From 119e30d3956046b6ae811c00a975ed3c0bf20b9c Mon Sep 17 00:00:00 2001 From: Luca Ceresoli Date: Mon, 14 May 2018 07:27:26 -0400 Subject: [PATCH] media: docs: selection: improve formatting Split section "Comparison with old cropping API" in paragraphs for easier reading and improve visible links text. Signed-off-by: Luca Ceresoli Signed-off-by: Hans Verkuil Signed-off-by: Mauro Carvalho Chehab --- .../media/uapi/v4l/selection-api-vs-crop-api.rst | 55 ++++++++++++---------- 1 file changed, 29 insertions(+), 26 deletions(-) diff --git a/Documentation/media/uapi/v4l/selection-api-vs-crop-api.rst b/Documentation/media/uapi/v4l/selection-api-vs-crop-api.rst index 2ad30a49184f..ba1064a244a0 100644 --- a/Documentation/media/uapi/v4l/selection-api-vs-crop-api.rst +++ b/Documentation/media/uapi/v4l/selection-api-vs-crop-api.rst @@ -6,31 +6,34 @@ Comparison with old cropping API ******************************** -The selection API was introduced to cope with deficiencies of previous -:ref:`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 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 -:c:type:`v4l2_pix_format`. -Introducing an image offsets could be done by modifying field ``m_userptr`` -at struct -:c:type:`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 +The selection API was introduced to cope with deficiencies of the +older :ref:`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 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 :c:type:`v4l2_pix_format`. +Introducing an image offsets could be done by modifying field +``m_userptr`` at struct :c:type:`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 :c:type:`v4l2_crop` -and struct :c:type:`v4l2_cropcap` have no reserved -fields. Therefore there is no way to extend their functionality. The new -struct :c:type:`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. +flags are introduced. Finally, struct :c:type:`v4l2_crop` and struct +:c:type:`v4l2_cropcap` have no reserved fields. Therefore there is no +way to extend their functionality. The new struct +:c:type:`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. -- 2.11.0