There are two ways to change the visibility of layers – turning it off using the little light-bulb, and freezing it using the little snowflake. Many times I’ve been asked, “What’s the difference between freezing a layer and turning it off?”. For years I had no idea what the difference was, or if in fact there was a difference at all! I simply got in the habit of just using the ON/OFF setting, but having found out what the subtle difference between them is, I’ve changed over to mainly freezing layers because it usually more accurately fits my intent.
Turning a layer off using the ON/OFF setting makes the objects on that layer hidden, but these object will still be considered part of the drawing. For example, you may have noticed that objects that have been turned off are still selectable in the drawing. Selecting it directly on screen of course still isn’t possible, as you’ve nothing to click on. But other ways of selecting objects will still pick it up – try a SELECTALL for example, and your objects that are turned off will be selected.
Frozen layers on the other hand are completely off. They are not considered part of the drawing at all, and are therefore not selectable.
Due to the different ways that AutoCAD® handles these layer properties, each carries a different overhead. Objects on an off layer are basically just as usable in a drawing as if that layer was on, and therefore AutoCAD® has to still prepare the objects in the same way as if they were on. For example, when AutoCAD® regens, these objects will be regenerated too. Frozen layers are meant to be completely ignored, and therefore carry no such overhead.
There are some other interesting consequences of these two layer visibility toggles. ZOOM EXTENTS for example will zoom to the extent of objects in the drawing. As layers that are turned off are still considered part of the drawing, they will affect zoom extents. Frozen layers on the other hand will not.
Finally, there are some differences to how visibility is affected within blocks. Lets say I have a block with objects on layer BLUE and GREEN, and the block itself is inserted on layer RED. Toggling the visibility of BLUE and GREEN works as expected for both methods. However, there is a difference when toggling the insertion layer of the block. If the layer RED is turned off using the ON/OFF toggle, the objects on BLUE and GREEN will still be visible. However, if RED is frozen, all of the objects in the block will be hidden, regardless of their layer.
But why have these two different methods? It is less of an issue now, but historically these were added when regenerating a drawing took a lot longer. Therefore it was helpful to have the option to turn things off without the drawing requiring regeneration every time. Thus, the ON/OFF toggle was helpful for showing what you want, but keeping them available to be displayed quickly.
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