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1 .. -*- coding: utf-8; mode: rst -*-
3 ********************************
4 Comparison with old cropping API
5 ********************************
7 The selection API was introduced to cope with deficiencies of previous
8 :ref:`API <crop>`, that was designed to control simple capture
9 devices. Later the cropping API was adopted by video output drivers. The
10 ioctls are used to select a part of the display were the video signal is
11 inserted. It should be considered as an API abuse because the described
12 operation is actually the composing. The selection API makes a clear
13 distinction between composing and cropping operations by setting the
14 appropriate targets. The V4L2 API lacks any support for composing to and
15 cropping from an image inside a memory buffer. The application could
16 configure a capture device to fill only a part of an image by abusing
17 V4L2 API. Cropping a smaller image from a larger one is achieved by
18 setting the field ``bytesperline`` at struct
19 :c:type:`v4l2_pix_format`.
20 Introducing an image offsets could be done by modifying field ``m_userptr``
21 at struct
22 :c:type:`v4l2_buffer` before calling
23 :ref:`VIDIOC_QBUF`. Those operations should be avoided because they are not
24 portable (endianness), and do not work for macroblock and Bayer formats
25 and mmap buffers. The selection API deals with configuration of buffer
26 cropping/composing in a clear, intuitive and portable way. Next, with
27 the selection API the concepts of the padded target and constraints
28 flags are introduced. Finally, struct :c:type:`v4l2_crop`
29 and struct :c:type:`v4l2_cropcap` have no reserved
30 fields. Therefore there is no way to extend their functionality. The new
31 struct :c:type:`v4l2_selection` provides a lot of place
32 for future extensions. Driver developers are encouraged to implement
33 only selection API. The former cropping API would be simulated using the
34 new one.