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diff --git a/Documentation/media/uapi/v4l/selection-api-005.rst b/Documentation/media/uapi/v4l/selection-api-005.rst new file mode 100644 index 000000000000..94731a13efdb --- /dev/null +++ b/Documentation/media/uapi/v4l/selection-api-005.rst @@ -0,0 +1,34 @@ +.. -*- 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 ``bytesperline`` at struct +:ref:`v4l2_pix_format <v4l2-pix-format>`. +Introducing an image offsets could be done by modifying field ``m_userptr`` +at struct +:ref:`v4l2_buffer <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 :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. |