Documentation/userspace-api/media/v4l/selection-api-vs-crop-api.rst
Source file repositories/reference/linux-study-clean/Documentation/userspace-api/media/v4l/selection-api-vs-crop-api.rst
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Documentation/userspace-api/media/v4l/selection-api-vs-crop-api.rst- Extension
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- Support Tooling And Documentation
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- Documentation
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- Support Tooling And Documentation: documentation
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- atlas-only
Why This File Exists
Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
- Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.
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- No C-style include directives detected by the generator.
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- No top-level syscall, struct, function, initcall, or export declaration detected by the generator.
Annotated Snippet
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
.. c:namespace:: V4L
.. _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.Annotation
- Atlas domain: Support Tooling And Documentation / Documentation.
- Implementation status: atlas-only.
Implementation Notes
- This generated page is the file-by-file coverage layer; curated subsystem chapters should link here when they synthesize a multi-file control flow.
- Core OS pages should be promoted from atlas-only to deep-reviewed when they explain data structures, invariants, locking, lifecycle, and C implementation snippets.
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