you want a grass tileset, a dirt tileset, and a winter tileset. You would make a set of active objects that looked like the grass tileset (or whatever). You would then make a set of files that corresponded with each tile and number them according to which tileset they belonged to; "FlatTile1.bmp" might be a flat grass tile, while "FlatTile2.bmp" would be a flat dirt tile and "FlatTile3.bmp" is a flat winter tile. (Likewise, an angular tile might be "AngularTile1.bmp", "AngularTile2.bmp" etc.)
Then you set a global value or somesuch to the number of the tileset you want to load; i.e. if you want the level to use a winter tileset, you set the value to 3.
You build your level using your active objects, using a different active object for each tile type (such as flat, angular left, angular light etc.)
Then you tell the active objects to load new files according to their tile type. For example, for FlatTile, you would load a new image from "FlatTile" + Str$(Tile Value) + ".bmp". Then make a backdrop out of FlatTile and delete the FlatTile object.
Voila, you've converted the level from one tileset to another using external files.
That's how I initially had my editor set up, Maggott, with different folders containing different tilesets so that I could switch between them by just changing a String.
And it works fine, but the trouble with it is just how many actions you have to use - one for each type of "active backdrop" object - when you're loading the level. And, as you mentioned, all images need to be in separate files, making them more difficult to edit at a glance.
Very interesting read about loading external graphics, one picture per tile. But I was more interested in knowing how to load several tiles from one picture. That's why I suggested to take a look at The Underside.
To achieve that, as someone mentioned in another thread, I believe that the Text Blitter object is essential.
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