Symbols & Attributes

What's it all about?

The geometry you create when making the prototype symbol will have normal attributes like color, layer, linetype, etc. These are included in the symbol/block definition. In programs modeled on the behavior of Autocad (like Rhino) each instance of the block you then place is like any other primitive—it has color, layer, and linetype too. Which raises this question: Which attributes control the appearance of the instance?

The "Autodesk strategy" built into Autocad recognizes that sometimes you want one answer to the "which one controls" question, and sometimes you want another. To pull this off colors and linetypes have two special variations: bylayer and byparent. In normal drawing use these have not significance, but when applied to geometry that is included in a block, they matter.

The results can be a little complicated (see the image below), but they do follow some rules (see below). The main thing this complexity gets you is the ability (for example) to make a chair block when fabric and metal finish attributes can be varied, or a door object with alternative trim packages on different layers, etc. If you are building a "one-off" model they aren't so important.

Visibility and layers

Objects drawn on layer 0 (aka "default") will have their visibility controlled by the visibility of the instance layer (active layer when the block is inserted). Objects drawn on other layers will have their visibility controlled by those layers when the instance layer is visible, but will be invisible if the instance layer is off.

Bylayer

Objects with color of linetype attributes of "bylayer" will adopt the color and linetype settings of the block instance layer. A "custom" color/linetype provides an absolute control.

Byparent

Objects may also have their own color and linetype assignments (i.e. not be controlled by their layer). Geometry with "byparent" attributes within a block will be colored according to the color or linetype attribute of the block instance.

The Illustration

Six concentric square surfaces were created in two stacks (upper left), on layer "default" and on layer "brown" (with a color setting of brown). The squares were colored "by layer","by parent", and "custom" (green). All six squares were then made into a block and the block was inserted (upper right) once on "default" and (three times--bottom row) on layer "red". These three instances were then given instance colors of by layer, by parent, and custom (teal).

[report bug]