Android supports a variety of touch screens and touch pads, including stylus-based digitizer tablets.
Touch screens are touch devices that are associated with a display such that the user has the impression of directly manipulating items on screen.
Touch pads are touch devices that aren't associated with a display, such as a digitizer tablet. Touch pads are typically used for pointing or for absolute indirect positioning or gesture-based control of a user interface.
Touch devices can have buttons whose functions are similar to mouse buttons.
Touch devices can sometimes be manipulated using a variety of different tools such as fingers or a stylus depending on the underlying touch sensor technology.
Touch devices are sometimes used to implement virtual keys. For example, on some Android devices, the touch screen sensor area extends beyond the edge of the display and serves dual purpose as part of a touch sensitive key pad.
Due to the great variety of touch devices, Android relies on a large number of configuration properties to describe the characteristics and desired behavior of each device.
Touch device classification
An input device is classified as a multi-touch device if both of the following conditions hold:
- The input device reports the presence of the
ABS_MT_POSITION_X
andABS_MT_POSITION_Y
absolute axes. - The input device does not have any gamepad buttons. This condition resolves an ambiguity with certain gamepads that report axes with codes that overlaps those of the MT axes.
An input device is classified as a single-touch device if both of the following conditions hold:
- The input device is not classified as a multi-touch device. An input device is either classified as a single-touch device or as a multi-touch device, never both.
- The input device reports the presence of the
ABS_X
andABS_Y
absolute axes, and the presence of theBTN_TOUCH
key code.
When an input device is classified as a touch device, the presence of virtual keys is determined by attempting to load the virtual key map file for the device. If a virtual key map is available, then the key layout file for the device is also loaded. Refer to [Virtual key map files](#virtual-key-map-files) for information about about the location and format of these files.
Next, the system loads the input device configuration file for the touch device.
All built-in touch devices should have input device configuration files. If no input device configuration file is present, the system chooses a default configuration that's appropriate for general-purpose touch peripherals such as external USB or Bluetooth HID touch screens or touch pads. These defaults aren't designed for built-in touch screens and can result in incorrect behavior.
After the input device configuration loaded, the system classifies the input device as a touch screen, touch pad, or pointer device.
- A touch screen device is used for direct manipulation of objects on the screen. The user directly touches the screen, so the system doesn't require any additional affordances to indicate the objects being manipulated.
- A touch pad device is used to provide absolute positioning information to an app about touches on a given sensor area. It can be useful for digitizer tablets.
- A pointer device is used for indirect manipulation of objects on the screen using a cursor. Fingers are interpreted as multi-touch pointer gestures. Other tools, such as styluses, are interpreted using absolute positions. See Indirect multi-touch pointer gestures for more information.
The following rules are used to classify the input device as a touch screen, touch pad, or pointer device.
- If the
touch.deviceType
property is set, then the device type is set as indicated. - If the input device reports the presence of the
INPUT_PROP_DIRECT
input property (through theEVIOCGPROP
ioctl), then the device type is set to touch screen. This condition assumes that direct input touch devices are attached to a display that is also connected. - If the input device reports the presence of the
INPUT_PROP_POINTER
input property (through theEVIOCGPROP
ioctl), then the device type set to pointer. - If the input device reports the presence of the
REL_X
orREL_Y
relative axes, then the device type is set to touch pad. This condition resolves an ambiguity for input devices that consist of both a mouse and a touch pad. In this case, the touch pad isn't used to control the pointer because the mouse already controls it. - Otherwise, the device type is set to pointer. This default ensures that touch pads that haven't been designated any other special purpose control the pointer.
Buttons
Buttons are optional controls that apps can use to perform additional functions. Buttons on touch devices behave similarly to mouse buttons and are mainly of use with pointer type touch devices or with a stylus.
The following buttons are supported:
BTN_LEFT
: Mapped toMotionEvent.BUTTON_PRIMARY
.BTN_RIGHT
: Mapped toMotionEvent.BUTTON_SECONDARY
.BTN_MIDDLE
: Mapped toMotionEvent.BUTTON_MIDDLE
.BTN_BACK
andBTN_SIDE
: Mapped toMotionEvent.BUTTON_BACK
. Pressing this button also synthesizes a key press with the key codeKeyEvent.KEYCODE_BACK
.BTN_FORWARD
andBTN_EXTRA
: Mapped toMotionEvent.BUTTON_FORWARD
. Pressing this button also synthesizes a key press with the key codeKeyEvent.KEYCODE_FORWARD
.BTN_STYLUS
: Mapped toMotionEvent.BUTTON_SECONDARY
.BTN_STYLUS2
: Mapped toMotionEvent.BUTTON_TERTIARY
.
Tools and tool types
A tool is a finger, stylus, or other apparatus that is used to interact with the touch device. Some touch devices can distinguish between different types of tools.
Elsewhere in Android, as in the MotionEvent
API, a tool is often referred
to as a pointer.
