Confirm behavior of symbol-placement: point and LineStrings

[jfirebaugh]

Currently, a symbol layer using symbol-placement: point renders a symbol for only the first coordinate pair in a LineString (see e.g. this test case).

Is this the intended behavior? It would seem more logical to render a symbol for each point in the LineString (or MultiLineString).

[lucaswoj]

This behaviour has not been explicitly specified or tested until recently.

Consistency between symbol placement and circle placement is desirable. This would be most easily acheived by adding a new symbol-placement: centroid mode.

[jfirebaugh]

I agree that this extends to circle layers as well, and the two should have equivalent placement behavior.

Can you say more about what the behavior of symbol-placement: centroid would be, and how it relates to the OP here (symbol-placement: point with LineStrings)?

[lucaswoj]

Quick table to clarify my point about centroids:

	circle	symbol w/ point placement
Point geometry	placed at vertex	placed at vertex
LineString geometry	placed at every vertex	placed at first vertex
Polygon geometry	placed at every vertex	placed at centroid

To get the placement of circles to match the placement of symbols in point placement mode, we'd need to replace the "placed at centroid" behaviour with "placed at every vertex" behaviour.

I'd advocate for keeping the "placed at centroid" behaviour available under a symbol-placement: centroid mode.

[mourner]

@lucaswoj an alternative suggestion from me is to have point mean centroid for all types. This means:

	circle	symbol w/ point placement
Point geometry	placed at vertex	placed at vertex
LineString geometry	placed at line centroid	placed at line centroid
Polygon geometry	placed at polygon centroid	placed at polygon centroid

I think this is more intuitive and generally more useful. For the previous behavior, I'd expect people to use different geometry types (e.g. multipoint).

[jfirebaugh]

By centroid, do you mean "pole of inaccessibility"? My proposal would be either of these two:

	circle, or symbol w/ point placement
Point geometry	placed at vertex
LineString geometry	placed at midpoint
Polygon geometry	placed at PoI

or

	circle, or symbol w/ point placement
Point geometry	placed at vertex
LineString geometry	placed at each vertex
Polygon geometry	placed at PoI

[lucaswoj]

By centroid, do you mean "pole of inaccessibility"?

Yup. I've been incorrectly using the term centroid. ("pole of inaccessibility" is so long and "POI" is so overloaded 😬)

My proposal would be either of these two

Those both look good.

@ansis Do you remember the motivation behind #2027? Curious if there are any important use cases for the current circle behaviour.