1.26.3. Configuration

Applications can use the selection API to select an area in a video signal or a buffer, and to query for default settings and hardware limits.

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 constraint flags.

1.26.3.1. Configuration of video capture

See figure Cropping and composing targets 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.

The range of coordinates of the top left corner, width and height of areas that can be sampled is given by the V4L2_SEL_TGT_CROP_BOUNDS target. It is recommended for the driver developers to put the top/left corner at position (0,0). The rectangle’s coordinates are expressed in pixels.

The top left corner, width and height of the source rectangle, that is the area actually sampled, is given by the V4L2_SEL_TGT_CROP target. It uses the same coordinate system as V4L2_SEL_TGT_CROP_BOUNDS. 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.

Each capture device has a default source rectangle, given by the V4L2_SEL_TGT_CROP_DEFAULT target. This rectangle shall cover 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.

The composing targets refer to a memory buffer. The limits of composing coordinates are obtained using V4L2_SEL_TGT_COMPOSE_BOUNDS. All coordinates are expressed in pixels. The rectangle’s top/left corner must be located at position (0,0). The width and height are equal to the image size set by VIDIOC_S_FMT.

The part of a buffer into which the image is inserted by the hardware is controlled by the V4L2_SEL_TGT_COMPOSE 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 constraint flags.

For capture devices the default composing rectangle is queried using V4L2_SEL_TGT_COMPOSE_DEFAULT. It is usually equal to the bounding rectangle.

The part of a buffer that is modified by the hardware is given by V4L2_SEL_TGT_COMPOSE_PADDED. It contains all pixels defined using V4L2_SEL_TGT_COMPOSE plus all padding data modified by hardware during insertion process. All pixels outside this rectangle must not 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.

1.26.3.2. Configuration of video output

For output devices targets and ioctls are used similarly to the video capture case. The composing 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.

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 V4L2_SEL_TGT_CROP_BOUNDS. All coordinates are expressed in pixels. The top/left corner is always point (0,0). The width and height is equal to the image size specified using VIDIOC_S_FMT ioctl.

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 V4L2_SEL_TGT_CROP. Its coordinates are expressed 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.

For output devices the default cropping rectangle is queried using V4L2_SEL_TGT_CROP_DEFAULT. It is usually equal to the bounding rectangle.

The part of a video signal or graphics display where the image is inserted by the hardware is controlled by V4L2_SEL_TGT_COMPOSE 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.

The device has a default composing rectangle, given by the V4L2_SEL_TGT_COMPOSE_DEFAULT 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 (0,0). Drivers shall set the active composing rectangle to the default one when the driver is first loaded.

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 V4L2_SEL_TGT_COMPOSE_PADDED identifier. It must contain all pixels from the V4L2_SEL_TGT_COMPOSE target.

1.26.3.3. Scaling control

An application can detect if scaling is performed by comparing the width and the height of rectangles obtained using V4L2_SEL_TGT_CROP and V4L2_SEL_TGT_COMPOSE targets. If these are not equal then the scaling is applied. The application can compute the scaling ratios using these values.