Author:         Harald Oehlmann <oehhar@users.sourceforge.net>
State:          Draft
Type:           Project
Vote:           Done
Created:        18-Mar-2020
Post-History:
Keywords:       Tk, bindings
Tcl-Version:    8.7

Abstract

Support for the gestures "finger scroll" and "pinch to zoom" are proposed in a platform neutral way.

NB: This tip is not ready specified yet. Anybody is invited to contribute.

Specification

Overview

Many systems (e.g. Windows, Linux Window Manager, Mac-OS, Android) nowdays support gestures in a generic manner. The gestures may be different and the input media may be different. Possible input media are a touch-screen or a touchpad.

The aim of this TIP is to make gestures provided by the system available within Tk.

The targeted gestures are:

• scrolling, for example by pointing two fingers on the touch-screen and moving it. This is often identical to the pan gesture.
• zooming, for example by pinching two fingers on a touch-screen.

There is a direct relation to the displayed image if a touch-screen is used. The scroll gesture gives a source and target point on the screen.

Platforms

The following discussion is about possible implementations on different platforms

Windows

Peter Spjuth has authored a Windows implementation in tk.branch pspjuth-touch

The two gestures are implemented by the following virtual bind tags:

• Zoom: <<PinchToZoom>> with substitutions %W %d %x %y %X %Y
  • %W: Window (below the fingers)
  • %d: descriptive dict (gesture=zoom, distance=finger distance)
  • %x,%y: position.
• Scroll: <<Gesture>> with substitutions %W %d %x %y %X %Y
  • %W: Window (below the fingers)
  • %d: descriptive dict (gesture=pan, distance=finger distance > 0 for two fingers)
  • %x,%y: position

AndroWish

Christian Werner has defined the following virtual binding tags. This information is copied and adapted from the Androwish documentation.

• Zoom: <<PinchToZoom>> with substitutions %X %Y %x %y %s

  • %X and %Y are substituted with the root window coordinate of the center of the two fingers
  • %x with the distance between the two fingers
  • %y with the angle measured in 64 times degrees CCW starting at 3 o'clock.
  • %s is the finger state: 0 (zoom motion), 1 (zoom start, i.e. 2nd finger down event), 2 (zoom end by 1st finger up event), 3 (zoom end by 2nd finger up event)

• Scroll: <<FingerDown>>, <<FingerUp>>, <<FingerMotion>>

  Scrolling by two fingers may be implemented by observing those events. The following substitutions are supported for all these three finger-related events:

  • %x and %y are the finger position scaled to {0...9999} of the device screen or viewport
  • %X and %Y are the motion difference scaled to {-9999...+9999}
  • %t is the pressure scaled to {0...9999}, and
  • %s is the finger identifier {1...10}

Linux

Mac-OS

Rationale

Implementation

The following fragments are known:

• tk.branch pspjuth-touch
• AndroWish

Discussion

Copyright

This document has been placed in the public domain.