Hello,
On Wed, Nov 23, 2022 at 03:17:16PM +0100, Hans Verkuil wrote:
> On 23/11/2022 10:52, Dave Stevenson wrote:
> > On Tue, 22 Nov 2022 at 14:23, Hans Verkuil wrote:
> >> On 11/22/22 12:49, Dave Stevenson wrote:
> >>> Hi Hans
> >>>
> >>> This has come about from a discussion with Laurent over how to handle
> >>> colorspace fields, whether a particular configuration is legitimate,
> >>> and whether we're looking at the correct behaviour. You're the go-to
> >>> person for that sort of question :-)
> >>>
> >>> - CAPTURE queue (in this case on a M2M ISP device, but that doesn't
> >>> really matter).
> >>> - Limited colorspace options are available from the device (standard
> >>> SDTV BT601 limited range, HDTV BT709 limited range, and JPEG's BT601
> >>> full range).
> >>> - VIDIOC_ENUM_FMT flags returns V4L2_FMT_FLAG_CSC_COLORSPACE. It does
> >>> NOT set V4L2_FMT_FLAG_CSC_XFER_FUNC, V4L2_FMT_FLAG_CSC_YCBCR_ENC, or
> >>> V4L2_FMT_FLAG_CSC_QUANTIZATION. (No documentation saying this isn't
> >>> permitted, and why have 4 flags if they aren't independent).
> >>> - VIDIOC_S_FMT called with V4L2_PIX_FMT_FLAG_SET_CSC set.
> >>>
> >>> Which colourspace fields from the format are applied?
> >>
> >> If only V4L2_FMT_FLAG_CSC_COLORSPACE is set, then only the colorspace
> >> field from userspace is applied. Which is almost certainly wrong
> >> sicne switching between the colorspace that is received and the
> >> colorspace that you want requires complex calculations. And if a
> >> device can do that, then it certainly can also allow userspace to
> >> override the other three colorimetry fields. So setting just
> >> V4L2_FMT_FLAG_CSC_COLORSPACE makes no sense.
I think part of the confusion is that we use the word "colorspace"
through the API to mean two things. The text below refers to capture
queues only, for output queues the story is different.
Historically, the colorspace field and the word colorspace has meant a
colour space preset, a single value combining the primary colours
chromaticities, the transfer function, and, for YUV formats, the YCbCr
encoding and the quantization range.
Later, explicit fields were added to describe the transfer function,
YCbCr encoding and quantization range. The colorspace field kept is
name, but, if my understanding is correct, shifted to describe the
primary colours chromaticities *only*.
Drivers that didn't support the new fields would set them to 0, which
maps to V4L2_MAP_XFER_FUNC_DEFAULT, V4L2_MAP_YCBCR_ENC_DEFAULT and
V4L2_MAP_QUANTIZATION_DEFAULT. To preserve backward compatibility,
userspace was required to interpret DEFAULT values as the default
implied by the colorspace field and the format type (RGB vs. YUV).
This scheme was designed to only cover drivers that do no allow
userspace to modify the colour space, as all this predates the addition
to the V4L2 specification of the V4L2_FMT_FLAG_CSC_* flags. Drivers that
support V4L2_FMT_FLAG_CSC_* must use the new scheme, and interpret the
colorspace field as meaning the primary colours chromaticities only.
Hans, is this correct ?
> >> The V4L2_FMT_FLAG_CSC_* flags come into play if you want to allow
> >> userspace to convert the colorimetry of the captured data to another
> >> colorimetry. That's often not supported, the colorimetry fields are
> >> just filled in based on what was captured. Often there is some support
> >> to convert between YCBCR/RGB/QUANTIZATION settings, so such devices
> >> can set V4L2_FMT_FLAG_CSC_YCBCR_ENC | V4L2_FMT_FLAG_CSC_QUANTIZATION
> >> if they want. This is typically a 3x3 matrix + one vector in the hardware.
> >
> > It's not a simple 3x3 matrix + vector in this case, and selection is
> > via an enum.
As far as I understand, for this specific device, the firmware API uses
an enum, but the hardware could do more than the enum values exposed by
the firmware, is that right ?
> >>> The driver is saying that only colorspace is supported, and [1] says that
> >>
> >> No, the driver is saying that it can convert to another colorspace
> >> if requested to do so by userspace, while leaving the xfer function,
> >> ycbcr encoding and quantization range untouched.
> >> That's a highly unlikely situation, and I think it's likely a bug
> >> in the driver. It should probably drop that flag.
