doc-rst: linux_tv DocBook to reST migration (docs-next)

This is the restructuredText (reST) migration of the ``media``
DocBook-XML set from the linux_tv project.

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/Documentation/linux_tv/media/v4l/selection-api-005.rst b/Documentation/linux_tv/media/v4l/selection-api-005.rst
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+.. -*- coding: utf-8; mode: rst -*-
+
+********************************
+Comparison with old cropping API
+********************************
+
+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 struct
+:ref:`v4l2_pix_format <v4l2-pix-format>```::bytesperline``.
+Introducing an image offsets could be done by modifying field struct
+:ref:`v4l2_buffer <v4l2-buffer>```::m_userptr`` before calling
+``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 :ref:`v4l2_crop <v4l2-crop>`
+and struct :ref:`v4l2_cropcap <v4l2-cropcap>` have no reserved
+fields. Therefore there is no way to extend their functionality. The new
+struct :ref:`v4l2_selection <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.
+
+
+.. ------------------------------------------------------------------------------
+.. This file was automatically converted from DocBook-XML with the dbxml
+.. library (https://github.com/return42/sphkerneldoc). The origin XML comes
+.. from the linux kernel, refer to:
+..
+.. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook
+.. ------------------------------------------------------------------------------