The following tool types are supported:
BTN_TOOL_FINGER
andMT_TOOL_FINGER
: Mapped toMotionEvent.TOOL_TYPE_FINGER
.BTN_TOOL_PEN
andMT_TOOL_PEN
: Mapped toMotionEvent.TOOL_TYPE_STYLUS
.BTN_TOOL_RUBBER
: Mapped toMotionEvent.TOOL_TYPE_ERASER
.BTN_TOOL_BRUSH
: Mapped toMotionEvent.TOOL_TYPE_STYLUS
.BTN_TOOL_PENCIL
: Mapped toMotionEvent.TOOL_TYPE_STYLUS
.BTN_TOOL_AIRBRUSH
: Mapped toMotionEvent.TOOL_TYPE_STYLUS
.BTN_TOOL_MOUSE
: Mapped toMotionEvent.TOOL_TYPE_MOUSE
.BTN_TOOL_LENS
: Mapped toMotionEvent.TOOL_TYPE_MOUSE
.BTN_TOOL_DOUBLETAP
,BTN_TOOL_TRIPLETAP
, andBTN_TOOL_QUADTAP
: Mapped toMotionEvent.TOOL_TYPE_FINGER
.
Hovering versus touching tools
Tools can either be in contact with the touch device or in range and hovering above it. Not all touch devices can sense the presence of a tool hovering above the touch device. Those that do, such as RF-based stylus digitizers, can often detect when the tool is within a limited range of the digitizer.
The InputReader
component distinguishes touching tools from hovering
tools. Likewise, touching tools and hovering tools are reported to apps
in different ways.
Touching tools are reported to apps as touch events
using MotionEvent.ACTION_DOWN
, MotionEvent.ACTION_MOVE
,
MotionEvent.ACTION_DOWN
,
MotionEvent.ACTION_POINTER_DOWN
and MotionEvent.ACTION_POINTER_UP
.
Hovering tools are reported to apps as generic motion events using
MotionEvent.ACTION_HOVER_ENTER
, MotionEvent.ACTION_HOVER_MOVE
,
and MotionEvent.ACTION_HOVER_EXIT
.
Touch device driver requirements
- Touch device drivers should register only the axes and key codes for the axes
and buttons that they support. Registering unsupported axes or key codes
can confuse the device classification algorithm or cause the system to incorrectly
detect the capabilities of the device. For example, if the device reports the
BTN_TOUCH
key code, the system assumes thatBTN_TOUCH
is always used to indicate whether the tool is touching the screen. Therefore,BTN_TOUCH
shouldn't be used to indicate that the tool is merely in the range and hovering. - Single-touch devices use the following Linux input events:
ABS_X
: (REQUIRED) Reports the X coordinate of the tool.ABS_Y
: (REQUIRED) Reports the Y coordinate of the tool.ABS_PRESSURE
: (optional) Reports the physical pressure applied to the tip of the tool or the signal strength of the touch contact.ABS_TOOL_WIDTH
: (optional) Reports the cross-sectional area or width of the touch contact or of the tool itself.ABS_DISTANCE
: (optional) Reports the distance of the tool from the surface of the touch device.ABS_TILT_X
: (optional) Reports the tilt of the tool from the surface of the touch device along the X axis.ABS_TILT_Y
: (optional) Reports the tilt of the tool from the surface of the touch device along the Y axis.BTN_TOUCH
: (REQUIRED) Indicates whether the tool is touching the device.BTN_LEFT
,BTN_RIGHT
,BTN_MIDDLE
,BTN_BACK
,BTN_SIDE
,BTN_FORWARD
,BTN_EXTRA
,BTN_STYLUS
,BTN_STYLUS2
: (optional) Reports button states.BTN_TOOL_FINGER
,BTN_TOOL_PEN
,BTN_TOOL_RUBBER
,BTN_TOOL_BRUSH
,BTN_TOOL_PENCIL
,BTN_TOOL_AIRBRUSH
,BTN_TOOL_MOUSE
,BTN_TOOL_LENS
,BTN_TOOL_DOUBLETAP
,BTN_TOOL_TRIPLETAP
,BTN_TOOL_QUADTAP
: (optional) Reports the tool type.
- Multi-touch devices use the following Linux input events:
ABS_MT_POSITION_X
: (REQUIRED) Reports the X coordinate of the tool.ABS_MT_POSITION_Y
: (REQUIRED) Reports the Y coordinate of the tool.ABS_MT_PRESSURE
: (optional) Reports the physical pressure applied to the tip of the tool or the signal strength of the touch contact.ABS_MT_TOUCH_MAJOR
: (optional) Reports the cross-sectional area of the touch contact, or the length of the longer dimension of the touch contact.ABS_MT_TOUCH_MINOR
: (optional) Reports the length of the shorter dimension of the touch contact. This axis should not be used ifABS_MT_TOUCH_MAJOR
is reporting an area measurement.ABS_MT_WIDTH_MAJOR
: (optional) Reports the cross-sectional area of the tool itself, or the length of the longer dimension of the tool itself. Don't use this axis unless you know the dimensions of the tool itself.ABS_MT_WIDTH_MINOR
: (optional) Reports the length of the shorter dimension of the tool itself. This axis should not be used ifABS_MT_WIDTH_MAJOR
is reporting an area measurement or if the dimensions of the tool itself are unknown.ABS_MT_ORIENTATION
: (optional) Reports the orientation of the tool.ABS_MT_DISTANCE
: (optional) Reports the distance of the tool from the surface of the touch device.ABS_MT_TOOL_TYPE
: (optional) Reports the tool type asMT_TOOL_FINGER
orMT_TOOL_PEN
.ABS_MT_TRACKING_ID
: (optional) Reports the tracking ID of the tool. The tracking ID is an arbitrary non-negative integer that is used to identify and track each tool independently when multiple tools are active. For example, when multiple fingers are touching the device, each finger should be assigned a distinct tracking ID that is used as long as the finger remains in contact. Tracking IDs can be reused when their associated tools move out of range.ABS_MT_SLOT
: (optional) Reports the slot ID of the tool, when using the Linux multi-touch protocol 'B'. Refer to the Linux multi-touch protocol documentation for more details.BTN_TOUCH
: (REQUIRED) Indicates whether the tool is touching the device.BTN_LEFT
,BTN_RIGHT
,BTN_MIDDLE
,BTN_BACK
,BTN_SIDE
,BTN_FORWARD
,BTN_EXTRA
,BTN_STYLUS
,BTN_STYLUS2
: (optional) Reports button states.BTN_TOOL_FINGER
,BTN_TOOL_PEN
,BTN_TOOL_RUBBER
,BTN_TOOL_BRUSH
,BTN_TOOL_PENCIL
,BTN_TOOL_AIRBRUSH
,BTN_TOOL_MOUSE
,BTN_TOOL_LENS
,BTN_TOOL_DOUBLETAP
,BTN_TOOL_TRIPLETAP
,BTN_TOOL_QUADTAP
: (optional) Reports the tool type.
