I am displaying symbols at the middle of lines and I would like them to be visible only when the length of the feature displayed on my screen is long enough (which I call "visible length" in the title of this message).
I found a workaround using a condition based on map scale:
if(@map_scale>5000, $length>50, $length>30)
Is there a way to do it at any scale? For example a condition saying "don't show the symbol if the feature length on screen is less than X cm"