> >>
> >>> "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"
> >>> so we have all 4 parameters defined via the defaults.
> >>> I read it that the ycbcr_enc, quantization, and xfer_func values
> >>> passed in should be ignored and replaced with the "default" values
> >>> derived from the colorspace value (use V4L2_MAP_XFER_FUNC_DEFAULT,
> >>> V4L2_MAP_YCBCR_ENC_DEFAULT, and V4L2_MAP_QUANTIZATION_DEFAULT)
> >>> Is this a valid interpretation?
> >>
> >> No. First of all, unless one or more of the V4L2_FMT_FLAG_CSC_* flags are
> >> set, the driver fills in these fields, and ignores what userspace
> >> filled in. So [1] just describes what these fields mean, and that you
> >> can use those DEFAULT macros to determine what the actual xfer_func,
> >> ycbcr_enc or quantization is if the driver left those values to 0.
> >>
> >> What those defaults are depends on the chosen colorspace field.
> >>
> >>> Confusion comes from [2] for V4L2_PIX_FMT_FLAG_SET_CSC saying:
> >>
> >> So if this flag is set, then userspace can ask the driver to convert
> >> to specific colorimetry settings, if supported (what is supported is
> >> indicated by the V4L2_FMT_FLAG_CSC_* flags returned by ENUM_FMT).
> >>
> >>> "If the colorimetry field (colorspace, xfer_func, ycbcr_enc, hsv_enc
> >>> or quantization) is set to *_DEFAULT, then that colorimetry setting
> >>> will remain unchanged from what was received."
> >>> What is "received" in this case?
> >>
> >> This relates to a capture device, so for an m2m device that means
> >> the colorimetry that userspace set for the output device.
> >
> > This is an ISP. What is the xfer function, ycbcr encoding and
> > quantization range settings for a Bayer image with colorspace
> > V4L2_COLORSPACE_RAW? The documentation does not work for that use
> > case.
>
> It wasn't clear to me that this was specifically about raw Bayer.
>
> For COLORSPACE_RAW there really is no xfer_func as such. V4L2_MAP_XFER_FUNC_DEFAULT
> sets it to NONE for COLORSPACE_RAW, but the documentation for NONE is a bit
> misleading since it says that that means linear. For COLORSPACE_RAW is really
> means custom. I am wondering if a XFER_FUNC_RAW or _CUSTOM should be introduced
> to properly signal this. I am certainly not opposed to that.
>
> The YCbCr encoding will be DEFAULT for Bayer (really, it is ignored, but
> it is good practice to set it to DEFAULT in that case). If you DO convert
> to YCbCr, then it has to follow some standard.
>
> The quantization range will be FULL for Bayer. Again, something else doesn't
> make sense for Bayer.
Note that it's the ISP input that is a raw bayer frame. The ISP output
is processed RGB or YUV, and has a colour space. In this case, the
documentation of the V4L2_PIX_FMT_FLAG_SET_CSC isn't great. Quoting it
again,
If the colorimetry field (colorspace, xfer_func, ycbcr_enc, hsv_enc or
quantization) is set to *_DEFAULT, then that colorimetry setting will
remain unchanged from what was received.
Clearly, when outputting RGB or YUV, even if userspace sets color
space-related fields to *_DEFAULT, the ISP will not output "what was
received". I think it should pick a suitable default. Does this make
sense to everybody ?
> >> A typical m2m device will just copy the colorimetry fields from output
> >> to capture format. If you want it to act as a csc device, then it will
> >> have to advertise the relevant V4L2_FMT_FLAG_CSC_* flags.
> >>
> >> If it converts between different pixelformats (e.g. RGB to YUV), then
> >> there is an implicit csc step, of course, but the result will be using
> >> the default rules.
> >>
> >> Example: userspace passes a frame with V4L2_COLORSPACE_BT2020, V4L2_XFER_FUNC_709,
> >> V4L2_YCBCR_ENC_DEFAULT (n/a) and V4L2_QUANTIZATION_FULL_RANGE, and wants
> >> a NV12 back. The colorimetry for that will be V4L2_COLORSPACE_BT2020,
> >> V4L2_XFER_FUNC_709, V4L2_YCBCR_ENC_BT2020 (most likely, hw limitations might
> >> force this to 709 or 601) and V4L2_QUANTIZATION_LIM_RANGE.
