I am assuming that since you are investigating LAYERS you are already familiar with CAD at a professional level.
The first thing I would like you to do is FORGET the common description of CAD LAYERS as “like transparent sheets on which objects are drawn”. While this analogy, evolved from manual drafting and is simple to explain and understand, it limits understanding of the real benefits and power of using layers.
Think instead of the wider figurative meaning of the term “layers”e.g. “layers” of society.
This database view is the foundation for more advanced layer management systems such as MycadHELPER’s composite-coincident-overlay modelling approach, because it treats layers as data classification and control mechanisms, not as drawing surfaces.
Let’s start with some terminology:
Drawings consist of GRAPHICAL OBJECTS represented on a computer screen or printed on paper. The source of the information is a DATABASE (file). Each object has a RECORD in the DATABASE. Each record is a list of the PROPERTIES of the object and instructions on how to represent it. The number and names of the properties vary according to the TYPE of object. The term TYPE is used in three different ways: 1. A LAYER definition, 2. PRODUCT-TYPE e.g. Brick Wall 3. GRAPHICAL-TYPE e.g. Line, Arc, Circle. In this context we are refering to product-type i.e. the definiton of a brick wall. Object properties may be a singular value (e.g. its layer-name), a switch (e.g. ON/OFF) or a selection LIST (e.g. Color Red, Yellow, Green). Each property has a NAME and a VALUE. The property-names are consistent and specific to objects of the same product-type. The property values may be assigned to an object in three ways: 1. by the the system – as the object is drawn, 2. by individual change, 3. from a LIST related to the product type. The list defines a LAYER. A group of Layers might be used to display a particular DRAWING FUNCTION e.g. Electrical plan level 02.
To understand “LAYERS” we first need to be familiar with the drawing database:
Key aspects of a drawing database:
- Drawing OBJECTS
- Object PROPERTIES
- Symbol Tables
- LAYERS
- BLOCK Definitions and References
- Layout and Paper Space Information
- Named Object Dictionaries
- Dimension and Annotation Systems
- Display and Visibility Controls
- External References (Xrefs)
- System Variables and Drawing Settings
- Relationships Between Database Elements
OBJECTS and LAYERS
At this time we don’t need to study the complete database structure BUT we do need to understand OBJECTS and LAYERS.
OBJECT types:
- Lines
- Polylines
- Arcs
- Circles
- Splines
- Ellipses
- Hatches
- Text and MText
- Dimensions
- Leaders
- Tables
- Blocks (inserted references)
- 3D solids
- Surfaces
- Meshes
- Point objects
Entries in an object record:
- A unique object ID (handle)
- A type definition
- Geometric data
- Object properties
- Relationships to other database objects
e.g. a record of a line consists of:
- Drawing space (paper or model)
- Layer name
- Start point coordinates
- End point coordinates
- Colour
- Linetype
- Line-type scale
- Lineweight
- Transparency
- Plot style information?
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 |
What does BYLAYER really mean?
In an OBJECT PROPERTY definition (record) property values can be assigned directly to the object or by reference to a layer-property-list e.g. the property of Color can be assigned directly to the object e.g. Color = Red or BYLAYER e.g. Color = bylayer.
The layer-name is the key to an object’s definition. An object’s layer-name points to a separate named database component defining the object’s such that an object with the reference layer “Brick-Wall” might have its property of “Color” with the Value “Red” but in the object’s own definition the property named “Color” would be shown as “bylayer” meaning that when the object is drawn or regenerated the object references the layer definition and its color will be Red.
Common practice recommends that wherever practicable object property values should be assigned by reference to a LAYER definition rather than directly to the object. To do this we need to distinguish between property NAMES (e.g. Color)and property VALUES(e.g. Red).
Each value in a layer description can be assigned in one of three ways. Just to remind us; “object properties may be a singular value (e.g. its layer-name), a switch (e.g. ON/OFF) or a selection LIST (e.g. Color Red, Yellow, Green).”
Keep in mind that the term “LAYER” can refer to:
- A List – of the properties describing a particular object-function.
- The Name – of an individual object-property-list
- The Value of the property named
- A Collection – of same type of objects defined by their layername.
Values assigned by reference to a defined layer are said to be BYLAYER. There is no formal description in AutoCAD for properties directly asigned to objects but some CAD users loosely call them by-object. Those values assigned from a layer-properties-list are said to be BYLAYER and the word “BYLAYER” is the value included in the object property list . e.g. property ID color property value BYLAYER. There is of course no color “BYLAYER” so when the object is drawn on the screen or printed its color is referenced from the specified layer definition e.g. RED.
An OBJECT database record looks like this:
(show image of a record)
(add link to web page showing a summaryof a drawing database)
It is worth emphasising that there could be as many different layers (property lists) as there are objects in a drawing but objects of the same object type can reference a same single named and saved layer-properties-list. Those property values that are referenced from the list instead of being assigned ditrectly to the object.
