[RFC libinput] Add an API for touchpad gesture events

Thu Jan 22 18:11:09 PST 2015

On Thu, Jan 22, 2015 at 04:52:50PM +0100, Hans de Goede wrote:
> For touchscreens we always send raw touch events to the compositor, and the
> compositor or application toolkits do gesture recognition. This makes sense
> because on a touchscreen which window / widget the touches are over is
> important context to know to interpret gestures.
> 
> On touchpads however we never send raw events since a touchpad is an absolute
> device which primary function is to send pointer motion delta-s, so we always
> need to do processing (and a lot of it) on the raw events.
> 
> Moreover there is nothing underneath the finger which influences how to
> interpret gestures, and there is a lot of touchpad and libinput configuration
> specific context necessary for gesture recognition. E.g. is this a clickpad,
> and if so are softbuttons or clickfinger used? What is the size of the
> softbuttons? Is this a true multi-touch touchpad or a semi multi-touch touchpad
> which only gives us a bounding box enclosing the fingers? Etc.
> 
> So for touchpads it is better to do gesture processing in libinput, this commit
> adds an initial implementation of a Gesture event API which only supports swipe
> gestures, other gestures will be added later following the same model wrt,
> having clear start and stop events and the number of fingers involved being
> fixed once a gesture sequence starts.

Overall, I think this is probably something we want, but I wonder if
this conflicts with the wl_touchpad low level Wayland protocol that
was discussed [0] as part of how to deal with touchpad hand writing (name
signing, Chinese character writing etc). I.e. *if* we'd provide all the
data to the client anyway, do we still need to put the gesture processing
in libinput as well? I'm not saying we shouldn't, just thinking out loud
whether these are orthogonal or not.

Anyhow, some comments inlined below:

> 
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
>  src/libinput.h | 155 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 154 insertions(+), 1 deletion(-)
> 
> diff --git a/src/libinput.h b/src/libinput.h
> index 7b7a2db..1507772 100644
> --- a/src/libinput.h
> +++ b/src/libinput.h
> @@ -173,7 +173,11 @@ enum libinput_event_type {
>  	 * Signals the end of a set of touchpoints at one device sample
>  	 * time. This event has no coordinate information attached.
>  	 */
> -	LIBINPUT_EVENT_TOUCH_FRAME
> +	LIBINPUT_EVENT_TOUCH_FRAME,
> +
> +	LIBINPUT_EVENT_GESTURE_SWIPE_START = 600,
> +	LIBINPUT_EVENT_GESTURE_SWIPE,
> +	LIBINPUT_EVENT_GESTURE_SWIPE_END,

This gesture is really similar to our already present scroll gesture,
and maybe for consistency maybe one single event would be enough? I mean
that SWIPE_START and SWIPE would be a single one, and SWIPE_END would
be, as with the scroll gesture, a 0-length vector representing the end
of a swipe.

A question that comes to mind is what level the swiping should be at.
For example, these swipe gestures, are they supposed to be simply just
"swiping" as in N fingers moving together in a single direction and then
releasing, or more like 2 finger scroll, where the complete motion is
more unspecified?

If we'd put the swipe gesture in the same "level" as scrolling, we could
include the 2-fingers case as a gesture as well so that applications
could make use of it whether the user has enabled 2-fingers scroll or
not.

On the other hand, I assume we cannot go too unspecified either, because
that'd mean a pinch gesture would just be a 2 finger motion gesture where
the fingers don't stay together. Then again, we could go the middle way
of saying that for a N fingers motion gesture one need to keep the
fingers more or less in the same relative distance to each other.

Anyway, do you have any ideas of what level you want to put it?

>  };
>  
>  /**
> @@ -350,6 +354,19 @@ libinput_event_get_touch_event(struct libinput_event *event);
>  /**
>   * @ingroup event
>   *
> + * Return the gesture event that is this input event. If the event type does
> + * not match the gesture event types, this function returns NULL.
> + *
> + * The inverse of this function is libinput_event_gesture_get_base_event().
> + *
> + * @return A gesture event, or NULL for other events
> + */
> +struct libinput_event_gesture *
> +libinput_event_get_gesture_event(struct libinput_event *event);
> +
> +/**
> + * @ingroup event
> + *
>   * Return the device event that is this input event. If the event type does
>   * not match the device event types, this function returns NULL.
>   *
> @@ -884,6 +901,142 @@ struct libinput_event *
>  libinput_event_touch_get_base_event(struct libinput_event_touch *event);
>  
>  /**
> + * @defgroup event_gesture Gesture events
> + *
> + * Gesture events are generated when a gesture is recognized on a touchpad.
> + *
> + * Gesture sequences always start with a LIBINPUT_EVENT_GESTURE_FOO_START
> + * event. All following gesture events will be of the
> + * LIBINPUT_EVENT_GESTURE_FOO type until a LIBINPUT_EVENT_GESTURE_FOO_STOP is
> + * generated which signals the end of the gesture.
> + */
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * @return The event time for this event
> + */
> +uint32_t
> +libinput_event_gesture_get_time(struct libinput_event_gesture *event);
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * @return The generic libinput_event of this event
> + */
> +struct libinput_event *
> +libinput_event_gesture_get_base_event(struct libinput_event_gesture *event);
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * Return the number of fingers used for a gesture. This can be used e.g.
> + * to differentiate between 3 or 4 finger swipes.

