[libcamera-devel] [PATCH] libcamera: color_space: Rename Jpeg to Sycc
Hans Verkuil
hverkuil-cisco at xs4all.nl
Fri Aug 19 11:12:56 CEST 2022
On 8/17/22 23:18, Laurent Pinchart wrote:
> 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.
>
> 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 at ideasonboard.com>
> ---
>
> Hans, following our discussion on IRC, I double-check the JFIF standard,
> and my findings are described in the commit message above. It seems that
> naming the color space "JPEG" in V4L2 is actually incorrect, as JFIF
> expects the 626-line ITU-R BT.601 chromaticities for the primary colors,
> which are slightly different than sRGB (the 525-line chromaticities
> match SMPTE 170M and SMPTE 240M, but not sRGB either). It seems that we
> are missing a V4L2_COLORSPACE_* value to describe the 625-line ITU-R
> BT.601 chromaticities, is that an oversight ?
We have that colorspace: V4L2_COLORSPACE_470_SYSTEM_BG, however modern
SDTV transmissions use V4L2_COLORSPACE_SMPTE170M these days.
The differences between the two are next to impossible to see.
However, I think you misinterpreted the JFIF standard: it does not specify
that the BT 601 chromaticities/transfer function should be used, it only
uses the YCbCr-RGB conversion that BT.601 describes.
Note 3 in section 7 of JFIF is relevant here:
"NOTE 3 − 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' , and E' 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. Moreover, as previously
noted, the colour space specification herein can provide only a basic level of colour fidelity. The use of supplemental metadata
such as an ICC profile (e.g., as specified in ISO 15076-1) may be necessary to provide a more accurate colour characterization."
In other words, it recommends that sYCC is followed as the default.
Note that I believe that the V4L2_COLORSPACE_JPEG define predates the sYCC standard,
so there was a good reason why SYCC wasn't used. :-)
I wonder if it would make sense to make a V4L2_COLORSPACE_SYCC as well. Probably
more effort than it is worth, though.
Regards,
Hans
>
> ---
> include/libcamera/color_space.h | 2 +-
> src/libcamera/color_space.cpp | 28 +++++++++----------
> .../pipeline/raspberrypi/raspberrypi.cpp | 10 +++----
> src/libcamera/v4l2_device.cpp | 4 +--
> src/py/libcamera/py_main.cpp | 2 +-
> 5 files changed, 23 insertions(+), 23 deletions(-)
>
> diff --git a/include/libcamera/color_space.h b/include/libcamera/color_space.h
> index 0d39fbc02220..8030a264c66f 100644
> --- a/include/libcamera/color_space.h
> +++ b/include/libcamera/color_space.h
> @@ -46,8 +46,8 @@ public:
> }
>
> static const ColorSpace Raw;
> - static const ColorSpace Jpeg;
> static const ColorSpace Srgb;
> + static const ColorSpace Sycc;
> static const ColorSpace Smpte170m;
> static const ColorSpace Rec709;
> static const ColorSpace Rec2020;
> diff --git a/src/libcamera/color_space.cpp b/src/libcamera/color_space.cpp
> index caf397607b10..6ace40dcb0fb 100644
> --- a/src/libcamera/color_space.cpp
> +++ b/src/libcamera/color_space.cpp
> @@ -121,8 +121,8 @@ namespace libcamera {
> * \brief Assemble and return a readable string representation of the
> * ColorSpace
> *
> - * If the color space matches a standard ColorSpace (such as ColorSpace::Jpeg)
> - * then the short name of the color space ("JPEG") is returned. Otherwise
> + * If the color space matches a standard ColorSpace (such as ColorSpace::Sycc)
> + * then the short name of the color space ("sYCC") is returned. Otherwise
> * the four constituent parts of the ColorSpace are assembled into a longer
> * string.
