40 lines
1.9 KiB
ReStructuredText
Executable file
40 lines
1.9 KiB
ReStructuredText
Executable file
.. -*- coding: utf-8; mode: rst -*-
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.. _selection-vs-crop:
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********************************
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Comparison with old cropping API
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********************************
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The selection API was introduced to cope with deficiencies of the
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older :ref:`CROP API <crop>`, that was designed to control simple
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capture devices. Later the cropping API was adopted by video output
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drivers. The ioctls are used to select a part of the display were the
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video signal is inserted. It should be considered as an API abuse
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because the described operation is actually the composing. The
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selection API makes a clear distinction between composing and cropping
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operations by setting the appropriate targets.
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The CROP API lacks any support for composing to and cropping from an
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image inside a memory buffer. The application could configure a
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capture device to fill only a part of an image by abusing V4L2
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API. Cropping a smaller image from a larger one is achieved by setting
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the field ``bytesperline`` at struct :c:type:`v4l2_pix_format`.
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Introducing an image offsets could be done by modifying field
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``m_userptr`` at struct :c:type:`v4l2_buffer` before calling
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:ref:`VIDIOC_QBUF <VIDIOC_QBUF>`. Those operations should be avoided
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because they are not portable (endianness), and do not work for
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macroblock and Bayer formats and mmap buffers.
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The selection API deals with configuration of buffer
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cropping/composing in a clear, intuitive and portable way. Next, with
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the selection API the concepts of the padded target and constraints
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flags are introduced. Finally, struct :c:type:`v4l2_crop` and struct
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:c:type:`v4l2_cropcap` have no reserved fields. Therefore there is no
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way to extend their functionality. The new struct
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:c:type:`v4l2_selection` provides a lot of place for future
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extensions.
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Driver developers are encouraged to implement only selection API. The
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former cropping API would be simulated using the new one.
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