Merge remote-tracking branch 'mmc-uh/next'

[deliverable/linux.git] / Documentation / media / uapi / v4l / selection-api-005.rst

.. -*- coding: utf-8; mode: rst -*-

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Comparison with old cropping API
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The selection API was introduced to cope with deficiencies of previous
:ref:`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 V4L2 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`. 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.