64 lines
2.3 KiB
ReStructuredText
64 lines
2.3 KiB
ReStructuredText
.. Permission is granted to copy, distribute and/or modify this
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.. document under the terms of the GNU Free Documentation License,
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.. Version 1.1 or any later version published by the Free Software
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.. Foundation, with no Invariant Sections, no Front-Cover Texts
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.. and no Back-Cover Texts. A copy of the license is included at
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.. Documentation/media/uapi/fdl-appendix.rst.
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..
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.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
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.. _touch:
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*************
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Touch Devices
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*************
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Touch devices are accessed through character device special files named
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``/dev/v4l-touch0`` to ``/dev/v4l-touch255`` with major number 81 and
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dynamically allocated minor numbers 0 to 255.
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Overview
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========
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Sensors may be Optical, or Projected Capacitive touch (PCT).
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Processing is required to analyse the raw data and produce input events. In
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some systems, this may be performed on the ASIC and the raw data is purely a
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side-channel for diagnostics or tuning. In other systems, the ASIC is a simple
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analogue front end device which delivers touch data at high rate, and any touch
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processing must be done on the host.
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For capacitive touch sensing, the touchscreen is composed of an array of
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horizontal and vertical conductors (alternatively called rows/columns, X/Y
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lines, or tx/rx). Mutual Capacitance measured is at the nodes where the
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conductors cross. Alternatively, Self Capacitance measures the signal from each
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column and row independently.
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A touch input may be determined by comparing the raw capacitance measurement to
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a no-touch reference (or "baseline") measurement:
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Delta = Raw - Reference
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The reference measurement takes account of variations in the capacitance across
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the touch sensor matrix, for example manufacturing irregularities,
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environmental or edge effects.
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Querying Capabilities
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=====================
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Devices supporting the touch interface set the ``V4L2_CAP_VIDEO_CAPTURE`` flag
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and the ``V4L2_CAP_TOUCH`` flag in the ``capabilities`` field of
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:c:type:`v4l2_capability` returned by the
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:ref:`VIDIOC_QUERYCAP` ioctl.
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At least one of the read/write or streaming I/O methods must be
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supported.
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The formats supported by touch devices are documented in
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:ref:`Touch Formats <tch-formats>`.
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Data Format Negotiation
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=======================
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A touch device may support any I/O method.
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