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Diffstat (limited to 'Documentation/DocBook/media/v4l/selection-api.xml')
| -rw-r--r-- | Documentation/DocBook/media/v4l/selection-api.xml | 95 |
1 files changed, 47 insertions, 48 deletions
diff --git a/Documentation/DocBook/media/v4l/selection-api.xml b/Documentation/DocBook/media/v4l/selection-api.xml index 4c238ce068b..28cbded766c 100644 --- a/Documentation/DocBook/media/v4l/selection-api.xml +++ b/Documentation/DocBook/media/v4l/selection-api.xml @@ -86,47 +86,47 @@ selection targets available for a video capture device. It is recommended to configure the cropping targets before to the composing targets.</para> <para>The range of coordinates of the top left corner, width and height of -areas that can be sampled is given by the <constant> V4L2_SEL_TGT_CROP_BOUNDS -</constant> target. It is recommended for the driver developers to put the -top/left corner at position <constant> (0,0) </constant>. The rectangle's +areas that can be sampled is given by the <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant> +target. It is recommended for the driver developers to put the +top/left corner at position <constant>(0,0)</constant>. The rectangle's coordinates are expressed in pixels.</para> <para>The top left corner, width and height of the source rectangle, that is -the area actually sampled, is given by the <constant> V4L2_SEL_TGT_CROP -</constant> target. It uses the same coordinate system as <constant> -V4L2_SEL_TGT_CROP_BOUNDS </constant>. The active cropping area must lie -completely inside the capture boundaries. The driver may further adjust the -requested size and/or position according to hardware limitations.</para> +the area actually sampled, is given by the <constant>V4L2_SEL_TGT_CROP</constant> +target. It uses the same coordinate system as <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>. +The active cropping area must lie completely inside the capture boundaries. The +driver may further adjust the requested size and/or position according to hardware +limitations.</para> <para>Each capture device has a default source rectangle, given by the -<constant> V4L2_SEL_TGT_CROP_DEFAULT </constant> target. This rectangle shall +<constant>V4L2_SEL_TGT_CROP_DEFAULT</constant> target. This rectangle shall over what the driver writer considers the complete picture. Drivers shall set the active crop rectangle to the default when the driver is first loaded, but not later.</para> <para>The composing targets refer to a memory buffer. The limits of composing -coordinates are obtained using <constant> V4L2_SEL_TGT_COMPOSE_BOUNDS -</constant>. All coordinates are expressed in pixels. The rectangle's top/left -corner must be located at position <constant> (0,0) </constant>. The width and -height are equal to the image size set by <constant> VIDIOC_S_FMT </constant>. +coordinates are obtained using <constant>V4L2_SEL_TGT_COMPOSE_BOUNDS</constant>. +All coordinates are expressed in pixels. The rectangle's top/left +corner must be located at position <constant>(0,0)</constant>. The width and +height are equal to the image size set by <constant>VIDIOC_S_FMT</constant>. </para> <para>The part of a buffer into which the image is inserted by the hardware is -controlled by the <constant> V4L2_SEL_TGT_COMPOSE </constant> target. +controlled by the <constant>V4L2_SEL_TGT_COMPOSE</constant> target. The rectangle's coordinates are also expressed in the same coordinate system as the bounds rectangle. The composing rectangle must lie completely inside bounds rectangle. The driver must adjust the composing rectangle to fit to the bounding limits. Moreover, the driver can perform other adjustments according to hardware limitations. The application can control rounding behaviour using -<link linkend="v4l2-selection-flags"> constraint flags </link>.</para> +<link linkend="v4l2-selection-flags"> constraint flags</link>.</para> <para>For capture devices the default composing rectangle is queried using -<constant> V4L2_SEL_TGT_COMPOSE_DEFAULT </constant>. It is usually equal to the +<constant>V4L2_SEL_TGT_COMPOSE_DEFAULT</constant>. It is usually equal to the bounding rectangle.</para> <para>The part of a buffer that is modified by the hardware is given by -<constant> V4L2_SEL_TGT_COMPOSE_PADDED </constant>. It contains all pixels -defined using <constant> V4L2_SEL_TGT_COMPOSE </constant> plus all +<constant>V4L2_SEL_TGT_COMPOSE_PADDED</constant>. It contains all pixels +defined using <constant>V4L2_SEL_TGT_COMPOSE</constant> plus all padding data modified by hardware during insertion process. All pixels outside this rectangle <emphasis>must not</emphasis> be changed by the hardware. The content of pixels that lie inside the padded area but outside active area is @@ -140,52 +140,51 @@ where the rubbish pixels are located and remove them if needed.</para> <title>Configuration of video output</title> <para>For output devices targets and ioctls are used similarly to the video -capture case. The <emphasis> composing </emphasis> rectangle refers to the +capture case. The <emphasis>composing</emphasis> rectangle refers to the insertion of an image into a video signal. The cropping rectangles refer to a memory buffer. It is recommended to configure the composing targets before to the cropping targets.</para> <para>The cropping targets refer to the memory buffer that contains an image to be inserted into a video signal or graphical screen. The limits of cropping -coordinates are obtained using <constant> V4L2_SEL_TGT_CROP_BOUNDS </constant>. +coordinates are obtained using <constant>V4L2_SEL_TGT_CROP_BOUNDS</constant>. All coordinates are expressed in pixels. The top/left corner is always point -<constant> (0,0) </constant>. The width and height is equal to the image size -specified using <constant> VIDIOC_S_FMT </constant> ioctl.