Chapter 2. Image Formats
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## Chapter 2. Image Formats

The V4L2 API was primarily designed for devices exchanging image data with applications. The v4l2_pix_format and v4l2_pix_format_mplane structures define the format and layout of an image in memory. The former is used with the single-planar API, while the latter is used with the multi-planar version (see the section called “Single- and multi-planar APIs”). Image formats are negotiated with the `VIDIOC_S_FMT` ioctl. (The explanations here focus on video capturing and output, for overlay frame buffer formats see also `VIDIOC_G_FBUF`.)

## Single-planar format structure

Table 2.1. struct v4l2_pix_format

__u32 | `width` | Image width in pixels.
__u32 | `height` | Image height in pixels. If `field` is one of `V4L2_FIELD_TOP`, `V4L2_FIELD_BOTTOM` or `V4L2_FIELD_ALTERNATE` then height refers to the number of lines in the field, otherwise it refers to the number of lines in the frame (which is twice the field height for interlaced formats).
Applications set these fields to request an image size, drivers return the closest possible values. In case of planar formats the `width` and `height` applies to the largest plane. To avoid ambiguities drivers must return values rounded up to a multiple of the scale factor of any smaller planes. For example when the image format is YUV 4:2:0, `width` and `height` must be multiples of two.
__u32 | `pixelformat` | The pixel format or type of compression, set by the application. This is a little endian four character code. V4L2 defines standard RGB formats in Table 2.18, “Packed RGB Image Formats”, YUV formats in the section called “YUV Formats”, and reserved codes in Table 2.22, “Reserved Image Formats”
enum v4l2_field | `field` | Video images are typically interlaced. Applications can request to capture or output only the top or bottom field, or both fields interlaced or sequentially stored in one buffer or alternating in separate buffers. Drivers return the actual field order selected. For more details on fields see the section called “Field Order”.
__u32 | `bytesperline` | Distance in bytes between the leftmost pixels in two adjacent lines.

Both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the value requested by the application, returning `width` times bytes per pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider cases where padding bytes after the last line of an image cross a system page boundary. Input devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the `bytesperline` value applies to the first plane and is divided by the same factor as the `width` field for the other planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a `bytesperline` value rounded up to a multiple of the scale factor.

For compressed formats the `bytesperline` value makes no sense. Applications and drivers must set this to 0 in that case.

__u32 | `sizeimage` | Size in bytes of the buffer to hold a complete image, set by the driver. Usually this is `bytesperline` times `height`. When the image consists of variable length compressed data this is the maximum number of bytes required to hold an image.
enum v4l2_colorspace | `colorspace` | This information supplements the `pixelformat` and must be set by the driver for capture streams and by the application for output streams, see the section called “Colorspaces”.

__u32

`priv`

This field indicates whether the remaining fields of the v4l2_pix_format structure, also called the extended fields, are valid. When set to `V4L2_PIX_FMT_PRIV_MAGIC`, it indicates that the extended fields have been correctly initialized. When set to any other value it indicates that the extended fields contain undefined values.

Applications that wish to use the pixel format extended fields must first ensure that the feature is supported by querying the device for the `V4L2_CAP_EXT_PIX_FORMAT` capability. If the capability isn't set the pixel format extended fields are not supported and using the extended fields will lead to undefined results.

To use the extended fields, applications must set the `priv` field to `V4L2_PIX_FMT_PRIV_MAGIC`, initialize all the extended fields and zero the unused bytes of the v4l2_format `raw_data` field.

When the `priv` field isn't set to `V4L2_PIX_FMT_PRIV_MAGIC` drivers must act as if all the extended fields were set to zero. On return drivers must set the `priv` field to `V4L2_PIX_FMT_PRIV_MAGIC` and all the extended fields to applicable values.

__u32 | `flags` | Flags set by the application or driver, see Table 2.23, “Format Flags”.
enum v4l2_ycbcr_encoding | `ycbcr_enc` | This information supplements the `colorspace` and must be set by the driver for capture streams and by the application for output streams, see the section called “Colorspaces”.
enum v4l2_quantization | `quantization` | This information supplements the `colorspace` and must be set by the driver for capture streams and by the application for output streams, see the section called “Colorspaces”.
enum v4l2_xfer_func | `xfer_func` | This information supplements the `colorspace` and must be set by the driver for capture streams and by the application for output streams, see the section called “Colorspaces”.

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