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diff --git a/Documentation/DocBook/media/v4l/selection-api.xml b/Documentation/DocBook/media/v4l/selection-api.xml new file mode 100644 index 000000000..28cbded76 --- /dev/null +++ b/Documentation/DocBook/media/v4l/selection-api.xml @@ -0,0 +1,324 @@ +<section id="selection-api"> + + <title>Experimental API for cropping, composing and scaling</title> + + <note> + <title>Experimental</title> + + <para>This is an <link linkend="experimental">experimental</link> +interface and may change in the future.</para> + </note> + + <section> + <title>Introduction</title> + +<para>Some video capture devices can sample a subsection of a picture and +shrink or enlarge it to an image of arbitrary size. Next, the devices can +insert the image into larger one. Some video output devices can crop part of an +input image, scale it up or down and insert it at an arbitrary scan line and +horizontal offset into a video signal. We call these abilities cropping, +scaling and composing.</para> + +<para>On a video <emphasis>capture</emphasis> device the source is a video +signal, and the cropping target determine the area actually sampled. The sink +is an image stored in a memory buffer. The composing area specifies which part +of the buffer is actually written to by the hardware. </para> + +<para>On a video <emphasis>output</emphasis> device the source is an image in a +memory buffer, and the cropping target is a part of an image to be shown on a +display. The sink is the display or the graphics screen. The application may +select the part of display where the image should be displayed. The size and +position of such a window is controlled by the compose target.</para> + +<para>Rectangles for all cropping and composing targets are defined even if the +device does supports neither cropping nor composing. Their size and position +will be fixed in such a case. If the device does not support scaling then the +cropping and composing rectangles have the same size.</para> + + </section> + + <section> + <title>Selection targets</title> + + <para> + <figure id="sel-targets-capture"> + <title>Cropping and composing targets</title> + <mediaobject> + <imageobject> + <imagedata fileref="selection.png" format="PNG" /> + </imageobject> + <textobject> + <phrase>Targets used by a cropping, composing and scaling + process</phrase> + </textobject> + </mediaobject> + </figure> + </para> + + <para>See <xref linkend="v4l2-selection-targets" /> for more + information.</para> + </section> + + <section> + + <title>Configuration</title> + +<para>Applications can use the <link linkend="vidioc-g-selection">selection +API</link> to select an area in a video signal or a buffer, and to query for +default settings and hardware limits.</para> + +<para>Video hardware can have various cropping, composing and scaling +limitations. It may only scale up or down, support only discrete scaling +factors, or have different scaling abilities in the horizontal and vertical +directions. Also it may not support scaling at all. At the same time the +cropping/composing rectangles may have to be aligned, and both the source and +the sink may have arbitrary upper and lower size limits. Therefore, as usual, +drivers are expected to adjust the requested parameters and return the actual +values selected. An application can control the rounding behaviour using <link +linkend="v4l2-selection-flags"> constraint flags </link>.</para> + + <section> + + <title>Configuration of video capture</title> + +<para>See figure <xref linkend="sel-targets-capture" /> for examples of the +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 +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> + +<para>Each capture device has a default source rectangle, given by the +<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>. +</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. +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> + +<para>For capture devices the default composing rectangle is queried using +<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 +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 +undefined. The application can use the padded and active rectangles to detect +where the rubbish pixels are located and remove them if needed.</para> + + </section> + + <section> + + <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 +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>. +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> + +<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 +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 +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 +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 +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> + + </section> + + <section> + + <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 +these are not equal then the scaling is applied. The application can compute +the scaling ratios using these values.</para> + + </section> + + </section> + + <section> + + <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 +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 +&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 +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, &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> + + </section> + + <section> + <title>Examples</title> + <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 +area)</para> + + <programlisting> + + &v4l2-selection; sel = { + .type = V4L2_BUF_TYPE_VIDEO_CAPTURE, + .target = V4L2_SEL_TGT_CROP_DEFAULT, + }; + ret = ioctl(fd, &VIDIOC-G-SELECTION;, &sel); + if (ret) + exit(-1); + sel.target = V4L2_SEL_TGT_CROP; + ret = ioctl(fd, &VIDIOC-S-SELECTION;, &sel); + if (ret) + exit(-1); + + </programlisting> + </example> + + <example> + <title>Simple downscaling</title> + <para>Setting a composing area on output of size of <emphasis> at most +</emphasis> half of limit placed at a center of a display.</para> + <programlisting> + + &v4l2-selection; sel = { + .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, + .target = V4L2_SEL_TGT_COMPOSE_BOUNDS, + }; + struct v4l2_rect r; + + ret = ioctl(fd, &VIDIOC-G-SELECTION;, &sel); + if (ret) + exit(-1); + /* setting smaller compose rectangle */ + r.width = sel.r.width / 2; + r.height = sel.r.height / 2; + r.left = sel.r.width / 4; + r.top = sel.r.height / 4; + sel.r = r; + sel.target = V4L2_SEL_TGT_COMPOSE; + sel.flags = V4L2_SEL_FLAG_LE; + ret = ioctl(fd, &VIDIOC-S-SELECTION;, &sel); + if (ret) + exit(-1); + + </programlisting> + </example> + + <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> + <programlisting> + + &v4l2-selection; compose = { + .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, + .target = V4L2_SEL_TGT_COMPOSE, + }; + &v4l2-selection; crop = { + .type = V4L2_BUF_TYPE_VIDEO_OUTPUT, + .target = V4L2_SEL_TGT_CROP, + }; + double hscale, vscale; + + ret = ioctl(fd, &VIDIOC-G-SELECTION;, &compose); + if (ret) + exit(-1); + ret = ioctl(fd, &VIDIOC-G-SELECTION;, &crop); + if (ret) + exit(-1); + + /* computing scaling factors */ + hscale = (double)compose.r.width / crop.r.width; + vscale = (double)compose.r.height / crop.r.height; + + </programlisting> + </example> + + </section> + +</section> |