[libcamera-devel,RFC,1/4] libcamera: colorspace: Rectify ColorSpace::Srgb preset

16918 diff mbox series

Rectify the ColorSpace::Srgb to denote that it does not use
any Y'Cbcr encoding and uses full range.

The kernel on the other hand, recommends to use Rec601 as the encoding
for V4L2_COLORSPACE_SRGB. It is not very explicit but it can be
inferred that the kernel assumes V4L2_COLORSPACE_SRGB is a YUV-encoded
one. However, when the data is in RGB, no encoding is required (and
this is denoted by YcbcrEncoding::None in libcamera).

Hence, to be clear on the libcamera colorspace API, rectify the
ColorSpace::Srgb preset to use YcbcrEncoding::None.

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
---
 src/libcamera/color_space.cpp | 9 +++------
 src/libcamera/v4l2_device.cpp | 2 --
 2 files changed, 3 insertions(+), 8 deletions(-)

Hi Umang,

Thank you for the patch.

On Wed, Aug 03, 2022 at 12:27:16AM +0530, Umang Jain via libcamera-devel wrote:
> Rectify the ColorSpace::Srgb to denote that it does not use
> any Y'Cbcr encoding and uses full range.
> 
> The kernel on the other hand, recommends to use Rec601 as the encoding
> for V4L2_COLORSPACE_SRGB. It is not very explicit but it can be
> inferred that the kernel assumes V4L2_COLORSPACE_SRGB is a YUV-encoded
> one. However, when the data is in RGB, no encoding is required (and
> this is denoted by YcbcrEncoding::None in libcamera).
> 
> Hence, to be clear on the libcamera colorspace API, rectify the
> ColorSpace::Srgb preset to use YcbcrEncoding::None.
> 
> Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
> ---
>  src/libcamera/color_space.cpp | 9 +++------
>  src/libcamera/v4l2_device.cpp | 2 --
>  2 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/src/libcamera/color_space.cpp b/src/libcamera/color_space.cpp
> index caf39760..73148228 100644
> --- a/src/libcamera/color_space.cpp
> +++ b/src/libcamera/color_space.cpp
> @@ -254,16 +254,13 @@ const ColorSpace ColorSpace::Jpeg = {
>  };
>  
>  /**
> - * \brief A constant representing the sRGB color space
> - *
> - * This is identical to the JPEG color space except that the Y'CbCr
> - * range is limited rather than full.
> + * \brief A constant representing the sRGB color space (non-YUV format)

I would write "(RGB formats only)" to be even clearer.

>   */
>  const ColorSpace ColorSpace::Srgb = {
>  	Primaries::Rec709,
>  	TransferFunction::Srgb,
> -	YcbcrEncoding::Rec601,
> -	Range::Limited
> +	YcbcrEncoding::None,
> +	Range::Full
>  };
>  
>  /**
> diff --git a/src/libcamera/v4l2_device.cpp b/src/libcamera/v4l2_device.cpp
> index 3fc8438f..a4446fbf 100644
> --- a/src/libcamera/v4l2_device.cpp
> +++ b/src/libcamera/v4l2_device.cpp
> @@ -746,7 +746,6 @@ 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 },

This will cause an issue, as V4L2Device::toColorSpace() will return
nullopt if the kernel reports V4L2_COLORSPACE_SRGB. You should instead
use the kernel definition of the SRGB color space here:

	{ V4L2_COLORSPACE_SRGB, {
		ColorSpace::Primaries::Rec709,
		ColorSpace::TransferFunction::Srgb,
		ColorSpace::YcbcrEncoding::Rec601,
		ColorSpace::Range::Limited
	} },

However, that won't be enough. V4L2Device::toColorSpace() also needs to
set colorSpace.ycbcrEncoding to YcbcrEncoding::None and colorSpace.range
to Range::Full if the format is an RGB format. I think it would be best
to do so in a patch before this one.

>  	{ V4L2_COLORSPACE_SMPTE170M, ColorSpace::Smpte170m },
>  	{ V4L2_COLORSPACE_REC709, ColorSpace::Rec709 },
>  	{ V4L2_COLORSPACE_BT2020, ColorSpace::Rec2020 },
> @@ -772,7 +771,6 @@ 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 },

This, on the other hand, should be fine.

>  	{ ColorSpace::Smpte170m, V4L2_COLORSPACE_SMPTE170M },
>  	{ ColorSpace::Rec709, V4L2_COLORSPACE_REC709 },
>  	{ ColorSpace::Rec2020, V4L2_COLORSPACE_BT2020 },