Remove hillshade-illumination-anchor "viewport"

[birkskyum]

Design Proposal: Remove hillshade-illumination-anchor option "viewport"

Motivation

The "viewport" is a very peculiar default to have, since it doesn't look anything like the real world, and looks buggy.

In addition the "viewport" behavior adds complexity and recalculations, that makes these goals hard to achieve:

• Performance improvements for hill-shading layer maplibre-gl-js#1082
• Support Fog in Mercator projection without Terrain3D maplibre-gl-js#4337 (comment)

Before / After

New default behavior ( 0 degrees, so north )

trimmed.map.mov

Old default behavior (notice all the hillshade changes, and tile boundaries, that looks like glitching)

trimmed.viewport.mov

[HarelM]

While this might be the case for terrain, as you showed in the video, this is not true for 2D maps.
The idea of hillshade for 2D is to get the user to understand the the topology intuitively. And viewport plays a large role for that, especially with the default of lighting from the top of the map.
Also, this is a breaking change to the spec and we decided not to do those.
Current terrain and hillshade is somewhat broken so maybe this makes sense only for terrain, but that would make things unpredictable.

[birkskyum]

if users effectively always would be better off having "viewport" in 2d and "map" (with custom bearing) in 3d terrain, then it's not great we lock down the style to either. Maybe the only way to do a non-breaking solution is to add a third option "viewport-2d-map-3d" that does that. Otherwise, we'd need to introduce another hillshade-illumination-anchor-3d that default "map" , but it's bit confusion with the reset of hillshade settings.

[HarelM]

I'm not sure how helpful this would be besides complicating things...
If a map is 3D the style author can set the hillshade to map.
If the map is 2D the style author can set the hillshade to viewport.
If there is a way to switch from 2D to 3D the developer who added this feature can switch the hillshade according to the current map state...
Otherwise, every viewport/map capable property may need this addition, and I'm not sure we want to go that route...

[birkskyum]

If there is a way to switch from 2D to 3D the developer who added this feature can switch the hillshade according to the current map state...

Our built-in terrain control toggle doesn't handle this currently, but maybe it should.

The way Google Maps solves this is to disable rotation in 2D mode, and in 0 bearing the map/viewport are identical so they don't need viewport.