93 lines
4.1 KiB
ReStructuredText
93 lines
4.1 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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.. _format:
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************
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Data Formats
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************
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Data Format Negotiation
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=======================
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Different devices exchange different kinds of data with applications,
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for example video images, raw or sliced VBI data, RDS datagrams. Even
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within one kind many different formats are possible, in particular an
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abundance of image formats. Although drivers must provide a default and
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the selection persists across closing and reopening a device,
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applications should always negotiate a data format before engaging in
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data exchange. Negotiation means the application asks for a particular
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format and the driver selects and reports the best the hardware can do
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to satisfy the request. Of course applications can also just query the
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current selection.
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A single mechanism exists to negotiate all data formats using the
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aggregate struct :c:type:`v4l2_format` and the
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and
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:ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctls. Additionally the
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:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` ioctl can be used to examine
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what the hardware *could* do, without actually selecting a new data
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format. The data formats supported by the V4L2 API are covered in the
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respective device section in :ref:`devices`. For a closer look at
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image formats see :ref:`pixfmt`.
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The :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl is a major turning-point in the
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initialization sequence. Prior to this point multiple panel applications
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can access the same device concurrently to select the current input,
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change controls or modify other properties. The first :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`
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assigns a logical stream (video data, VBI data etc.) exclusively to one
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file descriptor.
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Exclusive means no other application, more precisely no other file
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descriptor, can grab this stream or change device properties
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inconsistent with the negotiated parameters. A video standard change for
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example, when the new standard uses a different number of scan lines,
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can invalidate the selected image format. Therefore only the file
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descriptor owning the stream can make invalidating changes. Accordingly
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multiple file descriptors which grabbed different logical streams
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prevent each other from interfering with their settings. When for
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example video overlay is about to start or already in progress,
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simultaneous video capturing may be restricted to the same cropping and
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image size.
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When applications omit the :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl its locking side
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effects are implied by the next step, the selection of an I/O method
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with the :ref:`VIDIOC_REQBUFS` ioctl or implicit
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with the first :ref:`read() <func-read>` or
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:ref:`write() <func-write>` call.
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Generally only one logical stream can be assigned to a file descriptor,
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the exception being drivers permitting simultaneous video capturing and
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overlay using the same file descriptor for compatibility with V4L and
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earlier versions of V4L2. Switching the logical stream or returning into
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"panel mode" is possible by closing and reopening the device. Drivers
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*may* support a switch using :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>`.
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All drivers exchanging data with applications must support the
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:ref:`VIDIOC_G_FMT <VIDIOC_G_FMT>` and :ref:`VIDIOC_S_FMT <VIDIOC_G_FMT>` ioctl. Implementation of the
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:ref:`VIDIOC_TRY_FMT <VIDIOC_G_FMT>` is highly recommended but optional.
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Image Format Enumeration
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========================
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Apart of the generic format negotiation functions a special ioctl to
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enumerate all image formats supported by video capture, overlay or
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output devices is available. [#f1]_
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The :ref:`VIDIOC_ENUM_FMT` ioctl must be supported
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by all drivers exchanging image data with applications.
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.. important::
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Drivers are not supposed to convert image formats in kernel space.
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They must enumerate only formats directly supported by the hardware.
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If necessary driver writers should publish an example conversion
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routine or library for integration into applications.
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.. [#f1]
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Enumerating formats an application has no a-priori knowledge of
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(otherwise it could explicitly ask for them and need not enumerate)
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seems useless, but there are applications serving as proxy between
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drivers and the actual video applications for which this is useful.
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