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linux-next/Documentation/userspace-api/media/v4l/selection-api-vs-crop-api.rst
Mauro Carvalho Chehab 54f38fcae5 media: docs: move uAPI book to userspace-api/media
Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee38 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
2020-04-14 10:31:49 +02:00

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.. Permission is granted to copy, distribute and/or modify this
.. document under the terms of the GNU Free Documentation License,
.. Version 1.1 or any later version published by the Free Software
.. Foundation, with no Invariant Sections, no Front-Cover Texts
.. and no Back-Cover Texts. A copy of the license is included at
.. Documentation/userspace-api/media/fdl-appendix.rst.
..
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
.. _selection-vs-crop:
********************************
Comparison with old cropping API
********************************
The selection API was introduced to cope with deficiencies of the
older :ref:`CROP API <crop>`, that was designed to control simple
capture devices. Later the cropping API was adopted by video output
drivers. The ioctls are used to select a part of the display were the
video signal is inserted. It should be considered as an API abuse
because the described operation is actually the composing. The
selection API makes a clear distinction between composing and cropping
operations by setting the appropriate targets.
The CROP API lacks any support for composing to and cropping from an
image inside a memory buffer. The application could configure a
capture device to fill only a part of an image by abusing V4L2
API. Cropping a smaller image from a larger one is achieved by setting
the field ``bytesperline`` at struct :c:type:`v4l2_pix_format`.
Introducing an image offsets could be done by modifying field
``m_userptr`` at struct :c:type:`v4l2_buffer` before calling
:ref:`VIDIOC_QBUF <VIDIOC_QBUF>`. Those operations should be avoided
because they are not portable (endianness), and do not work for
macroblock and Bayer formats and mmap buffers.
The selection API deals with configuration of buffer
cropping/composing in a clear, intuitive and portable way. Next, with
the selection API the concepts of the padded target and constraints
flags are introduced. Finally, struct :c:type:`v4l2_crop` and struct
:c:type:`v4l2_cropcap` have no reserved fields. Therefore there is no
way to extend their functionality. The new struct
:c:type:`v4l2_selection` provides a lot of place for future
extensions.
Driver developers are encouraged to implement only selection API. The
former cropping API would be simulated using the new one.