ozone: evdev: Fix tilt calculation for tablets

ozone: evdev: Fix tilt calculation for tablets

This makes tilt get populated per the Pointer Events spec with a Wacom
Intuos Pro M Pen.

BUG=557952
TEST=Wacom Intuos Pro M Pen on Chromebook Pixel with
  --enable-blink-features=PointerEvent
  & http://rbyers.github.io/eventTest.html

Committed: https://crrev.com/b11a3a6c6f3a69a9bd39a79a9fe372d461ff76fc
Cr-Commit-Position: refs/heads/master@{#360587}

[spang, 2015-11-18 23:22:10]

spang@chromium.org changed reviewers:
+ mustaq@chromium.org, rbyers@chromium.org

[spang, 2015-11-18 23:22:10]

Feel free to commit after reviewing

[mustaq, 2015-11-19 15:17:18]

On 2015/11/18 23:22:10, spang wrote:
> Feel free to commit after reviewing

LGTM.

[mustaq, 2015-11-19 15:18:22]

The CQ bit was checked by mustaq@chromium.org

[commit-bot: I haz the power, 2015-11-19 15:18:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1456063003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1456063003/40001

[mustaq, 2015-11-19 15:26:35]

https://codereview.chromium.org/1456063003/diff/40001/ui/events/ozone/evdev/t...
File ui/events/ozone/evdev/tablet_event_converter_evdev.cc (right):

https://codereview.chromium.org/1456063003/diff/40001/ui/events/ozone/evdev/t...
ui/events/ozone/evdev/tablet_event_converter_evdev.cc:48: tilt_x_range_ =
info.GetAbsMaximum(ABS_TILT_X) - tilt_x_min_ + 1;
Have a quick question, didn't see it until CQing, sorry.

Does GetAbsMaximum() return the max possible raw "value" from device, or one off
from that? Because, intuitively, range = max-min, and thus the angle is +90 when
the "value" == max:
  180 * (max-min)/(max-min) - 90

[commit-bot: I haz the power, 2015-11-19 15:45:37]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-11-19 15:46:36]

Patchset 3 (id:??) landed as
https://crrev.com/b11a3a6c6f3a69a9bd39a79a9fe372d461ff76fc
Cr-Commit-Position: refs/heads/master@{#360587}

[spang, 2015-11-19 16:13:31]

On 2015/11/19 15:26:35, mustaq wrote:
>
https://codereview.chromium.org/1456063003/diff/40001/ui/events/ozone/evdev/t...
> File ui/events/ozone/evdev/tablet_event_converter_evdev.cc (right):
> 
>
https://codereview.chromium.org/1456063003/diff/40001/ui/events/ozone/evdev/t...
> ui/events/ozone/evdev/tablet_event_converter_evdev.cc:48: tilt_x_range_ =
> info.GetAbsMaximum(ABS_TILT_X) - tilt_x_min_ + 1;
> Have a quick question, didn't see it until CQing, sorry.
> 
> Does GetAbsMaximum() return the max possible raw "value" from device, or one
off
> from that? Because, intuitively, range = max-min, and thus the angle is +90
when
> the "value" == max:
>   180 * (max-min)/(max-min) - 90

We're not working with real numbers, this is an integer representation.

If we have K different integer values, then we've partitioned the 180 deg
interval into K equal subintervals each 180 deg / K degrees wide.

So we have to multiply by 180 / K in order to make our units degrees.

Here K = (max - min + 1) since the range is inclusive.

You're right that you can't get exactly +90deg out of this math, but we don't
need to. We haven't promised we can exactly represent any particular value, and
that's not actually possible starting from integers.