Hi Hans,
(CC'ing David)
Thank you for your answer.
On Thu, Nov 24, 2022 at 01:54:27PM +0100, Hans Verkuil wrote:
> On 23/11/2022 22:37, Laurent Pinchart wrote:
> > 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*.
>
> Correct. And technically, that is also correct: a colorspace describes
> just the primaries and whitepoint.
This dual definition of colorspace (historical vs current) really hurts
:-( Is it worth stating this *very* explicitly in the documentation ?
> > 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 ?
>
> Yes.
>
> >>>> 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 ?
>
> I assume that's a question for Dave, but that is what I would expect.
> My gut feeling is that the enum just selects one of three predefined
> 3x3 matrices + vector. In which case it does not touch the transfer
> function, let alone the colorspace.
>
> But without knowing the details, I can't be certain of course.
As far as I understand, the colorspace enum passed to the firmware also
influences the tone mapping LUT. It doesn't set the LUT values by
itself, as there are other imaging parameters that need to be taken into
account to calculate the LUT, but it influences it (the tone mapping LUT
is really two LUTs combined into one, a transfer function LUT and a tone
mapping LUT).
> >>>>> 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 ?
>
> Yes. Perhaps this would be better:
>
> "then that colorimetry setting will be ignored and the driver will set it
> to a suitable value."
>
> Or something along those lines...
How about this ?
Set by the application. It is only used for capture and is ignored for
output streams. If set, then request the device to do colorspace
conversion from the received colorspace to the requested colorspace
values. If the colorimetry field (colorspace, xfer_func, ycbcr_enc,
hsv_enc or quantization) is set to *_DEFAULT, then the driver will set
that field to a suitable value. Drivers may also adjust any of the
colorimetry fields when the requested values are not supported, in
isolation or combined together.
When possible and applicable, drivers should select the default values
to match the colorimetry of the data that was received (from a live
source for a captude device, or from memory for a memory-to-memory
device). In cases where this isn't possible drivers should select the
default values to match the known parameters. For instance, a driver
for an ISP converting raw Bayer data to YUV should select the default
YCbCr encoding based on the colorspace field.
As drivers are given a degree of freedom to select default colorimetry
values, applications should always set the four colorimetry fields to
avoid unexpected behaviours and maximize portability between devices.
> >>>> 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 ?
>
> This example describes what happens without any overrides from userspace.
> So either V4L2_PIX_FMT_FLAG_SET_CSC is cleared, or all four colorimetry
> fields are set to DEFAULT.
>
> And see above for a better description of what the spec should say.
Then my proposal above isn't quite right, as selecting LIM_RANGE on the
capture side when the received data has FULL_RANGE doesn't match the
text in my opinion. I'm not sure how to word it, how would you express
why a driver would select LIM_RANGE here ?
> >>>> 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.
>
> Without SET_CSC. Sorry, I should have been clearer.
>
> >> 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.
>
> It all depends what the device actually does: if it takes the raw data,
> then does color correction and produces a usable picture, then that
> would imply that the final image conforms to one of the colorspaces
> and transfer functions.
>
> If it just converts from Bayer to NV12, then it would produce, as you
> say, an unusable picture. Garbage in, garbage out.
Agreed. But it gets tricky :-)
For a device that only performs CFA interpolation, I agree, the
colorspace on the capture side should be RAW in the above example.
That's the easy part.
An ISP will, under normal circumstances, produce a well-defined
colorspace. For this to happen, the quantum efficiency of the camera
sensor to different wavelengths need to be taken into account, and
suitable values must be programmed in the linearization LUT and the RGB
to RGB matrix (possibly among other parameters) The process involves
calculations that are often done in userspace. The colorspace on the
capture video node is thus the result of a set of ISP parameters and
intrinsic sensor information, instead of being a value set by userspace
that drives the configuration of the ISP. It's not really feasible in
that case for the ISP driver to know what colorspace it outputs. That is
why the rkisp1 driver will blindly accept any colorspace set by
userspace on the ISP's subdev source pad, in order to propagate that
value along the pipeline up to the capture video node.
