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Layers, Colors and Printing in AutoCAD

Geoff Harrod

Layer naming and usage has always been hotly debated in AutoCAD circles. The question is, why has this always been such a big deal in the DWG CAD community and rarely so with users of other CAD systems?

In most CAD systems layers are used to group graphic objects for design convenience. Control of printed line widths is generally handled by separate mechanisms – such as by a ‘line-weight’ property or by a ‘pen’ property. However in DWG based CAD, layers have also been required to control printed line thickness because there is no separate mechanism for it.

In the DWG system, (until AutoCAD 2000) printed line widths have to be defined by means of on-screen line color. Originally, AutoCAD only provided for assigning colors and line types as properties of layers. This is now referred to as Color and line-type ‘By-layer’. Later, in v2.5 I think, AutoCAD introduced the option of assigning colors and line-types to individual graphic entities (‘By-entity’). But most users decided it was a conflicting rationale that introduced complications and was best avoided. This means that the layer naming scheme has to serve two quite separate functions – grouping objects logically, and setting printed line-width.

By-layer, By-entity or both?

It is a fairly generally held view that you should adopt either the By-layer or the By-entity colors, and never mix the two in the one drawing. But the rigid adoption of a By-Layer-only rule can complicate matters.

Despite the general avoidance of By-entity properties, there is a very common use of By-entity color that is built into AutoCAD - the setting of thicker lines for the figures in dimensions. This shows that you can mix By-entity and By-layer property techniques without creating confusion.

An obstacle to wider usage of color-by-entity has been the lack of adequate object selection tools in AutoCAD, other than by turning off layers. Autodesk have for some time supplied a LISP program called SSX for selecting objects by various entity properties. But the existence of SSX is not obvious and many users never find it. Also, it is a very awkward-to-use mechanism, enough to deter anyone.

The first menu that IntelliCAD pops up when you pick any Modify command, in this case Copy, enables you to select the objects to modify by various criteria including their properties such as color.

IntelliCAD, although mainly a ‘clone’ of AutoCAD, has improved on some of AutoCAD’s less effective mechanisms. It provides a much more flexible means of selecting groups of objects to edit than does AutoCAD up to R14. IntelliCAD’s Modify menu pops up a menu of selection options for all edit actions, and one of them is by color. This obviates most of the difficulties previously found in working with color-by-entity.

Selecting entities "by color" in IntelliCAD is a standard menu-driven selection procedure.

Layer Standards

A widely used layer naming scheme names layers in relation to line thickness and line type. These are deficient because they frustrate the use of layers for segregating object types or design aspects. If you try to combine layer naming for line width together with object grouping, it gets very cumbersome

There have been many proposed layer-naming standards, especially in the architectural field, but they are all rather complex and obscure and none have met with wide acceptance. They usually use some arbitrary coding scheme of letter groups. The result is a name that is a jumble of letters and numbers, where the meaning is not self-evident.

These ‘standards’ have also failed to be generally adopted industry-wide because they are inherently AutoCAD-specific and irrelevant to users of, for example, MicroStation.

3D

The logic of basing a layer scheme on pen thickness and line types reflects the earliest stage of CAD as an electronic drawing board. Now there is increasing use of CAD in ways that could never be done on a drawing board. In 3D, the model data is inherently quite separate from the generation of printable sheets which is why PaperSpace was introduced. This makes a layer scheme based on paper drafting practice inappropriate.

When working in 3D, it soon becomes obvious that you need to group things by the parts of the real-object. If renderings may be generated later, it is highly desirable for materials to be segregated by layers.

Color

No standard exists for what colors to use for the various line-widths. One rationale simply assigns AutoCAD color numbers in ascending numerical order to increasing line-widths. The other common rationale is based on the colors that were used for identifying Rotring pen sizes. The eight basic AutoCAD colors, that most users still limit themselves to, do not include all the Rotring colors exactly so there is more than one version of the Rotring-based system, causing further confusion.

[Rotring manufactures a popular line of technical drawing pens, with color-coded bands to distinguish line thickness. Line thicknesses and color-codes used are based on ISO standards. Ed]

The Future

It is interesting that Visio’s two product lines exemplify the old and new approaches to CAD. IntelliCAD must, because of its AutoCAD compatibility aim, perpetuate the older practices, while Visio Technical lacks that restriction and uses a fully Windows-oriented approach with on-screen line widths and colors as intended to be printed. I think that rationale ought to be the trend and AutoCAD is now equipped to support that logic in its 2000 version. But now, Autodesk are branching away from their DWG heritage and limitations with Actrix and Inventor. Inventor does the same things as Mechanical Desktop, but with no trace of DWG traditions and DOS-derived line-width concepts.

I expect all these long-running controversies about colors and layer naming schemes will end only when DWG is laid to rest as the new generations of CAD products eventually fully take over.

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