- If axes for both the single-touch and multi-touch protocol are defined, then only the multi-touch axes are used and the single-touch axes are ignored.
The minimum and maximum values of the
ABS_X
,ABS_Y
,ABS_MT_POSITION_X
, andABS_MT_POSITION_Y
axes define the bounds of the active area of the device in device-specific surface units. In the case of a touch screen, the active area describes the part of the touch device that actually covers the display.For a touch screen, the system automatically interpolates the reported touch positions in surface units to obtain touch positions in display pixels according to the following calculation:
displayX = (x - minX) * displayWidth / (maxX - minX + 1) displayY = (y - minY) * displayHeight / (maxY - minY + 1)
A touch screen might report touches outside of the reported active area.
Touches that are initiated outside the active area are not delivered to apps but can be used for virtual keys.
Touches that are initiated inside the active area, or that enter and exit the display area are delivered to apps. Consequently, if a touch starts within the bounds of an app and then moves outside of the active area, the app might receive touch events with display coordinates that are negative or beyond the bounds of the display. This is expected behavior.
A touch device should never clamp touch coordinates to the bounds of the active area. If a touch exits the active area, it should be reported as being outside of the active area, or it should not be reported at all.
For example, if the user's finger is touching near the top-left corner of the touch screen, it might report a coordinate of (minX, minY). If the finger continues to move further outside of the active area, the touch screen should either start reporting coordinates with components less than minX and minY, such as (minX - 2, minY - 3), or it should stop reporting the touch altogether. In other words, the touch screen should not be reporting (minX, minY) when the user's finger is really touching outside of the active area.
Clamping touch coordinates to the display edge creates an artificial hard boundary around the edge of the screen which prevents the system from smoothly tracking motions that enter or exit the bounds of the display area.
The values reported by
ABS_PRESSURE
orABS_MT_PRESSURE
, if they are reported at all, must be non-zero when the tool is touching the device and zero otherwise to indicate that the tool is hovering.Reporting pressure information is optional but strongly recommended. Apps can use pressure information to implement pressure-sensitive drawing and other effects.
The values reported by
ABS_TOOL_WIDTH
,ABS_MT_TOUCH_MAJOR
,ABS_MT_TOUCH_MINOR
,ABS_MT_WIDTH_MAJOR
, orABS_MT_WIDTH_MINOR
should be non-zero when the tool is touching the device and zero otherwise, but this isn't required. For example, the touch device might be able to measure the size of finger touch contacts but not stylus touch contacts.Reporting size information is optional but strongly recommended. Apps can use pressure information to implement size-sensitive drawing and other effects.
The values reported by
ABS_DISTANCE
orABS_MT_DISTANCE
should approach zero when the tool is touching the device. The distance can remain non-zero even when the tool is in direct contact. The exact values reported depend on the manner in which the hardware measures distance.Reporting distance information is optional but recommended for stylus devices.
The values reported by
ABS_TILT_X
andABS_TILT_Y
should be zero when the tool is perpendicular to the device. A non-zero tilt indicates that the tool is held at an incline.The tilt angles along the X and Y axes are assumed to be specified in degrees from perpendicular. The center point (perfectly perpendicular) is given by
(max + min) / 2
for each axis. Values smaller than the center point represent a tilt up or to the left, values larger than the center point represent a tilt down or to the right.InputReader
converts the X and Y tilt components into a perpendicular tilt angle ranging from 0 toPI / 2
radians and a planar orientation angle ranging from-PI
toPI
radians. This representation results in a description of orientation that is compatible with what is used to describe finger touches.Reporting tilt information is optional but recommended for stylus devices.
If the tool type is reported by
ABS_MT_TOOL_TYPE
, it supersedes any tool type information reported byBTN_TOOL_*
. If no tool type information is available at all, the tool type defaults toMotionEvent.TOOL_TYPE_FINGER
.A tool is determined to be active based on the following conditions:
When using the single-touch protocol, the tool is active if
BTN_TOUCH
, orBTN_TOOL_*
is 1.This condition implies that
InputReader
needs to have at least some information about the nature of the tool, either whether it is touching, or at least its tool type. If no information is available, then the tool is assumed to be inactive (out of range).- When using the multi-touch protocol 'A', the tool is active whenever it appears in the most recent sync report. When the tool stops appearing in sync reports, it ceases to exist.