That example is a bit underspecified, to clarify it, did you mean the
above for the case where userspace sets all four colorimetry fields to
*_DEFAULT on the capture side ? If so, I agree about the colorspace,
xfer_func and ycbcr_enc values. For the quantization values, following
the specification to the letter ("then that colorimetry setting will
remain unchanged from what was received"), shouldn't quantization be
V4L2_QUANTIZATION_FULL_RANGE on the capture side ?
> >> So this is all determined by the driver, not userspace.
> >
> > Now I pass a Bayer frame of colorspace V4L2_COLORSPACE_RAW,
> > V4L2_XFER_FUNC_NONE in, and ask for NV12 out. What properties do I use
> > on that NV12 frame please?
Let's be precise. Is that with or without V4L2_PIX_FMT_FLAG_SET_CSC ? If
with it, what values do you set for all the colorimetry fields,
*_DEFAULT ? I'll assume so in the comments below.
> Bayer frame:
> colorspace = RAW
> xfer_func = NONE
> ycbcr_enc = DEFAULT (ignored)
> quantization = FULL
>
> NV12 frame:
> colorspace = RAW
> xfer_func = NONE
While this is technically valid, the RAW primary colours chromaticities
would effectively be unusable for anyone who cares about colours, and
the linear transfer function would likely not be what a user would
expect.
One option would be to consider this to be userspace's fault, and
produce what has been requested, even if broken. I'm not opposed to
that.
When the hardware can not produce linear output, it would adjust the
colorimetry fields to what is supported. A suitable default would be
picked for xfer_func in that case. We could also pick suitable defaults
for fields set to *_DEFAULT instead of producing RAW and NONE.
Obviously, if the hardware supports linear output and xfer_func is set
to NONE on the capture side by userspace, then linear should be
produced.
> ycbcr_enc = 601 for SDTV, 709 for !SDTV
> quantization = LIMITED
>
> > Likewise I could pass in RGB with colorspace V4L2_COLORSPACE_SRGB and
> > ask for NV12 out that is to be JPEG encoded. How do I ensure I get the
> > appropriate conversion parameters?
>
> RGB frame:
> colorspace = SRGB
> xfer_func = SRGB
> ycbcr_enc = DEFAULT (ignored)
> quantization = FULL
>
> JPEG frame:
> colorspace = SRGB
> xfer_func = SRGB
> ycbcr_enc = 601
> quantization = FULL
>
> (or you just set colorspace to COLORSPACE_JPEG, which is a shorthand for this)
This last sentence makes me *very* uncomfortable, if the colorspace
field describes the primary colours chromaticities only in the recent
colorimetry API, we should stop talking about shorhands at all.
> >>> There is no inherent colourspace for
> >>> the device as it is M2M, so does that come back to being default
> >>> anyway, or reflecting the OUTPUT queue which might be Bayer and have
> >>> no range? Can we still ignore them all as the relevant
> >>> V4L2_FMT_FLAG_CSC_* flags aren't set?
> >>>
> >>> Hopefully you can enlighten us.
> >>
> >> So, to recap:
> >>
> >> 1) for an m2m device it is userspace that provides the colorimetry data
> >> in the output format. For a regular capture device it is the
> >> video receiver driver that sets it (typically determined by the
> >> sensor driver or a video receiver driver).
Agreed.
> >> 2) Implicit CSC conversion might take place when converting between
> >> different pixel formats. Typically this will only affect the
> >> ycbcr_enc and quantization fields, since that's usually all that
> >> is supported. The colorspace and xfer_func fields are just copied
> >> from the output pixelformat.
> >
> > As above, RGB or Bayer in to NV12 out makes no sense to copy
> > colorspace and xfer_func fields from the output pixelformat.
Why does it make no sense ? Especially for RGB inputs, the primary
colours chromaticities and the transfer function should be well-defined
on the output side, so they could be copied to the capture side.
For Bayer formats it makes less sense in that the output would then be
unusable for anyone who cares about proper colour rendering. As the ISP
will likely output images that have defined primary colours
chromaticities, it could replace the colorspace = DEFAULT on the capture
side with the real colorspace (chromaticities) as proposed above, and
also possibly replace xfer_func = DEFAULT with a proper non-linear
default instead of setting it to NONE. However, also as mentioned above,
I would be fine considering that if userspace wants to shoot itself in
the foot, we could let it do so. It would then be userspace's
responsibility to set the colorspace and xfer_func fields to proper
non-default values on the capture side.
