QGIS » QGIS Application

Hey Regis,

I've actually investigated this in the past, and it's not a huge change required.

The bulk of the changes come down to 3 places:

https://github.com/qgis/QGIS/blob/master/src/core/pal/feature.cpp#L746 (straight line labels, when being labeled on "straight" segments)
https://github.com/qgis/QGIS/blob/master/src/core/pal/feature.cpp#L901 (straight line labels, when being labeled over "curved" parts of a line (this is used when the above case fails))
and
https://github.com/qgis/QGIS/blob/master/src/core/pal/feature.cpp#L1240 (curved line labels)

These 3 places calculate a cost for the label candidate based on its distance from the center of the line.

What's needed is to add similar logic for other placements.

What I would suggest is that we add an "lineAnchorPointFraction" setting Pal::Layer. This would default to 0.5 - indicating that labels should be around the midpoint of the line. But it could then be set to any fraction - e.g. 0 to anchor them to the start of the line, 1 for the end, 0.25 for 1/4 of the way from the start, etc. The 3 cost calculation places in feature.cpp would then use this fraction instead of the hardcoded mid point of the line when calculating the costs.

That would allow the labeling engine to respect a "% along line" type setting (which could be exposed to users as mid/start/end/custom %, along with data-defined per-feature custom %).

Ideally we'd also want to allow a custom placement from start/end of line which is set in absolute units, e.g. mm/pixels/map units. This would add a bit to the complexity, but it comes down to adding a second lineAnchorPointOffset setting to Pal::Layer and also handling this in the 3 places in feature.cpp.

That was my earlier findings -- you can easily test these by altering the 3 places in feature.cpp and changing the hardcoded midpoint to something like 0, to force all labels to gravitate to the start of lines. See attached patch/screenshot.

https://github.com/nyalldawson/QGIS/commit/59d80fbc83ea3781f999e2bc0afde713e6862000

Please add center alignment and justify alignment as in https://issues.qgis.org/issues/21499