- When using the multi-touch protocol 'B', the tool is active as long as it has an active slot. When the slot it cleared, the tool ceases to exist.
- A tool is determined to be hovering based on the following conditions:
- If the tool is
BTN_TOOL_MOUSE
orBTN_TOOL_LENS
, then the tool is not hovering, even if either of the following conditions are true. - If the tool is active and the driver reports pressure information, and the reported pressure is zero, then the tool is hovering.
- If the tool is active and the driver supports the
BTN_TOUCH
key code andBTN_TOUCH
has a value of zero, then the tool is hovering.
- If the tool is
InputReader
supports both multi-touch protocol 'A' and 'B'. New drivers should use the 'B' protocol but either works.As of Android 4.0, touch screen drivers might need to be changed to comply with the Linux input protocol specification.
The following changes might be required:
When a tool becomes inactive (finger goes "up"), it should stop appearing in subsequent multi-touch sync reports. When all tools become inactive (all fingers go "up"), the driver should send an empty sync report packet, such as
SYN_MT_REPORT
followed bySYN_REPORT
.Previous versions of Android expected "up" events to be reported by sending a pressure value of 0. The old behavior was incompatible with the Linux input protocol specification and is no longer supported.
Physical pressure or signal strength information should be reported using
ABS_MT_PRESSURE
.Previous versions of Android retrieved pressure information from
ABS_MT_TOUCH_MAJOR
. The old behavior was incompatible with the Linux input protocol specification and is no longer supported.- Touch size information should be reported using
ABS_MT_TOUCH_MAJOR
.Previous versions of Android retrieved size information from
ABS_MT_TOOL_MAJOR
. The old behavior was incompatible with the Linux input protocol specification and is no longer supported.
Touch device operation
The following is a brief summary of the touch device operation on Android.
EventHub
reads raw events from theevdev
driver.InputReader
consumes the raw events and updates internal state about the position and other characteristics of each tool. It also tracks button states.- If BACK or FORWARD was pressed or released,
InputReader
notifiesInputDispatcher
about the key event. InputReader
determines whether a virtual key press occurred. If so, it notifiesInputDispatcher
about the key event.InputReader
determines whether the touch was initiated within the bounds of the display. If so, it notifiesInputDispatcher
about the touch event.- If there are no touching tools but there is at least one hovering tool,
InputReader
notifiesInputDispatcher
about the hover event. - If the touch device type is pointer,
InputReader
performs pointer gesture detection, moves the pointer and spots accordingly and notifiesInputDispatcher
about the pointer event. InputDispatcher
usesWindowManagerPolicy
to determine whether the events should be dispatched and whether they should wake the device. Then,InputDispatcher
delivers the events to the appropriate apps.
Touch device configuration
Touch device behavior is determined by the device's axes, buttons, input properties, input device configuration, virtual key map and key layout.
Refer to the following sections for more details about the files that participate in keyboard configuration:
Properties
The system relies on many input device configuration properties to configure and calibrate touch device behavior.
One reason for this is that the device drivers for touch devices often report the characteristics of touches using device-specific units.
For example, many touch devices measure the touch contact area using an internal device-specific scale, such as the total number of sensor nodes that were triggered by the touch. This raw size value would not be meaningful to apps because they would need to know about the physical size and other characteristics of the touch device sensor nodes.
The system uses calibration parameters encoded in input device configuration files to decode, transform, and normalize the values reported by the touch device into a simpler standard representation that apps can understand.
Documentation conventions
For documentation purposes, we use the following conventions to describe the values used by the system during the calibration process.
Raw axis values
The following expressions denote the raw values reported by the touch
device driver as EV_ABS
events.
raw.x
- The value of the
ABS_X
orABS_MT_POSITION_X
axis. raw.y
- The value of the
ABS_Y
orABS_MT_POSITION_Y
axis. raw.pressure
- The value of the
ABS_PRESSURE
orABS_MT_PRESSURE
axis, or 0 if not available. raw.touchMajor
- The value of the
ABS_MT_TOUCH_MAJOR
axis, or 0 if not available. raw.touchMinor
- The value of the
ABS_MT_TOUCH_MINOR
axis, orraw.touchMajor
if not available. raw.toolMajor
- The value of the
ABS_TOOL_WIDTH
orABS_MT_WIDTH_MAJOR
axis, or 0 if not available. raw.toolMinor
- The value of the
ABS_MT_WIDTH_MINOR
axis, orraw.toolMajor
if not available. raw.orientation
- The value of the
ABS_MT_ORIENTATION
axis, or 0 if not available. raw.distance
- The value of the
ABS_DISTANCE
orABS_MT_DISTANCE
axis, or 0 if not available. raw.tiltX
- The value of the
ABS_TILT_X
axis, or 0 if not available. raw.tiltY
- The value of the
ABS_TILT_Y
axis, or 0 if not available.
Raw axis ranges
The following expressions denote the bounds of raw values. They are obtained
by calling EVIOCGABS
ioctl for each axis.
raw.*.min
- The inclusive minimum value of the raw axis.
raw.*.max
- The inclusive maximum value of the raw axis.
raw.*.range
- Equivalent to
raw.*.max - raw.*.min
. raw.*.fuzz
- The accuracy of the raw axis. eg. fuzz = 1 implies values are accurate to +/- 1 unit.
raw.width
- The inclusive width of the touch area, equivalent to
raw.x.range + 1
. raw.height
- The inclusive height of the touch area, equivalent to
raw.y.range + 1
.