What about MycadHELPER?
Later I will show how a layer NAME can imply other layer properties not covered by CAD layer properties lists such as construction-phase, drawing-function, view point and drawing cross-section or floor-level. This will lead to a separate discussion of layer-naming standards and coincident-overlay-drafting.
(show example of several complex layer names)
(provide a link to relevant web page)
(show an object record – highlight the item “Layer”)
(show a layer being highlighted)
The layer name is one of the most important properties in an object’s record.
We now know that a “layer” has nothing to do with anything physical such as an overlay-sheet. We also know that while layernames can be used to identify like objects, a layer is not a specific group or number of objects. The confirmation being that a layer-property-list may be un-referenced, (empty).
How do OBJECTS and LAYERS get their property NAMES and VALUES
A key item in the object record is the name of a layer properties-list assigned at the time of drawing by the current layer system-variable CLAYER. In a NEW blank drawing the initial and only layer is Layer 0 (zero). Many lists (Layers) may be required to describe the various object types in a drawing. Thses are created and recorded in the database separately from object records. Once recorded, all layer-lists remain in the database unless PURGED.
A typical NEW object record might look like this:
| General Function | Layer Property | Purpose |
|---|---|---|
| Identification | Layer Name | Unique identifier used to assign objects to the layer. |
| Description (optional) | Human-readable explanation of the layer’s purpose. | |
| Appearance | Colour | Default display and plotting colour for objects. |
| Linetype | Continuous, Hidden, Centre, Phantom, etc. | |
| Lineweight | Thickness used for display and plotting. | |
| Transparency | Controls object transparency. | |
| Material | Rendering material for 3D objects. | |
| Plot Style (STB drawings) | Controls plotted appearance when named plot styles are used. | |
| Visibility | On/Off | Shows or hides the layer. |
| Freeze/Thaw | Removes or restores the layer from drawing regeneration. | |
| VP Freeze | Freeze layer in selected viewports only. | |
| Editing Control | Lock/Unlock | Prevents modification while remaining visible. |
| Printing Control | Plot/No Plot | Determines whether objects on the layer are printed. |
| Standards & Management | Layer Filters | Organize large layer collections. |
| Layer States | Save and restore groups of layer settings. | |
| Layer Reconciliation Status | Tracks newly created layers for standards compliance. |
Looking at these by function
The properties naturally fall into six broad categories:
1. Identification
These define what the layer is.
- Layer Name
- Description
2. Default Object Appearance
These determine how new BYLAYER objects will appear.
- Colour
- Linetype
- Lineweight
- Transparency
- Material
- Plot Style
These are the properties most users think of when discussing layers because they are inherited by objects whose properties are set to BYLAYER.
3. Visibility Control
These determine whether objects are displayed.
- On/Off
- Freeze/Thaw
- Viewport Freeze
4. Editing Control
These determine whether objects can be modified.
- Lock
5. Printing Control
These determine whether objects appear on printed output.
- Plot / No Plot
6. Management Functions
These assist in organizing and controlling complex drawings.
- Layer Filters
- Layer States
- Layer Reconciliation
A useful distinction
For teaching purposes, it is worth separating layer properties into two major groups:
A. Properties inherited by objects
These become object properties when an object is created BYLAYER.
- Colour
- Linetype
- Lineweight
- Transparency
- Material
- Plot Style
B. Properties that affect the layer itself
These are not inherited by objects.
- Layer Name
- Description
- On/Off
- Freeze/Thaw
- VP Freeze
- Lock
- Plot/No Plot
- Filters
- Layer States
- Reconciliation
8888888888888888888888888888888888888888888888888888888888888888888888
This distinction reinforces one of the points you’ve been making in your MycadHELPER material: a layer is not a graphical sheet or plane. It is a database record containing both default property values (which objects can inherit via BYLAYER) and management controls that govern how all objects assigned to that layer behave.
…….a LAYER is not a graphical sheet or plane or an individual object record, it is a separate, potentially shared, database record containing both property values and management controls that AutoCAD references when representing objects on the screen or when printing.
Some values relate to an object’s appearance and are said to be BYLAYER.
- Colour
- Linetype
- Lineweight
- Transparency
- Material
- Plot Style
Others control a layer’s visibility and edit-ability and hence management of objects for display and printing.
- Freeze/Thaw
- VP Freeze
- Lock
- Plot/No Plot
The resulting object properties are said to be BYLAYER
modification of both of these sets of properties these layer properties
others such as
- Filters
- Layer States
- Reconciliation
manage groups of layers
are assigned directly to an object record are commonly known as BYOBJECT
default
I think this classification aligns very well with your broader explanation that a layer should be understood as an object-property management system, rather than as a “transparent overlay.”
Drawing production concepts:
Workspaces – operator’s work enviroments (screen, menus etc)
Workzones – production drawing layer states – automated by MycadHELPER