IN DEPTH — Developing Digital Visual Literacy of Complex Digital Objects: Deconstructing CAD
- 5 days ago
- 11 min read
Updated: 4 days ago
This is the second of a three-part series by Aliza Leventhal and Julia Larson on enhancing archivists’ visual and technical literacy in handling complex digital objects.
Foundational Components: Objects, Attributes, Blocks, and Libraries.
The first article in this series introduced common features used in most digital design software that present challenges for archivists when files are transferred to an archival institution. This article focuses on the four foundational components of most computer-aided-design software: Objects, Attributes, Blocks, and Libraries. Objects are specific graphical components. Attributes, Blocks, and Libraries are tools used to organize, facilitate reuse, and manage associated data efficiently. These components may have varying names across CAD software (e.g., Object in AutoDesk is to Element in MicroStation), and there are nuances of how each component can be used. Here are preliminary definitions of these components that provide a foundational understanding of their basic qualities:
Objects (Elements in MicroStation) are the base graphical components of a drawing (e.g., lines, rectangles, arcs, circles, etc.).
Attributes (Item Types in MicroStation) are labels or tags that attach textual or numeric information to a block (e.g., part numbers, prices, manufacturer names, etc.) that can then be automatically extracted into external files like spreadsheets or databases.
Blocks (Cells in MicroStation) are named groups of 2D or 3D objects that are usually created for repeated use such as drawing symbols, common drawing components (e.g., doors and stairs), and standard details, to maintain consistency, save time, and reduce file size.
Libraries (for both AutoDesk and MicroStation products) are groupings of resources that are added into CAD drawings such as samples, set drawing components, blocks, details, and lots more!
These are often used in combination to build more complex components within and external to a CAD file. Most CAD software provides a core set of Objects, Blocks, and Libraries that are typically expanded by designers or firms who make or acquire additional packages of these components to support the specific typologies of their projects.
Archival professionals should be aware of how these components work both independently and together, in the same way they understand how formats and preset dropdown menus impact displayed data in a Microsoft Excel file.
Objects
Objects (also known as Elements) are the fundamental building blocks of a CAD drawing, categorized as graphical (visible entities) and non-graphical (invisible data). Visible graphical entities consist of lines, circles, arcs, polylines, splines (mathematically defined smooth curves), rectangles, polygons, points, and text. They are the earliest established Objects, which have expanded with the evolution of 3D capabilities and features in CAD software to include shapes like boxes, cones, cylinders, spheres, and surfaces. Non-graphic Objects are descriptive data about graphical Objects in a drawing, such as Linetypes (the visual pattern or style of lines—dashes, dots, symbols, or text used to represent different elements), DimStyles (Dimensional Styles), SelectionSets (groups of selected Objects that allow for simultaneous manipulation or modification), among others. Non-graphical Objects do not have a direct visual representation in the drawing, yet they contain crucial informational data stored within a drawing’s database that facilitates organizing, defining, and managing graphical objects.
Objects are managed via command-line or a combination of keyboard and mouse actions to graphically select groups of Objects. All graphical Objects are managed via the Properties palette (see Image A), allowing modification of geometry and appearance, such as layer, color, linetype, lineweight, and transparency. Additional specific properties depend on the Object type, such as Center, Radius, and Area. Within each CAD software, there are specific rules and restrictions for various Object types that apply to both graphical and non-graphical objects as well. There is a selection of Objects built into any CAD software that are often supplemented by Designers seeking additional specific drawing components from external vendors (both available for free and through purchase); though they are often in more evolved forms of preset combinations of Objects known as Blocks.
As the foundational components that form more complex structures in CAD, Objects can be overlooked as simple entities; however, there are significant long-term issues with CAD objects that archivists should be aware of, including significant file corruption, severe performance degradation (slowness, freezing), and increased file sizes due to accumulated junk data like unused layers or proxy objects. These issues cause slow rendering, a lag in responsiveness, and potential data loss. Archivists should inquire with creators about their cleanup practices to keep their files in order (a hope, not a likelihood) and keep Objects in mind when asking about a creator’s use of more complex objects—especially those independently made (rather than from the software company or an external vendor).
Attributes
Attributes are labels or tags that contain numeric and textual information (but not special characters or spaces!) about the entity they represent. This information can be linked to external databases, and these links will need to be maintained and followed to retrieve the data. Attributes can be hidden from view or can be assigned to different layers and have visibility switched on or off. Depending upon the software program, attributes can be embedded within blocks or other components, so an understanding of the structure of the program is necessary to view and retrieve attribute data.