In other cases, the ISP driver may expose a higher-level API that
accepts a colorspace value and calculates all the low-level ISP
parameters internally (likely in the firmware, possibly in the driver,
although the latter doesn't make much sense in my opinion). I don't
think this is what the Raspberry Pi ISP does for the colorspace, as it
has a CCM matrix exposed to userspace (Dave, please correct me if I'm
wrong).
For the Raspberry Pi ISP, I think the driver should turn colorspace =
DEFAULT into RAW as it can't know what colorspace will be output, and
accept any other colorspace value blindly. Unless I'm mistaken (David,
please correct me if I'm wrong), the tuning tool (part of libcamera)
computes the color correction matrix values in order to produce sRGB,
and the algorithms (still in libcamera) don't modify that at runtime.
The ISP will thus always produce sRGB, but I don't think we should
hardcode that in the driver.
> I have considered adding an XFER_FUNC_RAW setting. As opposed to NONE,
> which really just means a linear mapping. It might make sense to
> introduce that, I would have no problem with it.
I'm not opposed to it, as long as we carefully consider the implications
on userspace code (libcamera would be a great playground for that, our
colour space coding is quite complex already, it would be good to see if
a new value would be easily manageable in respect to both forward and
backward compatibility).
> > 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.
>
> Either is fine. But historically you will see a lot of COLORSPACE_JPEG usage,
> and that won't go away. So you'll have to support both in libcamera.
We've already spend lots of time trying to support colour space
management correctly, and I'm sure we will continue doing so :-) David
has already raised some issues related to code changes I've recently
proposed.
Let's first reach an agreement on how the Raspberry Pi ISP driver should
handle the V4L2 colorimetry fields upstream, and then we'll implement
this correctly in libcamera (with as much backward compatibility as
possible).
> >>>>> 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.
>
> Right.
>
> > 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.
>
> Exactly. The precise behavior of the driver will depend on what the
> device actually does.
>
> >> 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).
>
> True, but in order to know how to display final picture the ISP delivers,
> you need to know against what colorspace and transfer function have to be
> used to interpret the result.
>
> An ISP takes a raw image and processes it to deliver something that is
> using well-defined colorimetry. If it is not well defined you basically
> still have no idea how to interpret the colors.
We agree on this.
> >>>> 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 ?
>
> No, this is a valid thing to do.
OK, this is the only part where I'm confused, as it doesn't match my
recollection of our discussions that the colorspace field never acts as
a preset in the new API. I'll read on and comment just below.
> So if I understand it the device always produces YUV video, but in
The ISP has two outputs, which can independently produce RGB or YUV.
> three different modes: something that is suitable for a JPEG encoder
> that expects full range sRGB, SDTV-style colorimetry and HDTV-style
> colorimetry. In that case setting V4L2_FMT_FLAG_CSC_COLORSPACE is
> actually valid. If userspace doesn't override the colorspace, then
> the default behavior will be used (SDTV or HDTV, depending on the
> resolution), or it outputs according to the chosen colorspace and
> the settings for the other three fields implied by the preset.
First of all, if V4L2_COLORSPACE_JPEG shouldn't be used (as you
mentioned below), how would userspace do this ? Let's assume the
application wants to capture YUV data from the ISP in order to compress
it in JPEG. It needs the ISP to produce sRGB primary colours
chromaticities, sRGB transfer functio, BT.601 YCbCr encoding and full
quantization. If V4L2_COLORSPACE_JPEG can't be used, I see no other way
than requesting the following values :
.colorspace = V4L2_COLORSPACE_SRGB,
.xfer_func = V4L2_XFER_FUNC_SRGB,
.ycbcr_enc = V4L2_YCBCR_ENC_601,
.quantization = V4L2_QUANTIZATION_FULL_RANGE,
As a consequence, the driver would need to report all the
V4L2_FMT_FLAG_CSC_COLORSPACE, V4L2_FMT_FLAG_CSC_XFER_FUNC,
V4L2_FMT_FLAG_CSC_YCBCR_ENC and V4L2_FMT_FLAG_CSC_QUANTIZATION flags to
make it possible to set individual fields.