> Perhaps you should describe what your m2m device actually *does*? Ah,
> you did at the end :-)
>
> The m2m devices that I know just convert RGB to YUV or JPEG (or vice
> versa). In both cases the colorspace and transfer function remain as-is,
> so copying these two values from the output pixelformat is the right
> thing to do. It's all you have, nobody else can fill it in. Typically
> userspace will get this information from the sensor or video receiver.
>
> The process of converting a raw Bayer sensor to something presentable
> is something an ISP does, but the end result will have to specify
> valid (i.e. non-raw) colorspace/xfer_func/ycbcr_enc/quantization colorimetry,
> otherwise nobody will know how to interpret the resulting frame.
ISPs typically handle colorimetry in various places in the processing
pipeline. There's often a linearization LUT early on, followed by CFA
interpolation (debayering), a 3x3 RGB to RGB matrix (to compensate for
cross-talk and also to convert primary colours chromaticities), a tone
mapping LUT that handles the transfer function (plus other parameters),
a 3x3 matrix + 1x3 offset vector to convert RGB to YUV, and a clamp for
the quantization range. The 1x3 offset vector is sometimes automatically
computed by the hardware from the quantization range settings, and the
3x3 matrix may or may not take quantization into account. Given that the
Y line of the matrix has to have a sum of 1.0 and the U and V lines a
sum of 0.0, some hardware may not allow specifying all 9 values, but
derive some automatically.
All those parameters interact with colorimetry, but they can't be fully
controlled by the four colorimetry fields. Drivers may compute values
for all parameters based on the four colorimetry fields and program the
ISP accordingly, but in many cases the ISP driver will expose those
low-level parameters to provide full configurability. In that case the
colorimetry fields will be ignored. Some drivers also mix-and-match the
two options, exposing some low-level parameters directly (for instance
the linearization LUT, RGB to RGB matrix and tone mapping LUT), and
control other parameters through the colorimetry fields (in the same
example, the RGB to YUV 3x3 matrix and 1x3 vector and the clamp range,
exposed through the ycbcr_enc and quantization fields, while the
colorspace and xfer_func fields are ignored).
> >> 3) If explicit CSC conversion is signaled by the driver by setting
> >> V4L2_FMT_FLAG_CSC_* flags, then userspace can request specific
> >> colorimetry results, and the hardware will be configured by the
> >> driver to give that result.
> >
> > Not all results are achievable by all hardware.
> > So do we set all the V4L2_FMT_FLAG_CSC_* flags and just alter the bits
> > of the request that aren't supported in a way that the driver sees
> > fit?
If not all combinations are supported by the hardware (or firmware,
although in that case one may dream of getting the firmware updated to
offer more control of individual settings ;-)), that is what I would
recommend, yes. My understanding is that setting the
V4L2_FMT_FLAG_CSC_COLORSPACE flag only and using the colorspace field as
a preset doesn't comply with the V4L2 specification. Hans, is this
correct ?
> >> I hope this helps!
> >
> > Sort of, but only by muddying the waters.
> >
> > Flipping it around then and asking how it should be implemented.
> >
> > We have configuration for the ISP output path that produces 3
> > configurations regardless of input format corresponding to:
> > - V4L2_COLORSPACE_JPEG, V4L2_YCBCR_ENC_601, V4L2_XFER_FUNC_SRGB,
> > V4L2_QUANTIZATION_FULL_RANGE
>
> Here you request the JPEG pixelformat, so that forces the output
> configuration. So no need for any V4L2_FMT_FLAG_CSC_* for this pixel
> format.
The ISP doesn't produce JPEG data, it produces YUV.
Hans, do I understand correctly that, when producing YUV, for a driver
that have all the V4L2_FMT_FLAG_CSC_* flags set, if userspace sets
V4L2_PIX_FMT_FLAG_SET_CSC, the following two settings
.colorspace = V4L2_COLORSPACE_JPEG,
.xfer_func = V4L2_XFER_FUNC_SRGB,
.ycbcr_enc = V4L2_YCBCR_ENC_601,
.quantization = V4L2_QUANTIZATION_FULL_RANGE,
and
.colorspace = V4L2_COLORSPACE_SRGB,
.xfer_func = V4L2_XFER_FUNC_SRGB,
.ycbcr_enc = V4L2_YCBCR_ENC_601,
.quantization = V4L2_QUANTIZATION_FULL_RANGE,
will produce the exact same result ?