For example: a ‘door’ component may contain the following Attributes:- height, width, thickness, type, rating, and hardware. Each instance of the door could contain different values for those Attributes, and those instances could be linked to an external database or vendor site with additional information such as cost, availability, and manufacturer. An Attribute can be either variable or constant—if the same door component is used, a constant attribute can be assigned. If the door information changes in each instance, then variable Attributes will need to be used. By embedding data into the drawings, designers and engineers can encapsulate structural calculations into the base design to determine load variances needed for building permits and other baseline structural integrity issues. As seen in Image B, each specific construction component can contain Attribute data, which is linked to a database of parts.
For MicroStation, the vocabulary is (again) a bit problematic. What AutoCAD refers to as Attributes are named Item Types in MicroStation. An Item can be attached to an Element (Object) or Cell (Blocks) and can be exported as reports based on the definition or instance of the item. Users can set up Libraries of Item Types to be imported into drawing files to easily reuse existing Item values. In MicroStation V8, a Library Picklist can be used to define properties of an Item Type, for example, a door can have ‘wood’ as a picklist option, with associated properties of size and shape.
In analog drafting, Attribute data would be contained in separate physical materials, such as product brochures or samples, or the information would be listed on a specification sheet in table form. CAD files, in addition to being visual representations, are databases of nongraphical information that become integral project management tools for construction. Most programs allow for Attribute data to be extracted partially or en masse to create databases for specific portions of the file or building. In the early days of CAD (circa 1980s), the automatic extraction of specification data was a boon for productivity and efficiency in architecture and engineering firms.
Archivists need to be aware of the amount of data that Attributes can contain, and that the data might be externally linked. An archivist at an architecture firm or in a facilities department would need to ensure the data contained within the Attribute links were maintained for ongoing access to the external information. Ideally, the externally stored information should be included in the list of critical documents during a project closeout and when establishing a records retention schedule. This type of information can assist with alterations, replacement parts, or maintenance of a facility, since specific make and model data can be defined within an Attribute.
Blocks
Blocks are the AutoDesk term for drawing components that a user or office creates for repeated use throughout drawings (e.g., doors, windows, fixtures, targets, and labels). They are named groups of objects that are combined to create a single 2D or 3D Block. The use of Blocks helps to save time and maintain consistency by reusing existing content. Blocks can be organized into libraries and stored externally on a shared network drive or hard drive, or stored within a drawing, in a template file used in multiple drawings, or in entire Block libraries acquired from a vendor. A common analogy is that Blocks are the digital version of rubber stamps used in analog drafting, but much more flexible since digital Blocks can be resized, mirrored, rotated, arrayed, or displayed. These components efficiently manage the consistency of repetitive parts of a drawing and can contain attachments of graphic or nongraphic attributes, which can link to databases of information. As Image C from AutoCAD Web illustrates, the list shows the range of Block components– internal details such as toilets and track lights, external details such as trees, and drawing components such as the title Block and title border.
Blocks can also be dynamic, meaning they are assigned unique values within each copy of the component and added to a drawing rather than an identical instance (e.g., one-up numbering of an element such as windows or doors, or parametrically re-sizing a door to fit the width of an opening). Due to the repetitive nature of Blocks, they cannot easily be changed, unless ‘exploded’ – i.e., disassembled into constituent parts; when the component instance is changed, then the copied Blocks will change as well. Blocks can be ‘nested’ within other Blocks, for example, a framing detail might have multiple elements (Blocks each contain specific dimensional screws and framing) combined to create the detail, and these components can be tagged with specific attribute data.
Blocks may also be referred to as Cells in MicroStation or Symbols in other CAD programs. In Image D, a Cell Library is shown, with a list of some common Cell names and descriptions. MicroStation uses a different controlled vocabulary to indicate the placement of Cells in the Model, which is a grouping of elements and cells, and is part of a DGN file, of which there can be multiple cells, models, and other DGN files referencing the active DGN file. Much like Blocks, Cells can be inserted, replaced, removed, and altered in various ways.
It can be challenging to determine what software program archivists should use to view Blocks and associated components. The full (expensive) version of AutoCAD provides the complete suite of features to view and manipulate components, but does not offer comprehensive backwards compatibility to accurately render files made with earlier versions of the program. As of now, the alternative is the common free software reader, DWG TruView from AutoDesk, or a basic AutoDesk/AutoCAD free web application. The DWG TruView from AutoDesk does not display Block Libraries as separate entities, but does provide more detailed Layer information, allows for easily manipulated navigation controls, and multiple output possibilities. The AutoDesk/AutoCAD free web application does allow for exploration and exploding of Blocks, as well as extensive control over creating and modifying objects; however, navigation, output options, and layer definition are lacking. MicroStation also allows for the viewing and manipulation of Cells from its main interface (a free version is available to educators), which contains more robust features than the free AutoDesk counterparts. A limited amount of backwards compatibility is achieved with MicroStation; the AutoDesk versions will warn a user if external links are missing, but versioning can be problematic.