The next consequence is that, if the application sets
V4L2_PIX_FMT_FLAG_SET_CSC, and wants to use a preset by setting
colorspace to a preset value and set xfer_func, ycbcr_enc and
quantization to *_DEFAULT, then the driver will be free to select
defaults in an unspecified way (copying them from the input would be
valid behaviour for instance). This will lead to unpredictable behaviour
for userspace.
The only way I could see presets working is if the driver reports
V4L2_FMT_FLAG_CSC_COLORSPACE only, without the other three
V4L2_FMT_FLAG_CSC_* flags. In that case we could define the colorspace
field as a preset, as I don't think there will ever be hardware that can
modify the primary colours chromaticities only without also allowing
modifications of the other colorimetry values. This means that a driver
that uses presets will not be able to support any combination of
colorimetry parameters that do not match one of the presets. If a driver
wanted to support such non-standard combinations, it would need to set
multiple V4L2_FMT_FLAG_CSC_*, and we would be back to the problem
described in the previous paragraph.
This is probably doable, but I think it will significantly increase
complexity in userspace. Userspace will need to check if only
V4L2_FMT_FLAG_CSC_COLORSPACE is set or if other flags are set too, and
use that to decided whether to use the preset API or the full API
(hopefully that would work, but I'm always worried I'm forgetting about
some corner cases).
This also means that, among the available V4L2 presets, only
V4L2_COLORSPACE_REC709, V4L2_COLORSPACE_SRGB and V4L2_COLORSPACE_JPEG
would correctly match the fact that the ISP produces sRGB primary
colours chromaticities. I suppose we could ignore that, as the
chromaticities are not configured through the colorspace field (the
colour correction matrix is passed explicitly to the driver), but it
will make it even more difficult in userspace. If a user were to request
different chromaticities, libcamera could calculate a different colour
correction matrix, but it wouldn't be able to reflect that in the
presets it selects to program the ISP. There would be a need for
device-specific logic to convert the colorimetry information exposed to
applications to the V4L2 preset.
Documenting all this properly will also be difficult, and without proper
documentation, nobody will get it right (even with proper documentation
most drivers and applications get it wrong :-)).
As you can probably tell, I don't think all that is worth it. I think we
would be better off not dealing with presets and specifying the four
colorimetry fields explicitly. If someone really believes we need to
also support a preset API and is willing to document it (and make sure
it will work in all corner cases), then I'm fine with accepting support
for it in libcamera, if it can be done in a clean way.
> You wouldn't set the other V4L2_FMT_FLAG_CSC_ flags since you can't
> influence them separately, they are fixed depending on the colorspace
> (aka preset) itself.
At least the chromaticities can be (and actually are, in all cases)
influenced separately. It's not a blocker, we can probably live with the
device producing chromaticities that don't match the colour space preset
(at the price of device-specific logic in userspace), but I would prefer
keeping hacks as a last resort, not as the primary design option :-)
I still would prefer if the firmware allowed the YCbCr encoding and
quantization to also be set separately from the transfer function, but
that's not my call.
> >>>> 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.
>
> Ah, I didn't know that. Then you really should not use COLORSPACE_JPEG,
> that is something that should only be used for actual JPEG compressed
> video. It's very weird to see it used for uncompressed video. Legal,
> but weird.
In libcamera we call V4L2_COLORSPACE_JPEG "sYCC". It decouples the
colour space name from the image format.
> I think there are still a bunch of old sensor drivers that incorrectly
> report JPEG as well, even though they just transmit uncompressed video.
> I never dared to change them, it's hard to predict what might break.
>
> > 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 ?
>
> Yes.
>
> >>> - 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.
>
> The former, yes.
>
> >> 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 :-)
>
> I believe this is the first time an ISP needs this, so it's good to discuss
> this!
The rkisp1 driver was the first :-) It was the first to use the
V4L2_PIX_FMT_FLAG_SET_CSC flag. It didn't implement everything correctly
though, we have fixed that in v6.1.
> >>>>> [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