</para> +<constant>(0,0)</constant>. The width and height is equal to the image size +specified using <constant>VIDIOC_S_FMT</constant> ioctl.</para> <para>The top left corner, width and height of the source rectangle, that is the area from which image date are processed by the hardware, is given by the -<constant> V4L2_SEL_TGT_CROP </constant>. Its coordinates are expressed +<constant>V4L2_SEL_TGT_CROP</constant>. Its coordinates are expressed in in the same coordinate system as the bounds rectangle. The active cropping area must lie completely inside the crop boundaries and the driver may further adjust the requested size and/or position according to hardware limitations.</para> <para>For output devices the default cropping rectangle is queried using -<constant> V4L2_SEL_TGT_CROP_DEFAULT </constant>. It is usually equal to the +<constant>V4L2_SEL_TGT_CROP_DEFAULT</constant>. It is usually equal to the bounding rectangle.</para> <para>The part of a video signal or graphics display where the image is -inserted by the hardware is controlled by <constant> -V4L2_SEL_TGT_COMPOSE </constant> target. The rectangle's coordinates -are expressed in pixels. The composing rectangle must lie completely inside the -bounds rectangle. The driver must adjust the area to fit to the bounding -limits. Moreover, the driver can perform other adjustments according to -hardware limitations. </para> - -<para>The device has a default composing rectangle, given by the <constant> -V4L2_SEL_TGT_COMPOSE_DEFAULT </constant> target. This rectangle shall cover what +inserted by the hardware is controlled by <constant>V4L2_SEL_TGT_COMPOSE</constant> +target. The rectangle's coordinates are expressed in pixels. The composing +rectangle must lie completely inside the bounds rectangle. The driver must +adjust the area to fit to the bounding limits. Moreover, the driver can +perform other adjustments according to hardware limitations.</para> + +<para>The device has a default composing rectangle, given by the +<constant>V4L2_SEL_TGT_COMPOSE_DEFAULT</constant> target. This rectangle shall cover what the driver writer considers the complete picture. It is recommended for the -driver developers to put the top/left corner at position <constant> (0,0) -</constant>. Drivers shall set the active composing rectangle to the default +driver developers to put the top/left corner at position <constant>(0,0)</constant>. +Drivers shall set the active composing rectangle to the default one when the driver is first loaded.</para> <para>The devices may introduce additional content to video signal other than an image from memory buffers. It includes borders around an image. However, such a padded area is driver-dependent feature not covered by this document. Driver developers are encouraged to keep padded rectangle equal to active one. -The padded target is accessed by the <constant> V4L2_SEL_TGT_COMPOSE_PADDED -</constant> identifier. It must contain all pixels from the <constant> -V4L2_SEL_TGT_COMPOSE </constant> target.</para> +The padded target is accessed by the <constant>V4L2_SEL_TGT_COMPOSE_PADDED</constant> +identifier. It must contain all pixels from the <constant>V4L2_SEL_TGT_COMPOSE</constant> +target.</para> </section> @@ -194,8 +193,8 @@ V4L2_SEL_TGT_COMPOSE </constant> target.</para> <title>Scaling control</title> <para>An application can detect if scaling is performed by comparing the width -and the height of rectangles obtained using <constant> V4L2_SEL_TGT_CROP -</constant> and <constant> V4L2_SEL_TGT_COMPOSE </constant> targets. If +and the height of rectangles obtained using <constant>V4L2_SEL_TGT_CROP</constant> +and <constant>V4L2_SEL_TGT_COMPOSE</constant> targets. If these are not equal then the scaling is applied. The application can compute the scaling ratios using these values.</para> @@ -208,7 +207,7 @@ the scaling ratios using these values.</para> <title>Comparison with old cropping API</title> <para>The selection API was introduced to cope with deficiencies of previous -<link linkend="crop"> API </link>, that was designed to control simple capture +<link linkend="crop"> API</link>, 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 @@ -220,7 +219,7 @@ part of an image by abusing V4L2 API. Cropping a smaller image from a larger one is achieved by setting the field &v4l2-pix-format;<structfield>::bytesperline</structfield>. Introducing an image offsets could be done by modifying field &v4l2-buffer;<structfield>::m_userptr</structfield> -before calling <constant> VIDIOC_QBUF </constant>. Those +before calling <constant>VIDIOC_QBUF</constant>. 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 @@ -229,7 +228,7 @@ and constraints flags are introduced. Finally, &v4l2-crop; and &v4l2-cropcap; have no reserved fields. Therefore there is no way to extend their functionality. The new &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. </para> +The former cropping API would be simulated using the new one.</para> </section> @@ -238,9 +237,9 @@ The former cropping API would be simulated using the new one. </para> <example> <title>Resetting the cropping parameters</title> - <para>(A video capture device is assumed; change <constant> -V4L2_BUF_TYPE_VIDEO_CAPTURE </constant> for other devices; change target to -<constant> V4L2_SEL_TGT_COMPOSE_* </constant> family to configure composing + <para>(A video capture device is assumed; change +<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> for other devices; change target to +<constant>V4L2_SEL_TGT_COMPOSE_*</constant> family to configure composing area)</para> <programlisting> @@ -292,8 +291,8 @@ area)</para> <example> <title>Querying for scaling factors</title> - <para>A video output device is assumed; change <constant> -V4L2_BUF_TYPE_VIDEO_OUTPUT </constant> for other devices</para> + <para>A video output device is assumed; change +<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> for other devices</para> <programlisting> &v4l2-selection; compose = { |