On Tue, May 27, 2014 at 3:52 PM, Peter Hutterer <[email protected]> wrote: > On Tue, May 27, 2014 at 01:11:31PM -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 :) >> >> >> On Sun, May 25, 2014 at 7:13 PM, Peter Hutterer >> <[email protected]> wrote: >> > On Fri, May 23, 2014 at 10:34:10PM -0400, Chandler Paul wrote: >> >> On Fri, 2014-05-23 at 17:00 +1000, Peter Hutterer wrote: >> >> > I'm almost sold on normalization since it does reduce the likelihood of >> >> > things going wrong. We need to provide the axis resolution to convert >> >> > back >> >> > to the real data though where needed. >> >> > >> >> > once you provide the axis resolution, it doesn't matter if you provide >> >> > raw >> >> > data unless you also want to provide "raw" resolution, which is >> >> > excessive.. >> >> > >> >> > so, given that this would be sent down the protocol (and for the limited >> >> > resolution) the range should be normalised to uint16_t or uint32_t max, >> >> > with >> >> > the resolution in units/mm or canonicalized where more appropriate. This >> >> > would be what goes on the wayland protocol as well then. >> >> > >> >> > helper functions to convert that back to doubles, or elbows per square >> >> > ounce >> >> > would be part of the wl-client package that parses that protcol, not >> >> > libinput. >> >> > >> >> > that doesn't seem like the worst architecture for both libinput and the >> >> > protocol, any comments? >> >> >> >> 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. > > good point, keeping the zero point is important. > >> 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. >> >> >> 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). > > looking at linux/input.h, the axes where we should attach resolution in a > defined unit (units/mm or radians) are: > x/y/z, rx/ry/rz, distance, tilt x/y and their equivalents in the ABS_MT > range. The kernel already does that anyway if the device supports it, we're > just passing this on here. I'm not suggesting attaching a unit to e.g. > ABS_RUDDER, who knows what that is :) > > so just in case this isn't clear what I'm suggesting is to export the range > normalised in [LI_FIXED_MIN, LI_FIXED_MAX] or [0, LI_FIXED_MAX], with extra > information to map min/max into a physical dimension if possible. which is > what you're suggesting too afaict. > > Cheers, > Peter
Yeah, sounds like we're on the same page :) Jason --- Now instead of four in the eights place / you’ve got three, ‘Cause you added one / (That is to say, eight) to the two, / But you can’t take seven from three, / So you look at the sixty-fours.... _______________________________________________ wayland-devel mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/wayland-devel