> > - V4L2_COLORSPACE_SMPTE170M, V4L2_YCBCR_ENC_601, V4L2_XFER_FUNC_709,
> > V4L2_QUANTIZATION_LIM_RANGE
> > - V4L2_COLORSPACE_REC709, V4L2_YCBCR_ENC_709, V4L2_XFER_FUNC_709,
> > V4L2_QUANTIZATION_LIM_RANGE
>
> Here you select a YUV format (NV12 or similar). The default colorimetry
> it should select is V4L2_COLORSPACE_SMPTE170M for SDTV and V4L2_COLORSPACE_REC709
> otherwise. If you want to allow this to be overridden, then that can be
> done with V4L2_FMT_FLAG_CSC_COLORSPACE for these two pixelformats.
>
> If userspace selects an unsupported colorspace, then the driver should
> pick a supported colorspace (almost certainly you would want V4L2_COLORSPACE_REC709).
>
> > (Those should be the the same ycbcr_enc, xfer_func, and quantization
> > as the defaults for each colourspace, and translate to the normal
> > values used for JPEG, SDTV, and HDTV respectively).
> > These are selected by enum and we are not proposing on adding any extra ones.
> >
> > The input is going to be Bayer with colorspace set to
> > V4L2_COLORSPACE_RAW. There are therefore no colorspace parameters that
> > can be "left unchanged" or otherwise copied from the OUPTUT queue.
>
> So this device does a lot more than most m2m devices: it apparently knows
> how to convert the raw Bayer data to a proper picture.
>
> I do wonder if it is really converting correctly to these three colorimetry
> permutations. If you give it a test pattern, then save the result in these
> three format and visually compare them, do they look identical? If the ISP
> really does convert correctly, then you should not be able to see any
> difference.
Do you mean not seeing any difference when displaying the images on a
display that correctly takes the different colour spaces into account,
or not seeing any difference in the binary contents of the captured
images ? I would assume the former.
> There is a clear difference between V4L2_XFER_FUNC_SRGB and
> V4L2_XFER_FUNC_709 when seen side-by-side. Note that the difference between
> V4L2_COLORSPACE_SMPTE170M and V4L2_COLORSPACE_REC709 is pretty much impossible
> to see, they are very close.
>
> > What is the correct way to allow userspace selection of those options
> > within V4L2?
> >
> > Looking at the datasheet for TC358743 (a chip that your company
> > maintains the driver for), it can convert the incoming HDMI to 6
> > output formats via the VI_REP register field VOUT_COLOR_SEL:
> > RGB Full
> > RGB Limited
> > 601YCbCr Full
> > 601 YCbCr Limited
> > 709 YCbCr Full
> > 709 YCbCr Limited
> > This is the same situation as for our ISP - configuration through an
> > enum even though there is more complexity underneath.
> >
> > Currently the driver always selects 601 YCbCr Limited for UYVY output,
> > or RGB Full for 24bpp RGB.
> > If we were to extend that driver in a non-media controller system to
> > allow selection of the converted colourspace, how do we enforce those
> > restrictions?
>
> That chip can only convert between RGB/YUV and limited/full quantization
> range. The colorspace and xfer_func will be whatever is received in the AVI
> InfoFrame of the HDMI video stream. So this is a typical example of a
> 3x3 matrix + vector implementation. It comes with a bunch of default matrixes
> (hardcoded and selected by the enum), but I believe it can be manually set
> as well. I can't remember if that's done in this driver.
>
> If you want to manually set ycbcr_enc and quantization, then you would
> have to specify V4L2_FMT_FLAG_CSC_YCBCR_ENC | V4L2_FMT_FLAG_CSC_QUANTIZATION.
>
> > Under MC you get a similar set of flags in V4L2_MBUS_FRAMEFMT_SET_CSC
> > for struct v4l2_mbus_framefmt and set_format, and
> > V4L2_SUBDEV_MBUS_CODE_CSC_* in struct v4l2_subdev_mbus_code_enum, so
> > the same principles will presumably apply there too.
>
> Yes.
>
> > (I'm noting that the tc358743 driver currently doesn't bother setting
> > any of the colorimetry fields other than colorspace).
>
> I believe the driver predates these additional colorimetry fields.
Very few drivers implement the API correctly. Hopefully this ISP driver
will join the ranks of the good ones :-)
> >>> [1] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/colorspaces-defs.html
> >>> [2] https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/pixfmt-v4l2.html#v4l2-pix-fmt-flag-set-csc
--
Regards,
Laurent Pinchart
