libcamera: color_space: Rename Jpeg to Sycc

The JPEG color space is badly name, as the JPEG specification (ITU-T
T.81) doesn't define any particular color space:

    The interchange format does not specify a complete coded image
    representation. Application-dependent information, e.g. colour
    space, is outside the scope of this Specification.

The JFIF specification (ITU-T T.871) is clearer as it requires ITU-R
BT.601 YCbCr encoding and a full quantization range:

  The interpretations of Y, CB, and CR are derived from the E'Y, E'Cb,
  and E'Cr signals defined in the 625-line specification of Rec. ITU-R
  BT.601, but these signals are normalized so as to permit the usage of
  the full range of 256 levels of the 8-bit binary encoding of the Y
  component.

It however doesn't specify color primaries or a transfer function
explicitly. It only mentions the latter when describing the conversion
from YCbCr to RGB:

  The inverse relationship for computing full scale 8-bit per colour
  channel gamma pre-corrected RGB values (following Rec. ITU-R BT.601
  gamma pre-correction and colour primary specifications) from YCbCr
  colours (with 256 levels per component) can be computed as follows:
  [...]

Given that ITU-R BT.601-5 (1995) didn't specify color primaries or a
transfer function, and that the later ITU-R BT.601-7 (2011) version
specifies color primaries for the 625-line variant that do not match
sRGB, the JPEG color space in libcamera is badly named. This is
confirmed by ITU-T T.871:

  As this Recommendation | International Standard is based on the prior
  informally-circulated JFIF version 1.02 specification that was
  produced in 1992, which referenced Rec. ITU-R BT.601 (formerly CCIR
  601), it references that specification for definition of the E'Y,
  E'Cb, and E'Cr signals that correspond to the YCBCR values specified
  herein. However, since the development of the prior JFIF version 1.02
  specification, additional industry specifications have been developed,
  Rec. ITU-R BT.601 has been updated, and common industry practice has
  emerged which often follows the sYCC specification in IEC
  61966-2-1/Amd.1. The difference between the use of the colour
  interpretation specification in this Recommendation | International
  Standard and that of the sYCC specification may be considered
  negligible in practice.

Rename the color space to sYCC, as its definition matches the sYCC
standard, and indicate that it is typically used to encode JPEG images.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
index 3fc8438f..b22a981f 100644
--- a/src/libcamera/v4l2_device.cpp
+++ b/src/libcamera/v4l2_device.cpp
@@ -745,8 +745,8 @@ void V4L2Device::eventAvailable()
 
 static const std::map<uint32_t, ColorSpace> v4l2ToColorSpace = {
 	{ V4L2_COLORSPACE_RAW, ColorSpace::Raw },
-	{ V4L2_COLORSPACE_JPEG, ColorSpace::Jpeg },
 	{ V4L2_COLORSPACE_SRGB, ColorSpace::Srgb },
+	{ V4L2_COLORSPACE_JPEG, ColorSpace::Sycc },
 	{ V4L2_COLORSPACE_SMPTE170M, ColorSpace::Smpte170m },
 	{ V4L2_COLORSPACE_REC709, ColorSpace::Rec709 },
 	{ V4L2_COLORSPACE_BT2020, ColorSpace::Rec2020 },
@@ -771,8 +771,8 @@ static const std::map<uint32_t, ColorSpace::Range> v4l2ToRange = {
 
 static const std::vector<std::pair<ColorSpace, v4l2_colorspace>> colorSpaceToV4l2 = {
 	{ ColorSpace::Raw, V4L2_COLORSPACE_RAW },
-	{ ColorSpace::Jpeg, V4L2_COLORSPACE_JPEG },
 	{ ColorSpace::Srgb, V4L2_COLORSPACE_SRGB },
+	{ ColorSpace::Sycc, V4L2_COLORSPACE_JPEG },
 	{ ColorSpace::Smpte170m, V4L2_COLORSPACE_SMPTE170M },
 	{ ColorSpace::Rec709, V4L2_COLORSPACE_REC709 },
 	{ ColorSpace::Rec2020, V4L2_COLORSPACE_BT2020 },