1.27.4. Comparison with old cropping API

The selection API was introduced to cope with deficiencies of the older CROP API, 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 v4l2_pix_format. Introducing an image offsets could be done by modifying field m_userptr at struct v4l2_buffer before calling 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 v4l2_crop and struct v4l2_cropcap have no reserved fields. Therefore there is no way to extend their functionality. The new struct 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.