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Saturday, May 10, 2008

Type and Instance Parameters

A parametric element is something that can change size, material, and graphic look but is still the same fundamental element. Most elements in Revit are designed with parameters that allow for the creation of variations of a base type. Take a typical Revit door family as an example. Each family can have many types built into it. Each type typically represents a variation in size, material, color, or other defining characteristic. Although each type can vary in shape and size, the base geometry for each type is derived from the same family.

The Difference between Blocks and Families
In a typical CAD environment, you might create each door as a block; each of those blocks would be a separate element unrelated to any of the others. So, 20 door sizes would mean 20 floor-plan blocks,20 section blocks, 20 elevation blocks, and 20 3D blocks if you were going to use them as liberally as we use them in Revit. All of those in Revit are represented with one family that can display itself in 2D and 3D and whose size, material, and visibility can be changed at any time.

Depending on how the family is built, parameters can affect either the type
or the instance.Type parameters affect all families used in the model, whereas instance parameters affect only the family you’ve selected. This is an important distinction: You can change instance properties only when you have an element selected, but you can change type properties without selecting anything.Consider a round table. You might define its shape using a type parameter for the radius.

If you placed 20 types of tables with a 2´ radius and then changed that radius to 3´, all 20 tables would update automatically. Now, if the radius parameter was an instance parameter, changing the radius would affect only the type of table you currently had selected. The same logic can be applied to other dimensional constraints and materiality. Revit forces you to consider what an element is and what it means to change the element’s defining characteristics. For example, most content in Revit doesn’t let you arbitrarily change dimensions of every instance, on the fly, whenever you want—this would make tracking the notion of object type difficult and would make mass-updating more tedious. Think of a type as something you’ll eventually have to schedule, spec, and install as a real-world commodity.Bidirectional Relationships
Objects with parameters that can be edited are nothing new in the world of software. But what makes Revit unique is its ability to go beyond mere parameters and create relationships between objects.This ability has been referred to as the parametric change engine, and it’s a core technological advantage built into Revit.

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