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.
Note
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_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]. |