Output ranges
The following expressions denote the characteristics of the output coordinate system. The system uses linear interpolation to translate touch position information from the surface units used by the touch device into the output units that are reported to apps such as display pixels.
output.width
- The output width. For touch screens (associated with a display), this
is the display width in pixels. For touch pads (not associated with a display),
the output width equals
raw.width
, indicating that no interpolation is performed. output.height
- The output height. For touch screens (associated with a display), this
is the display height in pixels. For touch pads (not associated with a display),
the output height equals
raw.height
, indicating that no interpolation is performed. output.diag
- The diagonal length of the output coordinate system, equivalent to
sqrt(output.width ^2 + output.height ^2)
.
Basic configuration
The touch input mapper uses many configuration properties in the input device configuration file to specify calibration values. The following table describes some general purpose configuration properties. All other properties are described in the following sections along with the fields they are used to calibrate.
touch.deviceType
Definition: touch.deviceType
= touchScreen
|
touchPad
| pointer
| default
Specifies the touch device type.
-
If the value is
touchScreen
, the touch device is a touch screen associated with a display. -
If the value is
touchPad
, the touch device is a touch pad not associated with a display. -
If the value is
pointer
, the touch device is a touch pad not associated with a display, and its motions are used for indirect multi-touch pointer gestures. -
If the value is
default
, the system automatically detects the device type according to the classification algorithm.
Refer to the Classification section for more details about how the device type influences the behavior of the touch device.
In Android 3 and lower, all touch devices were assumed to be touch screens.
touch.orientationAware
Definition: touch.orientationAware
= 0
| 1
Specifies whether the touch device should react to display orientation changes.
-
If the value is
1
, touch positions reported by the touch device are rotated whenever the display orientation changes. -
If the value is
0
, touch positions reported by the touch device are immune to display orientation changes.
The default value is 1
if the device is a touch screen, 0
otherwise.
The system distinguishes between internal and external touch screens and displays. An orientation aware internal touch screen is rotated based on the orientation of the internal display. An orientation aware external touch screen is rotated based on the orientation of the external display.
Orientation awareness is used to support rotation of touch screens on devices like the Nexus One. For example, when the device is rotated clockwise 90 degrees from its natural orientation, the absolute positions of touches are remapped such that a touch in the top-left corner of the touch screen's absolute coordinate system is reported as a touch in the top-left corner of the display's rotated coordinate system. This is done so that touches are reported with the same coordinate system that apps use to draw their visual elements.
Prior to Honeycomb, all touch devices were assumed to be orientation aware.
touch.gestureMode
Definition: touch.gestureMode
= pointer
| spots
|
default
Specifies the presentation mode for pointer gestures. This configuration property is only relevant when the touch device is of type pointer.
-
If the value is
pointer
, the touch pad gestures are presented by way of a cursor similar to a mouse pointer. -
If the value is
spots
, the touch pad gestures are presented by an anchor that represents the centroid of the gesture and a set of circular spots that represent the position of individual fingers.
The default value is pointer
when the INPUT_PROP_SEMI_MT
input property
is set, or spots
otherwise.
X and Y fields
The X and Y fields provide positional information for the center of the contact area.
Calculation
The calculation is straightforward: positional information from the touch driver is linearly interpolated to the output coordinate system.
xScale = output.width / raw.width yScale = output.height / raw.height If not orientation aware or screen rotation is 0 degrees: output.x = (raw.x - raw.x.min) * xScale output.y = (raw.y - raw.y.min) * yScale Else If rotation is 90 degrees: output.x = (raw.y - raw.y.min) * yScale output.y = (raw.x.max - raw.x) * xScale Else If rotation is 180 degrees: output.x = (raw.x.max - raw.x) * xScale output.y = (raw.y.max - raw.y) * yScale Else If rotation is 270 degrees: output.x = (raw.y.max - raw.y) * yScale output.y = (raw.x - raw.x.min) * xScale End If
touchMajor, touchMinor, toolMajor, toolMinor, size fields
The touchMajor
and touchMinor
fields describe the approximate dimensions
of the contact area in output units (pixels).
The toolMajor
and toolMinor
fields describe the approximate dimensions
of the tool itself in output units (pixels).
The size
field describes the normalized size of the touch relative to
the largest possible touch that the touch device can sense. The smallest
possible normalized size is 0.0 (no contact, or it is unmeasurable), and the largest
possible normalized size is 1.0 (sensor area is saturated).
When both the approximate length and breadth can be measured, then the
touchMajor
field
specifies the longer dimension and the touchMinor
field specifies the shorter dimension
of the contact area. When only the approximate diameter of the contact area can be measured,
then the touchMajor
and touchMinor
fields are equal.
Likewise, the toolMajor
field specifies the longer dimension and the
toolMinor
field specifies the shorter dimension of the tool's cross-sectional area.
If the touch size is unavailable but the tool size is available, then the tool size is set equal to the touch size. Conversely, if the tool size is unavailable but the touch size is available, then the touch size is set equal to the tool size.
Touch devices measure or report the touch size and tool size in various ways. The current implementation supports three different kinds of measurements: diameter, area, and geometric bounding box in surface units.