See above about number of fingers question.

> + *
> + * This function can be called on all gesture events including
> + * LIBINPUT_EVENT_GESTURE_FOO_START and the returned finger count value will
> + * not change during a sequence. IOW libinput_event_gesture_get_finger_count
> + * will only return a different value then a previous call after a
> + * LIBINPUT_EVENT_GESTURE_FOO_STOP has been received.
> + *
> + * @return the number of fingers used for a gesture
> + */
> +enum libinput_key_state
> +libinput_event_gesture_get_finger_count(struct libinput_event_gesture *event);
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * Return the delta between the last event and the current event. For gesture
> + * events that are not of type @ref LIBINPUT_EVENT_GESTURE_SWIPE, this
> + * function returns 0.
> + *
> + * If a device employs pointer acceleration, the delta returned by this
> + * function is the accelerated delta.

Would we really want this? For the 2 fingers scroll, we do not use the
accelerated motion.

Jonas

[0] https://bugs.freedesktop.org/show_bug.cgi?id=85879

> + *
> + * Relative motion deltas are normalized to represent those of a device with
> + * 1000dpi resolution. See @ref motion_normalization for more details.
> + *
> + * @note It is an application bug to call this function for events other than
> + * @ref LIBINPUT_EVENT_GESTURE_SWIPE.
> + *
> + * @return the relative x movement since the last event
> + */
> +double
> +libinput_event_gesture_get_dx(struct libinput_event_gesture *event);
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * Return the delta between the last event and the current event. For gesture
> + * events that are not of type @ref LIBINPUT_EVENT_GESTURE_SWIPE, this
> + * function returns 0.
> + *
> + * If a device employs pointer acceleration, the delta returned by this
> + * function is the accelerated delta.
> + *
> + * Relative motion deltas are normalized to represent those of a device with
> + * 1000dpi resolution. See @ref motion_normalization for more details.
> + *
> + * @note It is an application bug to call this function for events other than
> + * @ref LIBINPUT_EVENT_GESTURE_SWIPE.
> + *
> + * @return the relative y movement since the last event
> + */
> +double
> +libinput_event_gesture_get_dy(struct libinput_event_gesture *event);
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * Return the absolute x coordinate of the current center position of a
> + * swipe gesture. This can be used e.g. to determine if a swipe is starting
> + * close to a touchpad edge, or to synchronize an animation with how much
> + * percent of the width of a touchpad a swipe gesture has travelled.
> + *
> + * The returned value is in the range of 0.0 to 1.0 with 0.0 indicating that
> + * the center is on the right edge of the touchpad, and 1.0 indicating that it
> + * is on the left edge.
> + *
> + * For gesture events that are not of type
> + * @ref LIBINPUT_EVENT_GESTURE_SWIPE_START, @ref LIBINPUT_EVENT_GESTURE_SWIPE
> + * or @ref LIBINPUT_EVENT_GESTURE_SWIPE_STOP this function returns 0.
> + *
> + * @note It is an application bug to call this function for events other than
> + * @ref LIBINPUT_EVENT_GESTURE_SWIPE_START, @ref LIBINPUT_EVENT_GESTURE_SWIPE
> + * or @ref LIBINPUT_EVENT_GESTURE_SWIPE_STOP.
> + *
> + * @return the current absolute x coordinate
> + */
> +double
> +libinput_event_gesture_get_absolute_x(struct libinput_event_gesture *event);
> +
> +/**
> + * @ingroup event_gesture
> + *
> + * Return the absolute y coordinate of the current center position of a
> + * swipe gesture. This can be used e.g. to determine if a swipe is starting
> + * close to a touchpad edge, or to synchronize an animation with how much
> + * percent of the height of a touchpad a swipe gesture has travelled.
> + *
> + * The returned value is in the range of 0.0 to 1.0 with 0.0 indicating that
> + * the center is on the top edge of the touchpad, and 1.0 indicating that it
> + * is on the bottom edge.
> + *
> + * For gesture events that are not of type
> + * @ref LIBINPUT_EVENT_GESTURE_SWIPE_START, @ref LIBINPUT_EVENT_GESTURE_SWIPE
> + * or @ref LIBINPUT_EVENT_GESTURE_SWIPE_STOP this function returns 0.
> + *
> + * @note It is an application bug to call this function for events other than
> + * @ref LIBINPUT_EVENT_GESTURE_SWIPE_START, @ref LIBINPUT_EVENT_GESTURE_SWIPE
> + * or @ref LIBINPUT_EVENT_GESTURE_SWIPE_STOP.
> + *
> + * @return the current absolute y coordinate
> + */
> +double
> +libinput_event_gesture_get_absolute_y(struct libinput_event_gesture *event);
> +
> +/**
>   * @defgroup base Initialization and manipulation of libinput contexts
>   */
>  
> -- 
> 2.1.0
> 
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