LAYER BYLAYER

Introduction

“LAYERS” is probably the most important but least well explained concept in CAD. We hear the terms CAD, CAM and BIM and can imagine PIM and CAPIM/ (Computer Aided Product Information Modelling).

CHATGTP sumarises CAD and BIM as follows: “CAD represents a building primarily as geometry. BIM represents a building as information-bearing objects and their relationships, with the geometry being one part of that information model.” The MycadLAYERS approach is an attempt to “give conventional CAD a structured information layer without requiring the full BIM environment.”

Thus if you want to pursue BIM you will need to consider REVIT, ARCHICAD or BENTLY SYSTEMS but CHATGTP points out that BIM is not one magical database containing everything about a building. In a real project there may be separate models for architecture, mechanical, electrical, hydraulic etc. rather than one enormous model containing everything. MycadLAYERS, however, is a modest step towards a BIM-like solution using AutoCAD LAYERS to provide a single integrated model of multiple trades on multiple levels.

What is CAD and how does MycadLAYERS help? Computer Aided Drafting (CAD) is used to produce illustrations of how to build or manufacture products. This often includes drawings such as floor plans, elevations, construction details and may require many types of information combined into views such as an electrical plan of level 02.

The key difference between the common explanation of a layer as “a transparent overlay” and MycadLAYERS is that MycadLAYERS recognises that:

Drawings are made up of OBJECTS held as RECORDS in a drawing DATABASE.
OBJECTS have many properties that identify their appearance and function.
The key to an object’s identity is an OBJECT PROPERTY called “Layer”.
The VALUE of the property “Layer” is a LAYERNAME.
LAYERNAMES can be used to group objects of the same LAYERNAME or to distinguish between objects with dissimilar LAYERNAMES.
Objects have many properties such as geometry, appearance and editability.
Some properties such as geometry are “fixed” to the object and cannot be changed without editing the object whereas others such as appearance and editability can be given property values by reference to a named list of property values.
The list name is the object’s layer name.
Object values assigned assigned by reference to a layer properties list are described as BYLAYER.
Object properties BYLAYER permit the object’s appearance to be changed by redefining the property value in the layer list.

Perhaps the most important function of layer names is that “Layering” can be used to classify differing objects so that each type can be separated for editing or selectively combined into combinations for production drawings.

To fully benefit from layers we need to distinguish between LAYERS and OBJECTS.

CAD OBJECTS (the stuff that you draw) have many different properties describing their function and appearance but the key OBJECT PROPERTY is its LAYERNAME. A layer name is unique to objects of the same function. It is this property that is used to manage the visibility, editability and printing of objects for use in production drawings.

By way of confirmation of the distinction between OBJECTS and LAYERS…PROPERTIES belong to OBJECTS not LAYERS. Some object property VALUES are directly applied to the object while others may be referenced from the named layer-property-value-list. Thus a LAYERNAME is both a property of the object and also the name of a list of object property VALUES. Such lists can exist without any objects referencing them or can apply to many objects.

Another way of saying it….Objects have a full list of properties including geometric and other properties and a reference LAYERNAME. Several object properties, (particularly geometric properties) aquire their values from the drawing process and the current system settings (including “CLAYER” – the current-layer). Appearance, visibility and editability property values may alternatively be assigned directly to the object; for example the VALUE of object property “Color” may be optionally applied directly to the object, e.g. Color = Red, or referenced from its layer property list, e.g. Color = BYLAYER. (the color is drawn from the Layer property value list with the LAYERNAME recorded in the object’s properties.

AutoCAD is commonly confiigured so that, when starting a new blank drawing, property values default to BYLAYER e.g. property ID = Color, property value = BYLAYER. There is of course no color “BYLAYER” so when the object is displayed on the screen or printed its color is referenced from the specified layer definition. The alternative is to directly assign a value to a property in th object’s record e.g. ID = Color, value = Red.

Notes:

There is no formal description in AutoCAD for properties directly assigned to objects but some CAD users loosely call them by-object.
While layernames can be used to identify like objects by their properties, a layer is not a specific group or number of objects.
A layer has nothing to do with anything physical such as an overlay-sheet.
The confirmation being that a layer-property-list may be un-referenced, (empty).
There can be as many different layer definitions as there are objects in a drawing.
Objects of the same object type can reference and be identified by a single named and saved layer-properties-list.
Once created, all layer-lists remain in the database unless PURGED.
All objects on a layer look the same.
Change the layer once instead of many individual objects.
Most CAD standards expect objects to use BYLAYER.
Lineweights and colors can be controlled through layers.

Layer 0

All drawings, including blank new drawings, by default have a layer 0 (zero). i.e. the OBJECT has the property of LAYERNAME. i.e. “Layer” VALUE “0”. Other layers can be created as the drawing progresses. Whatever layer is current when an object is created becomes the VALUE of that object’s LAYERNAME. By default the values of Layer 0 in a new blank drawing will be Color white or black, Line-type continuous, Line-weight 0. (Is that why it is called “0” or is it because technically the first digit in a number series begins with 0?)

Layer 0 might look like this:

A typical LAYER record

Function	Controls
Identification	Name, Description
Appearance	Colour, Linetype, Lineweight, Transparency, Material
Output	Plot Style, Plot/No Plot
Visibility	On/Off, Freeze/Thaw, VP Freeze
Editing	Lock/Unlock
Viewport Management	New VP Freeze
Organisation	Current, In Use, Xref-dependent, Filters

Typical object property list using BYLAYER

Typical record of an INSERT

Properties that can be inherited BYLAYER

Appearance properties

Colour
Linetype
Lineweight
Transparency
Material
Plot Style

Other properties that affect layers

Layer Name
Description
On/Off
Freeze/Thaw
VP Freeze
Lock
Plot/No Plot
Filters
Layer States
Reconciliation

Controls that mange groups of layers

Filters
Layer States
Reconciliation

“Renaming” layers

Although AutoCAD purports to “rename” layers as follows:

The layer definition receives a new name.
All objects referencing that layer automatically update the VALUEof their Layer property to the new name.
The visual appearance does not change unless the new layer definition is changed.

In effect a new layer is created with the new name with properties defined to match the original layer. All objects with the original layer name have their layer name property updated to the new name and the original layer is purged from the system.

Moving objects between layers

A layer is an environmental condition with a list of property settings (with or without objects) not a LOCATION from or to which objects may be moved. The object’s Layer property is changed to reference a different layer definition. “Renaming” a layer effectively creates a NEW layer with a new name albeit it may have the same properties as the former. LAYER definitions remain in the system whether or not there are any objects that reference them. Unreferenced layers may be purged.

What does “by-object” mean?

Changing appearance properties of an object from BYLAYER to a specific value does not change the LAYER’s application to other objects with the same LAYER-NAME.

CHPROP lets you change:

Color
Layer
Linetype
Linetype Scale
Lineweight
Thickness
Transparency
Material
Annotative setting
Plot Style (when named plot styles are used)

Other useful commands:

MATCHPROP — copy properties between objects
LAYMATCH — match objects from another layer
LMAN — manage layer states

Opinion. Some tutors recomend creation of a template drawing with pre-named and pre-defined layers before any objects are created thus avoiding the delay of having to repeatedly create new LAYER-NAMES and define new property lists. A counter effect is that the drawing database may become overloaded with irrelevant lists of properties that must then later be PURGED. In the opinion of the author It is better to have a layer naming standard that can easily create and manage layers on-the-run rather than searching lists to find an appropriate name with appropriate properties. MycadLAYERS provides such a system. (see page on MycadLAYERS)