Definition: touch.size.calibration
= none
|
geometric
| diameter
| area
| default
Specifies the kind of measurement used by the touch driver to report the touch size and tool size.
-
If the value is
none
, the size is set to zero. -
If the value is
geometric
, the size is assumed to be specified in the same surface units as the position, so it is scaled in the same manner. -
If the value is
diameter
, the size is assumed to be proportional to the diameter (width) of the touch or tool. -
If the value is
area
, the size is assumed to be proportional to the area of the touch or tool. -
If the value is
default
, the system uses thegeometric
calibration if theraw.touchMajor
orraw.toolMajor
axis is available, otherwise it uses thenone
calibration.
touch.size.scale
Definition: touch.size.scale
= <a non-negative floating point number>
Specifies a constant scale factor used in the calibration.
The default value is 1.0
.
touch.size.bias
Definition: touch.size.bias
= <a non-negative floating point number>
Specifies a constant bias value used in the calibration.
The default value is 0.0
.
touch.size.isSummed
Definition: touch.size.isSummed
= 0
| 1
Specifies whether the size is reported as the sum of the sizes of all active contacts, or is reported individually for each contact.
-
If the value is
1
, the reported size is divided by the number of contacts prior to use. -
If the value is
0
, the reported size is used as is.
The default value is 0
.
Some touch devices, particularly "Semi-MT" devices cannot distinguish the
individual dimensions of multiple contacts so they report a size measurement
that represents their total area or width. This property should only be set to
1
for such devices. If in doubt, set this value to 0
.
Calculation
The calculation of the touchMajor
, touchMinor
, toolMajor
,
toolMinor
,
and size
fields depends on the specified calibration parameters.
If raw.touchMajor and raw.toolMajor are available: touchMajor = raw.touchMajor touchMinor = raw.touchMinor toolMajor = raw.toolMajor toolMinor = raw.toolMinor Else If raw.touchMajor is available: toolMajor = touchMajor = raw.touchMajor toolMinor = touchMinor = raw.touchMinor Else If raw.toolMajor is available: touchMajor = toolMajor = raw.toolMajor touchMinor = toolMinor = raw.toolMinor Else touchMajor = toolMajor = 0 touchMinor = toolMinor = 0 size = 0 End If size = avg(touchMajor, touchMinor) If touch.size.isSummed == 1: touchMajor = touchMajor / numberOfActiveContacts touchMinor = touchMinor / numberOfActiveContacts toolMajor = toolMajor / numberOfActiveContacts toolMinor = toolMinor / numberOfActiveContacts size = size / numberOfActiveContacts End If If touch.size.calibration == "none": touchMajor = toolMajor = 0 touchMinor = toolMinor = 0 size = 0 Else If touch.size.calibration == "geometric": outputScale = average(output.width / raw.width, output.height / raw.height) touchMajor = touchMajor * outputScale touchMinor = touchMinor * outputScale toolMajor = toolMajor * outputScale toolMinor = toolMinor * outputScale Else If touch.size.calibration == "area": touchMajor = sqrt(touchMajor) touchMinor = touchMajor toolMajor = sqrt(toolMajor) toolMinor = toolMajor Else If touch.size.calibration == "diameter": touchMinor = touchMajor toolMinor = toolMajor End If If touchMajor != 0: output.touchMajor = touchMajor * touch.size.scale + touch.size.bias Else output.touchMajor = 0 End If If touchMinor != 0: output.touchMinor = touchMinor * touch.size.scale + touch.size.bias Else output.touchMinor = 0 End If If toolMajor != 0: output.toolMajor = toolMajor * touch.size.scale + touch.size.bias Else output.toolMajor = 0 End If If toolMinor != 0: output.toolMinor = toolMinor * touch.size.scale + touch.size.bias Else output.toolMinor = 0 End If output.size = size
pressure field
The pressure
field describes the approximate physical pressure applied to the
touch device as a normalized value between 0.0 (no touch) and 1.0 (normal pressure).
A zero pressure indicates that the tool is hovering.
touch.pressure.calibration
Definition: touch.pressure.calibration
= none
|
physical
| amplitude
| default
Specifies the kind of measurement used by the touch driver to report the pressure.
-
If the value is
none
, the pressure is unknown so it is set to 1.0 when touching and 0.0 when hovering. -
If the value is
physical
, the pressure axis is assumed to measure the actual physical intensity of pressure applied to the touch pad. -
If the value is
amplitude
, the pressure axis is assumed to measure the signal amplitude, which is related to the size of the contact and the pressure applied. -
If the value is
default
, the system uses thephysical
calibration if the pressure axis available, otherwise usesnone
.
touch.pressure.scale
Definition: touch.pressure.scale
= <a non-negative floating point number>
Specifies a constant scale factor used in the calibration.
The default value is 1.0 / raw.pressure.max
.
Calculation
The calculation of the pressure
field depends on the specified calibration parameters.
If touch.pressure.calibration == "physical" or "amplitude": output.pressure = raw.pressure * touch.pressure.scale Else If hovering: output.pressure = 0 Else output.pressure = 1 End If End If
orientation and tilt fields
The orientation
field describes the orientation of the touch and tool as an
angular measurement. An orientation of 0
indicates that the major axis is
oriented vertically, -PI/2
indicates that the major axis is oriented to the left,
PI/2
indicates that the major axis is oriented to the right. When a stylus
tool is present, the orientation range can be described in a full circle range
from -PI
or PI
.
