separate zooming logic for trackpads

[ansis]

@kkaefer via #179 (comment)

I was thinking about implementing different zoom behavior when the user uses a trackpad: Since the user is touching directly, we should tie the zoom to the finger movement directly, i.e. not apply transitions in case of trackpad zooming. However, I'm not sure whether it's possible to distinguish trackpads from mousewheels in all browsers. Typically, the deltas for trackpad zoom are much lower, but they can get in the same area as mousewheel scroll deltas when the user zooms fast. Maybe we can do a heuristic detection that puts zooming into trackpad mode when the first couple of mousewheel events have a low (< 40 or so) delta, and in scrollwheel mode when it has high values.

I'm seeing two ways the current mousewheel smoothing isn't ideal for trackpads:

• Very frequent events may make the transition not speed up fast enough (since the first part of the transition curve is slower).
• Some trackpads have built-in smoothing at the end of a scroll. This would essentially handle the transition part for you.

[ansis]

@kkaefer how big of a priority is this (how bad does the current approach feel on macbooks)?

[kkaefer]

It's very awkward because there is no direct relation between the distance you move your finger and the zooming.

[mourner]

Leaflet had the same discussion recently, and there was a similar heuristic proposed that seems to work — if the interval between the first two mousewheel events is <40ms, assume a trackpad.

[ansis]

I'll leave this for someone else. My trackpad isn't good enough for this to be a problem

I think the distance should be proportional, its just that its delayed. Making the duration of the transition proportional to the delta (or 0 if mousewheel is detected) could fix this: https://github.com/mapbox/llmr/blob/gl/js/ui/handlers.js#L49

Or feel free to disable easing until this is fixed, if its annoying

[kkaefer]

Well, IMHO, the best would be to completely disable transitions for high-resolution trackpads; any sort of easing is weird because you get the feeling that it your touch doesn't immediately control the zoom level, or that the renderer is laggy.

[mourner]

@kkaefer that's a pretty good idea. I'd love to see how that feels, and maybe try to use the same approach in Leaflet it it's good.

[kkaefer]

Dang, Firefox uses double the scroll amount when the window is on a retina screen...