[PATCH libinput] evdev: fix device_transform_ functions
peter.hutterer at who-t.net
Mon Feb 17 20:28:53 PST 2014
On Mon, Feb 17, 2014 at 01:42:52PM -0500, Benjamin Tissoires wrote:
> X and Y are li_fixed_t, which is 24.8 fixed point real number.
> li_fixed_t max is thus ~8388607.
> On a touchscreen with a range of 32767 values (like a 3M sensor), and
> mapped on monitor with a resolution of 1920x1080, we currently have:
> (x - li_fixed_from_int(device->abs.min_x)) * width == 62912640
> which is 7 times bigger than li_fixed_t max.
> To keep the precision of the sensor, first compute the uniformized
> coordinate (in range 0 .. 1.0) of the touch point, then multiply it
> by the screen dimension, and revert it to a li_fixed_t.
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires at gmail.com>
> I have hit this problem by playing with a touchscreen reporting 4096 values, on
> xf86-input-libinput. xf86-input-libinput does not use the real screen size, but
> 0xffff instead. This allows to report a touchscreen with a range of 128 values
> to work properly :(
> I went through the multitouch device database, and took one example of a more
> real use-case (xf86-input-libinput is still in an early shape).
fwiw, this is exactly the type of use-case where it would be simple and
worth it to knock up a test for a single device and make sure that the
coordinates are correct. which gives us a nice reproducer and prevents us
from errors like this in the future.
> src/evdev.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
> diff --git a/src/evdev.c b/src/evdev.c
> index d8dff65..0d033b8 100644
> --- a/src/evdev.c
> +++ b/src/evdev.c
> @@ -91,8 +91,9 @@ evdev_device_transform_x(struct evdev_device *device,
> li_fixed_t x,
> uint32_t width)
> - return (x - li_fixed_from_int(device->abs.min_x)) * width /
> + double x_scaled = (li_fixed_to_double(x) - device->abs.min_x) /
> (device->abs.max_x - device->abs.min_x + 1);
> + return li_fixed_from_double(x_scaled * width);
A simple 1L * .... should suffice, right?
> @@ -100,8 +101,9 @@ evdev_device_transform_y(struct evdev_device *device,
> li_fixed_t y,
> uint32_t height)
> - return (y - li_fixed_from_int(device->abs.min_y)) * height /
> + double y_scaled = (li_fixed_to_double(y) - device->abs.min_y) /
> (device->abs.max_y - device->abs.min_y + 1);
> + return li_fixed_from_double(y_scaled * height);
> static void
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