The tilt
field describes the inclination of the tool as an angular measurement.
A tilt of 0
indicates that the tool is perpendicular to the surface.
A tilt of PI/2
indicates that the tool is flat on the surface.
touch.orientation.calibration
Definition: touch.orientation.calibration
= none
|
interpolated
| vector
| default
Specifies the kind of measurement used by the touch driver to report the orientation.
- If the value is
none
, the orientation is unknown so it is set to 0. - If the value is
interpolated
, the orientation is linearly interpolated such that a raw value ofraw.orientation.min
maps to-PI/2
and a raw value ofraw.orientation.max
maps toPI/2
. The center value of(raw.orientation.min + raw.orientation.max) / 2
maps to0
. - If the value is
vector
, the orientation is interpreted as a packed vector consisiting of two signed 4-bit fields. This representation is used on Atmel Object Based Protocol parts. When decoded, the vector yields an orientation angle and confidence magnitude. The confidence magnitude is used to scale the size information, unless it is geometric. - If the value is
default
, the system uses theinterpolated
calibration if the orientation axis available, otherwise usesnone
.
Calculation
The calculation of the orientation
and tilt
fields depends on the
specified calibration parameters and available input.
If touch.tiltX and touch.tiltY are available: tiltXCenter = average(raw.tiltX.min, raw.tiltX.max) tiltYCenter = average(raw.tiltY.min, raw.tiltY.max) tiltXAngle = (raw.tiltX - tiltXCenter) * PI / 180 tiltYAngle = (raw.tiltY - tiltYCenter) * PI / 180 output.orientation = atan2(-sin(tiltXAngle), sinf(tiltYAngle)) output.tilt = acos(cos(tiltXAngle) * cos(tiltYAngle)) Else If touch.orientation.calibration == "interpolated": center = average(raw.orientation.min, raw.orientation.max) output.orientation = PI / (raw.orientation.max - raw.orientation.min) output.tilt = 0 Else If touch.orientation.calibration == "vector": c1 = (raw.orientation & 0xF0) >> 4 c2 = raw.orientation & 0x0F If c1 != 0 or c2 != 0: If c1 >= 8 Then c1 = c1 - 16 If c2 >= 8 Then c2 = c2 - 16 angle = atan2(c1, c2) / 2 confidence = sqrt(c1*c1 + c2*c2) output.orientation = angle If touch.size.calibration == "diameter" or "area": scale = 1.0 + confidence / 16 output.touchMajor *= scale output.touchMinor /= scale output.toolMajor *= scale output.toolMinor /= scale End If Else output.orientation = 0 End If output.tilt = 0 Else output.orientation = 0 output.tilt = 0 End If If orientation aware: If screen rotation is 90 degrees: output.orientation = output.orientation - PI / 2 Else If screen rotation is 270 degrees: output.orientation = output.orientation + PI / 2 End If End If
distance field
The distance
field describes the distance between the tool and the touch device
surface. A value of 0.0 indicates direct contact and larger values indicate
increasing distance from the surface.
touch.distance.calibration
Definition: touch.distance.calibration
= none
|
scaled
| default
Specifies the kind of measurement used by the touch driver to report the distance.
-
If the value is
none
, the distance is unknown so it is set to 0. -
If the value is
scaled
, the reported distance is multiplied by a constant scale factor. -
If the value is
default
, the system uses thescaled
calibration if the distance axis available, otherwise usesnone
.
touch.distance.scale
Definition: touch.distance.scale
= <a non-negative floating point number>
Specifies a constant scale factor used in the calibration.
The default value is 1.0
.
Calculation
The calculation of the distance
field depends on the specified calibration
parameters.
If touch.distance.calibration == "scaled": output.distance = raw.distance * touch.distance.scale Else output.distance = 0 End If
Example
# Input device configuration file for a touch screen that supports pressure, # size and orientation. The pressure and size scale factors were obtained # by measuring the characteristics of the device itself and deriving # useful approximations based on the resolution of the touch sensor and the # display. # # Note that these parameters are specific to a particular device model. # Different parameters need to be used for other devices. # Basic Parameters touch.deviceType = touchScreen touch.orientationAware = 1 # Size # Based on empirical measurements, we estimate the size of the contact # using size = sqrt(area) * 28 + 0. touch.size.calibration = area touch.size.scale = 28 touch.size.bias = 0 touch.size.isSummed = 0 # Pressure # Driver reports signal strength as pressure. # # A normal index finger touch typically registers about 80 signal strength # units although we don't expect these values to be accurate. touch.pressure.calibration = amplitude touch.pressure.scale = 0.0125 # Orientation touch.orientation.calibration = vector
Compatibility notes
The configuration properties for touch devices changed significantly in Android Ice Cream Sandwich 4.0. All input device configuration files for touch devices must be updated to use the new configuration properties.
Older touch device drivers might also need to be updated.
Virtual key map files
Touch devices can be used to implement virtual keys.
There are several ways of doing this, depending on the capabilities of the touch controller. Some touch controllers can be directly configured to implement soft keys by setting firmware registers. Other times it is desirable to perform the mapping from touch coordinates to key codes in software.
When virtual keys are implemented in software, the kernel must export a virtual key map
file called virtualkeys.<devicename>
as a board property. For example,
if the touch screen device drivers reports its name as "touchyfeely" then
the virtual key map file must have the path
/sys/board_properties/virtualkeys.touchyfeely
.
