The Layer List (Layer Views)

An important concept in KLayout are the layer views. KLayout displays the layers of a layout by default in a list on the right side of the main window. This list however, does not directly reflect the layers in the layout database. Instead this list is a collection of "views". A view is a description of what is to be displayed and how.

Essentially, the entries in the layer list are pointers to layers in the database, together with a description how to paint the shapes on these layers (the "layer properties").

The pointer is the "source" of a layer view. This is typically a GDS layer and datatype, but can be a layer name (for DXF for example). There are also abstract sources (such as cell boundaries) and the sources can include selectors or modifiers. Selectors are used to only display shapes with certain user properties or from certain hierarchy levels. Modifiers transform the shapes before they are drawn for example. The source is defined by a "source specification" - this is a string describing the database layer and selectors and modifiers. A simple source string is "1/0" which is for GDS layer 1, datatype 0 without and selectors or modifiers. To change the source, use "Change Source" from the layer list's context menu.

See Transforming Views And Property Selectors for some source specification string applications and more details.

Beside the source, a layer entry has a display name. This is an arbitrary text providing a description for the user. By default - when no such name is present - the source of the layer will be displayed. To change the display name, use "Rename" from the layer list's context menu.

Plus of course, the layer views have many options to specify the drawing style, animations and decorations.

The concept of separating views from the database layers opens some interesting options:

Layer views can refer to individual layouts from multi-layout views (through the "@1", "@2", ... notation in the source). Hence, multiple layouts can be mixed in a single layer list.
Layers can be present in the list which do not need to be present in the database. Such a layer is shown as empty. This is important as in GDS an empty layer is equivalent to non-existing. Still you may want to have it shown in the layer list - the views offer this option.
Vice versa, database layer may not be listed in the layer list if no corresponding layer view is present. This way, auxiliary or debug layers can be omitted from the layer list. A "wildcard specification" is available to make sure, all layers are shown if you need to see all.
Multiple tabs can be present to provide multiple views on the same layouts. This is just an alternative set of layer views.
Layer grouping, sorting etc. are just operations on the views, no database change is involved.

The concept on the other hand is slightly counter-intuitive at first. Here are some hints:

Renaming a layer does not change the source - if you rename a layer to something like "1/0", you are likely to fool yourself thinking this is layer 1, datatype 0.
Changing a layer view's source does not change the database too - it will just change the pointer. To change a layer's information in the database, use Edit/Layer/Edit Layer Specification.
Deleting a layer from the layer list does not delete the layer from the database. Use Edit/Layer/Delete Layer instead.
Additing a new layer does not immediately create the layer in the database. Only once you draw something on that layer, it is generated in the database.