> *
> @@ -134,8 +134,8 @@ std::string ColorSpace::toString() const
>
> static const std::array<std::pair<ColorSpace, const char *>, 6> colorSpaceNames = { {
> { ColorSpace::Raw, "RAW" },
> - { ColorSpace::Jpeg, "JPEG" },
> { ColorSpace::Srgb, "sRGB" },
> + { ColorSpace::Sycc, "sYCC" },
> { ColorSpace::Smpte170m, "SMPTE170M" },
> { ColorSpace::Rec709, "Rec709" },
> { ColorSpace::Rec2020, "Rec2020" },
> @@ -242,17 +242,6 @@ const ColorSpace ColorSpace::Raw = {
> Range::Full
> };
>
> -/**
> - * \brief A constant representing the JPEG color space used for
> - * encoding JPEG images
> - */
> -const ColorSpace ColorSpace::Jpeg = {
> - Primaries::Rec709,
> - TransferFunction::Srgb,
> - YcbcrEncoding::Rec601,
> - Range::Full
> -};
> -
> /**
> * \brief A constant representing the sRGB color space
> *
> @@ -266,6 +255,17 @@ const ColorSpace ColorSpace::Srgb = {
> Range::Limited
> };
>
> +/**
> + * \brief A constant representing the sYCC color space, typically used for
> + * encoding JPEG images
> + */
> +const ColorSpace ColorSpace::Sycc = {
> + Primaries::Rec709,
> + TransferFunction::Srgb,
> + YcbcrEncoding::Rec601,
> + Range::Full
> +};
> +
> /**
> * \brief A constant representing the SMPTE170M color space
> */
> diff --git a/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp b/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
> index e895584d4fbc..b4094898ca6c 100644
> --- a/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
> +++ b/src/libcamera/pipeline/raspberrypi/raspberrypi.cpp
> @@ -593,11 +593,11 @@ CameraConfiguration *PipelineHandlerRPi::generateConfiguration(Camera *camera,
> fmts = data->isp_[Isp::Output0].dev()->formats();
> pixelFormat = formats::NV12;
> /*
> - * Still image codecs usually expect the JPEG color space.
> + * Still image codecs usually expect the sYCC color space.
> * Even RGB codecs will be fine as the RGB we get with the
> - * JPEG color space is the same as sRGB.
> + * sYCC color space is the same as sRGB.
> */
> - colorSpace = ColorSpace::Jpeg;
> + colorSpace = ColorSpace::Sycc;
> /* Return the largest sensor resolution. */
> size = sensorSize;
> bufferCount = 1;
> @@ -628,7 +628,7 @@ CameraConfiguration *PipelineHandlerRPi::generateConfiguration(Camera *camera,
> case StreamRole::Viewfinder:
> fmts = data->isp_[Isp::Output0].dev()->formats();
> pixelFormat = formats::ARGB8888;
> - colorSpace = ColorSpace::Jpeg;
> + colorSpace = ColorSpace::Sycc;
> size = { 800, 600 };
> bufferCount = 4;
> outCount++;
> @@ -835,7 +835,7 @@ int PipelineHandlerRPi::configure(Camera *camera, CameraConfiguration *config)
> format.size = maxSize;
> format.fourcc = dev->toV4L2PixelFormat(formats::YUV420);
> /* No one asked for output, so the color space doesn't matter. */
> - format.colorSpace = ColorSpace::Jpeg;
> + format.colorSpace = ColorSpace::Sycc;
> ret = dev->setFormat(&format);
> if (ret) {
> LOG(RPI, Error)
> diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> index 3fc8438f6579..b22a981ff1a6 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 },
> diff --git a/src/py/libcamera/py_main.cpp b/src/py/libcamera/py_main.cpp
> index 505cc3dc1a0d..2a58e34a7cdb 100644
> --- a/src/py/libcamera/py_main.cpp
> +++ b/src/py/libcamera/py_main.cpp
> @@ -602,8 +602,8 @@ PYBIND11_MODULE(_libcamera, m)
> .def_readwrite("ycbcrEncoding", &ColorSpace::ycbcrEncoding)
> .def_readwrite("range", &ColorSpace::range)
> .def_static("Raw", []() { return ColorSpace::Raw; })
> - .def_static("Jpeg", []() { return ColorSpace::Jpeg; })
> .def_static("Srgb", []() { return ColorSpace::Srgb; })
> + .def_static("Sycc", []() { return ColorSpace::Sycc; })
> .def_static("Smpte170m", []() { return ColorSpace::Smpte170m; })
> .def_static("Rec709", []() { return ColorSpace::Rec709; })
> .def_static("Rec2020", []() { return ColorSpace::Rec2020; });
>
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