mirror of https://gitee.com/openkylin/linux.git
35 lines
1.9 KiB
ReStructuredText
35 lines
1.9 KiB
ReStructuredText
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.. -*- coding: utf-8; mode: rst -*-
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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 previous
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:ref:`API <crop>`, that was designed to control simple capture
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devices. Later the cropping API was adopted by video output drivers. The
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ioctls are used to select a part of the display were the video signal is
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inserted. It should be considered as an API abuse because the described
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operation is actually the composing. The selection API makes a clear
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distinction between composing and cropping operations by setting the
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appropriate targets. The V4L2 API lacks any support for composing to and
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cropping from an image inside a memory buffer. The application could
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configure a capture device to fill only a part of an image by abusing
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V4L2 API. Cropping a smaller image from a larger one is achieved by
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setting the field ``bytesperline`` at struct
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:ref:`v4l2_pix_format <v4l2-pix-format>`.
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Introducing an image offsets could be done by modifying field ``m_userptr``
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at struct
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:ref:`v4l2_buffer <v4l2-buffer>` before calling
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:ref:`VIDIOC_QBUF`. Those operations should be avoided because they are not
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portable (endianness), and do not work for macroblock and Bayer formats
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and mmap buffers. 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 :ref:`v4l2_crop <v4l2-crop>`
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and struct :ref:`v4l2_cropcap <v4l2-cropcap>` have no reserved
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fields. Therefore there is no way to extend their functionality. The new
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struct :ref:`v4l2_selection <v4l2-selection>` provides a lot of place
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for future extensions. Driver developers are encouraged to implement
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only selection API. The former cropping API would be simulated using the
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new one.
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