The opacity property accepts a value from 0 to 100. Where 0 set an element to completely invisible while 100 means it's completely opaque.

The Screenshot shows Opacitie values of 0, 25, 50, 75 and 100.

Please note: when you use a shader, the opacity is set in the shader. The widget's “normal” opacity is ignored then.

Opacity can be used to switch widgets on an off programatically.

Example: to show an element only on the afternoon, you can use the following expression for the opacity value:

({dh23} > 12) and 100 or 0