On Tue, May 27, 2014 at 04:32:14PM -0400, Chandler Paul wrote: > On Tue, 2014-05-27 at 13:11 -0700, Jason Gerecke wrote: > > I've been away from my computer for most of the (long) weekend up > > here, so apologies for being a bit quiet :) > > > There's a subtlety on the protocol side of things that can't be > > ignored. When normalizing data, you want to be careful to preserve > > information about the zero point. Without that, you can't meaningfully > > pass the data along. Lets imagine that we have some sensor that will > > report values between 10 and 100, with a resolution of 1 unit = 1 > > elbow per square ounce. If we normalize that to the range [0, > > UINT32_MAX] we've lost information about where "zero" is. A normalized > > value of zero does not correspond to zero elbows per square ounce as > > you might expect, and the resolution info is insufficient to correct > > the offset. > > > > Now, if we've done our jobs properly in libinput, that shouldn't be a > > problem. We would have normalized that sensor's values to [0.1, 1] and > > announced the axis to have a resolution of 1 unit = 100 elbows per > > square ounce. Because the zero point is offset like it originally was, > > it's preserved through the scaling done for the protocol and so the > > original 10-100 range can be recovered. The only amendment I'd make is > > to use a signed integer type rather than an unsigned one, since we may > > have negative normalized values that need to be sent through the > > protocol. > I just wrote code to normalize it to INT_MAX, but since everything's in > fixed point integers the actual values it's being scaled to are > 0-8388607.99609375 when the fixed point axis value is converted back > into a double, which as I'm sure you probably realize is kind of a > strange value, and I'm starting to think something like 0.1-1.0 would be > a lot better, trying to normalize to INT_MAX results in something that > sounds really weird to work with.
we need a LI_FIXED_MAX then. Normalising to 0-1 in a 24.8 fixed point only leaves us with 256 value per axis. > Also, what exactly is a "zero-point" in this context? whatever the neutral state of an axis is. e.g. tilt goes in both directions so the effective range is -value ... 0 ... +value. > > >> Seems fine to me. As for normalizing values to units/mm or the like, is > > >> there any known conversion for the units the tablet returns for distance > > >> to metric? > > > > > > Benjamin answered that on IRC, but for the archives: the distance is in > > > mm, > > > though in reality the data is inprecise. > > > > > > Cheers, > > > Peter > > > > > > > I would avoid attaching units or resolution to axes which do not > > already declare them. The distance values on our pens do roughly > > correspond to millimeters from the sensor (which itself is usually a > > few mm below the surface) but we should be reporting a non-zero > > resolution through evdev if the data were reasonably accurate :D > > > > Also, libinput shouldn't generally be "normalizing values to units/mm > > or the like." Data should be normalized to some range within [-1, 1] > > so that the zero point is preserved. Resolution data should be > > provided through another means which relates normalized values to > > real-world units (and should probably be documented to be zero if the > > resolution is unknown). The only exception to this /might/ be > > something like tilt or rotation (though the more I think about it, the > > less I believe it to be worthy of exception given how apps actually > > use the data). > Just to get an idea, how many applications do you think would actually > need to get the resolution information for the tablet? well, we only need a one to make it a necessity to support this :) Cheers, Peter _______________________________________________ wayland-devel mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/wayland-devel