A virtual key map file describes the coordinates and Linux key codes of virtual keys on the touch screen.
In addition to the virtual key map file, there must be a corresponding key layout
file and key character map file to map the Linux key codes to Android key codes and
to specify the type of the keyboard device (usually SPECIAL_FUNCTION
).
Syntax
A virtual key map file is a plain text file consisting of a sequence of virtual key layout descriptions either separated by newlines or by colons.
Comment lines begin with '#' and continue to the end of the line.
Each virtual key is described by 6 colon-delimited components:
0x01
: A version code. Must always be0x01
.- <Linux key code>: The Linux key code of the virtual key.
- <centerX>: The X pixel coordinate of the center of the virtual key.
- <centerY>: The Y pixel coordinate of the center of the virtual key.
- <width>: The width of the virtual key in pixels.
- <height>: The height of the virtual key in pixels.
All coordinates and sizes are specified in terms of the display coordinate system.
Here is a virtual key map file all written on one line.
# All on one line 0x01:158:55:835:90:55:0x01:139:172:835:125:55:0x01:102:298:835:115:55:0x01:217:412:835:95:55
The same virtual key map file can also be written on multiple lines.
# One key per line 0x01:158:55:835:90:55 0x01:139:172:835:125:55 0x01:102:298:835:115:55 0x01:217:412:835:95:55
In the above example, the touch screen has a resolution of 480x800. Accordingly, all of the virtual keys have a <centerY> coordinate of 835, which is a little bit below the visible area of the touch screen.
The first key has a Linux scan code of 158
(KEY_BACK
), centerX of
55
, centerY of 835
, width of 90
, and height of
55
.
Example
Virtual key map file: /sys/board_properties/virtualkeys.touchyfeely
.
0x01:158:55:835:90:55 0x01:139:172:835:125:55 0x01:102:298:835:115:55 0x01:217:412:835:95:55
Key layout file: /system/usr/keylayout/touchyfeely.kl
.
key 158 BACK key 139 MENU key 172 HOME key 217 SEARCH
Key character map file: /system/usr/keychars/touchyfeely.kcm
.
type SPECIAL_FUNCTION
Indirect multi-touch pointer gestures
In pointer mode, the system interprets the following gestures:
- Single finger tap: click.
- Single finger motion: move the pointer.
- Single finger motion plus button presses: drag the pointer.
- Two finger motion both fingers moving in the same direction: drag the area under the pointer in that direction. The pointer itself does not move.
- Two finger motion both fingers moving towards each other or apart in different directions: pan/scale/rotate the area surrounding the pointer. The pointer itself does not move.
- Multiple finger motion: freeform gesture.
Palm rejection
As of Android 13, the system can automatically reject inputs from palms
when the built-in framework is enabled. In-house, custom-built solutions are still supported,
though they might need to be modified to return the TOOL_TYPE_PALM
flag when a palm
is detected. The built-in framework also works in conjunction with custom solutions.
The actual model looks at the first 90 ms of gesture data, at the current pointer, and at
the surrounding pointers, then considers how far away from the display edge the touches are.
It then determines, on a per-pointer basis, which of the pointers are palms. It also takes into
account the size of each contact, as reported by touchMajor
and
touchMinor
. The Android framework then removes the pointers that are marked as
palms from the touch stream.
If a pointer was already sent to the apps, then the system either:
- (If there are other active pointers) Cancels the pointer with
ACTION_POINTER_UP
andFLAG_CANCELED
set. - (If this is the only pointer) Cancels the pointer with
ACTION_CANCEL
.
A public API, MotionEvent.FLAG_CANCELED
, indicates that the current
event shouldn't trigger user action. This flag is set for both ACTION_CANCEL
and ACTION_POINTER_UP
.
If the palm pointer wasn't sent to apps, then the system simply drops the pointer.
Enable palm rejection
- In your touch driver, use the
input_abs_set_res
macro to set the resolutions for the following fields (units are pixels per mm):ABS_MT_POSITION_X
ABS_MT_POSITION_Y
ABS_MT_TOUCH_MAJOR
ABS_MT_TOUCH_MINOR
Support for
ABS_MT_TOUCH_MINOR
is optional. However, if your device does support it, make sure the resolution is set correctly. - To confirm the fields are set correctly, run:
$ adb shell getevent -li
- To enable the feature during runtime, run:
$ adb shell device_config put input_native_boot palm_rejection_enabled 1
- Restart the
system_server
process.$ adb shell stop && adb shell start
- Confirm that
adb shell dumpsys input
shows that there are palm rejectors insideUnwantedInteractionBlocker
. If it doesn't, check the input-related logs to find clues on what might be misconfigured.See the following example for reference:
UnwantedInteractionBlocker: mEnablePalmRejection: true isPalmRejectionEnabled (flag value): true mPalmRejectors: deviceId = 3: mDeviceInfo: max_x =
max_y = x_res = 11.00 y_res = 11.00 major_radius_res = 1.00 minor_radius_res = 1.00 minor_radius_supported = true touch_major_res = 1 touch_minor_res = 1 mSlotState: mSlotsByPointerId: mPointerIdsBySlot: mSuppressedPointerIds: {} - To permanently enable the feature, add the corresponding sysprop command in your
init**rc
file:setprop persist.device_config.input_native_boot.palm_rejection_enabled 1