Flip orientation of Font Markers on line when upside down using QGIS

Question

I would like to have font markers flip when they are upside down but otherwise follow the line angle. Here is an image of how they look now:

I would like them to look like this:

I achieved this by switching the line direction but I want to do it through the style and I would like to do it at the marker level not the line level. I have tried a geometry generator to switch the line.

if(y_max( $geometry)>y_min($geometry),$geometry,reverse($geometry))

But this only switches the issue to the bottom line.

I also tried a data defined override on the font marker rotation with these expressions.

if( @symbol_angle < 180,0,180)
if(y_max( $geometry)<  y_min($geometry),0,180)

Answer 1

If you're working with "font markers" for your text, set the following for data defined override of the rotation:

if(line_interpolate_angle($geometry,0)>180,180,0)

This checks, whether the line is "flipped" and if it is, tells the symbology to rotate the text by 180 degrees, otherwise it stays, as it is.

I did some additional testing, and this solution does not take changes of line direction into account.