2.16. Defining Colorspaces in V4L2¶

In V4L2 colorspaces are defined by four values. The first is the colorspace identifier (enum v4l2_colorspace) which defines the chromaticities, the default transfer function, the default Y'CbCr encoding and the default quantization method. The second is the transfer function identifier (enum v4l2_xfer_func) to specify non-standard transfer functions. The third is the Y'CbCr encoding identifier (enum v4l2_ycbcr_encoding) to specify non-standard Y'CbCr encodings and the fourth is the quantization identifier (enum v4l2_quantization) to specify non-standard quantization methods. Most of the time only the colorspace field of struct v4l2_pix_format or struct v4l2_pix_format_mplane needs to be filled in.

On HSV formats the Hue is defined as the angle on the cylindrical color representation. Usually this angle is measured in degrees, i.e. 0-360. When we map this angle value into 8 bits, there are two basic ways to do it: Divide the angular value by 2 (0-179), or use the whole range, 0-255, dividing the angular value by 1.41. The enum v4l2_hsv_encoding specifies which encoding is used.

注釈

The default R'G'B' quantization is full range for all colorspaces except for BT.2020 which uses limited range R'G'B' quantization.

v4l2_colorspace¶

V4L2 Colorspaces¶
Identifier	Details
`V4L2_COLORSPACE_DEFAULT`	The default colorspace. This can be used by applications to let the driver fill in the colorspace.
`V4L2_COLORSPACE_SMPTE170M`	See Colorspace SMPTE 170M (V4L2_COLORSPACE_SMPTE170M).
`V4L2_COLORSPACE_REC709`	See Colorspace Rec. 709 (V4L2_COLORSPACE_REC709).
`V4L2_COLORSPACE_SRGB`	See Colorspace sRGB (V4L2_COLORSPACE_SRGB).
`V4L2_COLORSPACE_OPRGB`	See Colorspace opRGB (V4L2_COLORSPACE_OPRGB).
`V4L2_COLORSPACE_BT2020`	See Colorspace BT.2020 (V4L2_COLORSPACE_BT2020).
`V4L2_COLORSPACE_DCI_P3`	See Colorspace DCI-P3 (V4L2_COLORSPACE_DCI_P3).
`V4L2_COLORSPACE_SMPTE240M`	See Colorspace SMPTE 240M (V4L2_COLORSPACE_SMPTE240M).
`V4L2_COLORSPACE_470_SYSTEM_M`	See Colorspace NTSC 1953 (V4L2_COLORSPACE_470_SYSTEM_M).
`V4L2_COLORSPACE_470_SYSTEM_BG`	See Colorspace EBU Tech. 3213 (V4L2_COLORSPACE_470_SYSTEM_BG).
`V4L2_COLORSPACE_JPEG`	See Colorspace JPEG (V4L2_COLORSPACE_JPEG).
`V4L2_COLORSPACE_RAW`	The raw colorspace. This is used for raw image capture where the image is minimally processed and is using the internal colorspace of the device. The software that processes an image using this 'colorspace' will have to know the internals of the capture device.

v4l2_xfer_func¶

V4L2 Transfer Function¶
Identifier	Details
`V4L2_XFER_FUNC_DEFAULT`	Use the default transfer function as defined by the colorspace.
`V4L2_XFER_FUNC_709`	Use the Rec. 709 transfer function.
`V4L2_XFER_FUNC_SRGB`	Use the sRGB transfer function.
`V4L2_XFER_FUNC_OPRGB`	Use the opRGB transfer function.
`V4L2_XFER_FUNC_SMPTE240M`	Use the SMPTE 240M transfer function.
`V4L2_XFER_FUNC_NONE`	Do not use a transfer function (i.e. use linear RGB values).
`V4L2_XFER_FUNC_DCI_P3`	Use the DCI-P3 transfer function.
`V4L2_XFER_FUNC_SMPTE2084`	Use the SMPTE 2084 transfer function. See Transfer Function SMPTE 2084 (V4L2_XFER_FUNC_SMPTE2084).

v4l2_ycbcr_encoding¶

V4L2 Y'CbCr Encodings¶
Identifier	Details
`V4L2_YCBCR_ENC_DEFAULT`	Use the default Y'CbCr encoding as defined by the colorspace.
`V4L2_YCBCR_ENC_601`	Use the BT.601 Y'CbCr encoding.
`V4L2_YCBCR_ENC_709`	Use the Rec. 709 Y'CbCr encoding.
`V4L2_YCBCR_ENC_XV601`	Use the extended gamut xvYCC BT.601 encoding.
`V4L2_YCBCR_ENC_XV709`	Use the extended gamut xvYCC Rec. 709 encoding.
`V4L2_YCBCR_ENC_BT2020`	Use the default non-constant luminance BT.2020 Y'CbCr encoding.
`V4L2_YCBCR_ENC_BT2020_CONST_LUM`	Use the constant luminance BT.2020 Yc'CbcCrc encoding.
`V4L2_YCBCR_ENC_SMPTE_240M`	Use the SMPTE 240M Y'CbCr encoding.

v4l2_hsv_encoding¶

V4L2 HSV Encodings¶
Identifier	Details
`V4L2_HSV_ENC_180`	For the Hue, each LSB is two degrees.
`V4L2_HSV_ENC_256`	For the Hue, the 360 degrees are mapped into 8 bits, i.e. each LSB is roughly 1.41 degrees.

v4l2_quantization¶

V4L2 Quantization Methods¶
Identifier	Details
`V4L2_QUANTIZATION_DEFAULT`	Use the default quantization encoding as defined by the colorspace. This is always full range for R'G'B' (except for the BT.2020 colorspace) and HSV. It is usually limited range for Y'CbCr.
`V4L2_QUANTIZATION_FULL_RANGE`	Use the full range quantization encoding. I.e. the range [0…1] is mapped to [0…255] (with possible clipping to [1…254] to avoid the 0x00 and 0xff values). Cb and Cr are mapped from [-0.5…0.5] to [0…255] (with possible clipping to [1…254] to avoid the 0x00 and 0xff values).
`V4L2_QUANTIZATION_LIM_RANGE`	Use the limited range quantization encoding. I.e. the range [0…1] is mapped to [16…235]. Cb and Cr are mapped from [-0.5…0.5] to [16…240].