[Wayland-bugs] [Bug 85715] wl_fixed is not precise enough for high dpi mice

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=85715

--- Comment #5 from Pekka Paalanen <ppaalanen at gmail.com> ---
(In reply to Derek Foreman from comment #3)
> Actually, I think even normalized, with the most ridiculous mouse I've seen
> (8200dpi) it's still like .049 units per "dot" without the accel curve's
> intervention.  If I'm doing my math right we're good to 102400DPI before we
> can't represent a dot in the absence of an accel curve.  I don't think
> anyone will argue a need for that.  It won't be me in any event.
> 
> Storing doubles internally and sending wl_fixed over the wire seems like it
> should be a solution to me.

I missed something. Why don't you see a problem with relative motion, when you
filed this bug in the first place? Assuming both would use wl_fixed_t.

Or was the comment not about relative motion events?

(In reply to Jonas Ådahl from comment #2)
> If deltas and coordinates in wl_fixed_t is enough for clients to work
> properly, we could just cut off the left over fraction from converting from
> double to wl_fixed_t and append it to the next event. This would fix the
> issue of small movements being dropped.

I'd rather not go mangling the event data like this. I can't explain it, but it
feels "dirty". You do not deliver all of a physical (raw) event, but let it
somehow affect following events, that is, the physical vs. protocol events are
not exactly N:1. I can't give an example when that matters. I do expect, that
not all clients will linearly accumulate the relative motion. They could do
something else, like control a 3D-rotation where the order of operations
matters. I just have a feeling that "carrying fractions over" will shoot
someone's foot in the future.

> What it wouldn't fix is clients
> receiving these small movements, and to fix that we either need a new larger
> data type in the Wayland wire protocol, or have a non-fraction and fraction
> values as separate ints or something.

Yeah. Let's go with this. For instance, int32_t for integer part plus uint32_t
for fractional part. It takes the same amount of bytes as a double. It's
simple. It's obvious. It's not surprising.

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