When creating objects there are many options to consider, this article reviews some of the considerations in the creation of parametric objects. Other information will follow that will discuss additional considerations and modeling techniques.

The concept behind highly parametric objects is simple; if one geometric shape can be morphed into thousands of configurations by setting parameters then one parametric object is all you need. This is very efficient for the architect that needs an object. It is much faster to insert one object into the project and then modify that object as the design matures and changes than constantly needing to find, insert and substitute new objects as the design process proceeds. If hundreds of non parametric (static) objects are needed to cover all the configuration options not only does storage for the objects increase but finding the specific desired object becomes time consuming and difficult.

Traditionally the objects available to architects have not been highly parametric which results in the architect either accepting an inexact object (a double hung window with no divider bars when they want divider bars) or they need to devote part of their fee to building the objects desired. Even when architects build their own objects they rarely build highly parametric objects because it can take 5 to 10 times longer to build than a static object. In this case every time the object needs to change the architect must rebuild the new version, this is inefficient, stifles design creativity and uses up the architects fee.

It is not logical for architects to be building objects, they are not paid to create objects, they tend to build and rebuild static objects. This paper will review the approach of the parametric object creation.

Clearly not all objects need to be parametric; a toilet that comes in one style does not need to be parametric. However, most building products are highly configurable; a 2x4 drop in fluorescent fixture comes with different numbers of tubes and diffusers. As a generic fixture it may not be necessary to build parametrics into this object. However when the same fixture is created for a manufacturer it may be very important to build in parametrics so the schedules can accurately document model numbers, tube quantities/types, and the specific diffuser. A double hung window can be configured in thousands of different ways so there is little question that the desired object should be parametric, the question in this case is how parametric.

Once the decision is made to make the object parametric the question is how parametric. Parametric objects come with a price, not just the price for the time it takes to create the object, but a performance price. A static double hung window that you can only change the height and width of will be a small file size, it will load into the project in a few seconds and can then be inserted into a wall in a few more seconds. On the other hand if we create a highly parametric window in may take over 30 seconds to load into the project and 20 seconds to insert into a wall. If you then want to change this highly parametric object it may take another 30 seconds to morph into the new design. An immediate reaction may be why create parametric objects when you pay this time penalty. When analyzed, however, the parametric object takes less real time, so it is the best solution. If you had to search through all the static versions of a window you would be looking through thousands of static objects and if every time you made a design change you had to repeat the "hunt" you will spend much more time than the 20 and 30 seconds associated with the parametric object. Next year when computers speeds double, and then again the following year the performance price you pay for parametrics will continue to decline.

As the complexity of the parametric object increases so does the file size. A static casement window will have a file size of .5MB and a highly parametric casement window will be around 2MB. I recently created a custom house design, with no windows the file size was 31.5 MB, when static windows were inserted the file grew to 34.6 MB, when parametric windows were inserted the project grew to 47.9 MB. At first you may think this is a heavy price to pay, but I am able to change everything I want on these windows (sill design, height, width, recess into the wall, finishes interior and exterior, divider bars layout and design, and more). I am able to design the look that I want without looking for the exact window on the internet (it most likely does not exist) or needing to spend hours rebuilding windows to get what I am looking for. This increased file size is a small price to pay for the ability to quickly conduct design studies and improve the design.

Once you determine that the object should be a parametric object you have two fundamental options, configure the parameters outside of Revit or configure the parameters inside of Revit. The outside approach requires the creation of a configuration software program, identifying the options and creating the rules of allowable configurations. External configurators tend to be a more expensive approach. Although the configurator will create only an object that follows the allowable options once the user places the object within the project they can manually morph the object and break "all' the rules. External configurators are slower to use when you decide to reconfigure the object as you need to repeat the external configuration process and reinsert the object back into the project.

With internal configurators the parametrics are available all the time during the modeling session which is an advantage as changes are quicker. The interface to the parametric options are limited to the Revit interface so the options are not able to be presented as nicely as the external solution. (Although if every creator of external configurators uses a different interface you will need to learn multiple interfaces which is far from ideal). At this time Reed feels that the internal configurator has the advantage of less development expense for the manufacturer and faster speed when making changes to the object.

Looking at a current object creation project for window and doors: one object has the following configuration options.

1. Sizes — The width and heights can be changed.
2. The window and doors can be placed in any thickness wall. A sill extension can be included to automatically resize for variable wall thicknesses
3. Casings (trim) on the interior and exterior will be included, multiple profiles are included
4. The exterior sill will be modifiable for different configurations and profiles
5. Interior and Exterior Different Finishes - Frame, sash, divider bars will be modeled as interior components and exterior components (split into two pieces) so individual finishes can be assigned to the interior and exterior of the windows and doors
6. Window Offset — The entire window (door) can be moved toward the interior or exterior of the building
7. Divider Bar Resizing — The user can configure the number of horizontal and vertical divider bars. The bars will automatically reconfigure when the widow size is changed
8. Divider Bar Layout Options - The windows are modeled with a traditional grid pattern (horizontal and vertical with equal spacing) and asymmetrical designs such as prairie style.
9. Divider Bars Profiles and Placement — The model will include divider bars on the exterior of the glass, interior of the glass and between the glasses. The user can select the bar profile. The user can turn on or off any combination of these placements.
10. Linked CAD Details — The appropriate CAD detail will be linked to the object, when the user zooms into the window or door they will see a realistic detailed view of the product.
11. Combinations — Side by sides and transoms are included.

Objects with this degree of configuration are very powerful and give the architect almost unlimited design options.

Objects are the foundation for the power of Revit. Properly modeled parametric objects allow the designer to review their designs electronically and make design changes quickly.

Dennis Neeley, AIA was a principal of Neeley/Lofrano Architects, San Francisco for over 20 years. He was a developer of commercial and residential projects and a principal in a construction company for over 10 years. In 1985 he developed software for architects and engineers and licensed it to Autodesk, this product, AutoCAD AEC is credited with being a major factor in the rapid adoption of CAD in architecture. He has also started and/or managed several successful software companies (ArchSOFT, ASG, Softdesk, CADSpec, VisCOMM, Bricsnet, Tectonic) focused on CAD applications, manufacturers electronic information, and internet based facilities management. He has taught at UC Berkeley, Carnegie Mellon and the University of Washington. In February, 2008, Reed Construction Data purchased Tectonic, he now assists in the development and implementation of the Reed BIM applications and services.