Archivists should look out for the use of repeated elements as a sign that the files have an associated library of Blocks/Cells/Symbols that need to be included as part of the accession and/or accounted for when errors indicate a network drive file path is missing or not loading. Blocks can also contain block attributes, which are externally linked files that can contain data such as part numbers, costs, dimensions, or other text or numeric information. Blocks are an integral part of creating a CAD file by combining and grouping objects into Blocks, adding numeric and textual attribute data, and copying/resizing/altering the components. Blocks/Cells are the literal building blocks of CAD drawings.
Libraries
Libraries are collections of digital components that designers and engineers use to efficiently insert reproducible Blocks, components, details, symbols, specifications, or other stylizing elements into a born-digital drawing file. These groupings of resources can be either open source or proprietary assets that may be included with a software program or separately hosted, such as a shared central server or online databases and websites. Many digital design software companies offer some libraries of standard 2D and 3D components, such as standardized symbols, within their product. Archival professionals need to know whether libraries were used in drawings and whether the libraries were internal or external to the software environment. It is helpful to understand how the use of libraries evolved over the course of a creator’s career or even within a project.
Libraries (often referred to as Blocks, Symbols, or Content Libraries) have been a part of many CAD products since the 1980s, and part of AutoCAD's ecosystem since its inception in 1982. The early development of Libraries over almost two decades (circa 1980s to mid-1990s) responded to the strong interest by architects and designers to reduce the time required for rote and redundant tasks and responsiveness to an increasingly graphic interface. By the late 1990s/early 2000s, the graphically browsable and searchable palette view reflected the user needs for better structured and accessed resources. It is imperative that archival professionals take the greater context of trends in the evolving ecosystem of design and engineering software as part of the explanation for notable changes in records across time.
In addition to the issues mentioned above for Objects, Attributes, and Blocks, which would impact CAD Libraries, archivists should also inquire about the source and storage of libraries, as these two factors can impact long-term access and viability of resources saved to a CAD Library and transferred to an archival institution. Libraries are often developed from a combination of sources: from built-in resources included with the software, provided by a vendor (e.g., Toto providing Blocks of their products), sourced from specialized websites that can be product-specific or product-agnostic, or created by the individual or firm. Similar to the source, the storage location of CAD Libraries can cause issues for future access. Archival professionals should ask creators about the breadth of their office’s CAD Libraries and where they are stored, including the project folder, a central repository on a network drive, a cloud platform, or a private location.
Conclusion
The four foundational components of CAD files outlined in this article contain many of the baseline graphical and non-graphical data parts for many 2D CAD files that archivists should be aware of when investigating design records. Differences in software nomenclature, as well as visual representations, can make parsing the data difficult if a user is not familiar with the various types of components. Whenever possible, archival professionals should leverage original software manuals from vendors, as well as any internal documentation of workflow, best practices, or standards used by an individual or firm as part of their work.
This article provided an overview of Objects, Attributes, Blocks, and Libraries used in CAD with the intention of increasing efficiency and consistency in drawings. The next article will break down the visual components that make up a CAD software interface, including navigation bars, command line, and drawing spaces.
Additional Resources
[1] Managing Design Information in enterprise-wide CAD using ‘smart drawings’: https://www.sciencedirect.com/science/article/pii/S001044859700095X
[2] A Closer Look at Creating and Using Attributes in AutoCAD: https://www.engineering.com/a-closer-look-at-creating-and-using-attributes-in-autocad/
[3] The Power of Fields and Attributes in AutoCAD: https://static.au-uw2-prd.autodesk.com/handout_11005_GEN11005-L_20-_20The_20Power_20of_20Fields_20and_20Attributes_20in_20AutoCAD_20Handsout.pdf
[4] AutoCAD Defining Blocks: https://help.autodesk.com/view/ACD/2024/ENU/?guid=GUID-F81D7F1E-1F0A-45AD-AC7E-891A85A0033A
[5] MicroStation Cell Definition: https://docs.bentley.com/LiveContent/web/MicroStation%20PowerDraft%20Help-v15/en/GUID-047E349D-9B91-509B-F56D-CDD4E37C2D15.html
Aliza Leventhal, Library Director, Fleet Library - Rhode Island School of Design
Julia D Larson, Collections and Digital Archivist, Environmental Design Archives - University of California, Berkeley
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