Symbols (aka, "Super-primitives" or "Blocks")

What's it all about?

If you are designing a school you're likely to need a lot of desks. Wouldn't it be nice if the program came with a "desk" primitive? Or several -- left handed, right handed, student, or teacher? That's what "super-primitives" are about. They're more often called "blocks" because that's what Autocad calls them or "symbols" but the idea is the same: you define them, and they can be placed like a primitive and then scaled, rotated, moved and all the rest.

One of the main reasons blocks are useful is that one definition (which might be big) can be used to draw many many instances (which are small), which makes the file smaller, quicker to load, etc.

Creating them

It is, of course, impossible to guess all the needed symbols for a project, so software vendors provide a means of making your own. You create one of them, give it a name and define an insertion point (and possibly other attributes, like cost or color). Then you can place as many of them as you want.

Changing them

If you change the super-primitive "master copy" after you've placed a few of them, the neat thing is that all the existing placements (called "instances") are changed too. That means you can manage a certain amount of ambiguity (we know it's a 30" x 60" desk, but not much else....) in the project with a minmum of fuss (just update the block once the decision has been made!).

Of (possibly conflicting) attributes

The geometry you create when making the prototype symbol will have normal attributes like color, layer, linetype, etc. These are included in the symbol/block definition. In programs modeled on the behavior of Autocad (like Rhino) each instance of the block you then place is like any other primitive—it has color, layer, and linetype too. Which raises this question: Which one controls the appearance of the instance?

BIM: Way more attributes

Of course, the desk might actually be an office cubicle, and there might be an occupant, phone number, and computer ip-address associated with it. Those will be different for each desk. The very best super-primitives begin to represent database systems, with both variable and inflexible parts. BIM software is the main example of this these days.

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