[RFC 0/1] Color manager calibration protocol v1

Pekka Paalanen ppaalanen at gmail.com
Wed Apr 17 09:32:38 UTC 2019

On Tue, 16 Apr 2019 23:42:30 +0200
Erwin Burema <e.burema at gmail.com> wrote:

> On Tue, 16 Apr 2019 at 17:03, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> >
> > On Tue, 16 Apr 2019 13:33:02 +0000
> > Erwin Burema <e.burema at gmail.com> wrote:
> >  
> > > On Tuesday, 16 April 2019, Pekka Paalanen wrote:  
> > > > On Tue, 16 Apr 2019 13:11:06 +0200
> > > > Erwin Burema <e.burema at gmail.com> wrote:
> > > >  
> > > > > On Tue, 16 Apr 2019 at 12:45, Pekka Paalanen <ppaalanen at gmail.com> wrote:  
> > > > > >
> > > > > > On Sun, 14 Apr 2019 12:57:47 +0200
> > > > > > Erwin Burema <e.burema at gmail.com> wrote:
> > > > > >  
> > > > > > > Without a way to calibrate/profile screens an color management
> > > > > > > protocol looses a lot of its value. So to add this missing feature I
> > > > > > > wrote the following protocol.
> > > > > > >
> > > > > > > The idea is that the calibration/profiling SW only sets the RGB
> > > > > > > triplet and then the compositor is responsible to draw a rectanglular
> > > > > > > region on the selected output screen, since not all calibration tools
> > > > > > > will be at the center of the screen a user should be able to modify
> > > > > > > the placement of this rectanglular region. Unless specified the
> > > > > > > monitor profile (if any) should not be applied but the GPU curve
> > > > > > > should, currently to set a new curve the calibration tool should
> > > > > > > generate a new ICC profile with the wanted curve in the VCGT tag (I
> > > > > > > am not sure if this is the best option but would make the most
> > > > > > > universal one). In the end after profiling the last uploaded ICC
> > > > > > > could then be saved (although a compositor is not required to honor
> > > > > > > the request in that case it should send the not saved error). If the
> > > > > > > compositor doesn't save or the connection with this protocol is
> > > > > > > broken the compositor should restore previous settings.  
> > > > > >
> > > > > > Hi,
> > > > > >
> > > > > > I only took a very quick glance, but I do like where this design is
> > > > > > going. I'll refrain from commenting on wl_surface vs. not for now
> > > > > > though.
> > > > > >
> > > > > > Forgive me my ignorance, but why is the "GPU curve" needed to be a
> > > > > > custom curve provided by the client?  
> >
> > Hi,
> >
> > ok, below we are finally getting to the point.
> >  
> > > > Ok, you start with an identity curve and iterate. Why only the "GPU
> > > > curve" instead of a "full" color correction transformation?
> > > >  
> > >
> > > Since we are trying to setup the "GPU curve" in this case a full
> > > transform would only get in the way.  
> >
> > Why?
> >  
> Good question!

Hi Erwin,

there seems to be two very different cases, and below we discuss one or
the other paragraph by paragraph, which might be confusing.

	Case 1: Calibrating and/or profiling the monitor only

This case is what I see as controlling the pixel values on the wire,
and the identity LUT instead of "GPU curve" access. I explain why no
"GPU curve" in my opinion further below.

	Case 2: Calibrating and/or profiling the full color pipeline
		from application to a specific monitor

This would be using the normal compositor color mapping pipeline and
adjusting the full output calibration and profile to achieve a desired
color reproduction.

These two are mutually exclusive approaches, so they have independent
rationales and protocol requirements. I am still not sure which one you
want. In some old discussions with Graeme I think he advocated for Case
2, so that a user will actually get what he wants even with a faulty

While Case 2 might be good for the end user, it also aims to hide any
compositor bugs in their color pipelines, and makes the resulting
profile potentially invalid for other compositors.

I'm arguing a lot in the below. It's not so much that I think the
proposal is wrong, I'm not knowledgeable enough to fully judge that. My
aim is to avoid blindly repeating the design of X11-based software
stacks in case there might be a better design now that we have a
display server that is expected to participate in color management and
do color mapping on behalf of apps. With X11 that was not really
possible until the advent of X11 compositing managers. It is such a
fundamental design change, that I really want to prod people to try to
see the new possibilities. I just hope I don't become a "difficult
person" because of that. :-)

> > The goal is to create a full color transform from some client chosen
> > color space into the output color space, so that we can determine the
> > color profile of the output. You might iterate only on the curve at
> > first, but do you not then iterate on the full transform as well to
> > ensure you get the result you want, at least to check once? How will do
> > that with your extension?
> >  
> No here we are not interested in the full transform that would be to
> limited we are only interested in the output part, especially we are
> interested in how a certain RGB triplet send to the display maps to
> X'Y'Z' (which is an absolute color space used as one of the two
> options for the color connection space in the icc profile) the X'Y'Z'
> triplet will be provided by the colorimeter/spectrometer which measure
> the patch. This of course means that before we have a profile we have
> to iterate over many patches. My extension is meant to be able for
> calibration software to set a certain RGB triplet for the patch after
> which the software will read out the X'Y'Z' value from the calibration
> tool (most likely via USB these days, although serial is also an
> option)

This sounds like Case 1. In other words, you want to be able to set the
pixel values directly on the wire, so that only the monitor will affect
the measurement results.

> The above is what is called characterization or profiling, another
> step that (optionally) happens before this is called calibration and
> is where we use the screen settings and the video card LUT to setup
> the display in a preferred state with a nice response if we have a
> really nice screen we might be able to get away with only calibrating
> and telling software it is sRGB/AdobeRGB in practice we need both
> (especially on cheaper screens).

Screen hardware knobs sure, but the video card LUT I do not understand
why that (or the equivalent in case the compositor implements it with
code instead of hardware configuration) has to be controlled or
non-identity. See my further comments to the article by Elle Stone
explaining why I do not see the LUT as calibration.

If you need to apply a curve for measurement purposes, why not do that
in the client before sending the pixel values to the compositor?

> > Or asking in other words: why do you need the compositor to implement a
> > special-case path where it applies only a part of an ICC profile?
> >  
> We are skipping the whole ICC profile, or at least the parts that will
> be actually used for color management, the VCGT tag is an optional
> non-standard extension useful to keep calibration and profiling
> information for a screen together. Color management will work fine
> without it (just need another way to store calibration information but
> that is a solvable problem)

Yes. It is a special-case path. Usually the VCGT and the profile are
both applied by a compositor. This is very much the opposite of how X11
worked, and I believe the reasons why things on X11 worked like they do
is that there simply was no concept of the display server doing any
kind of color mapping. But on Wayland, we want the compositor to do the
color mapping and be good at it.

> > Why not let the compositor apply the output profile you submit in full,
> > and be smart about what you put in that profile?
> >  
> Since we don't have an output profile yet? And this is all about
> creating one (+ calibration)

Assuming you wanted to do full calibration and color profiling, I would

1. Choose a base color profile in which you send you test patch color
   to the compositor.

2. Initialize the output color profile to the same profile as in step
   1. This will result in identity pass-through in the compositor.

3. Measure a set of colors.

4. Correct the output color profile according to measurement results.

5. If error between desired and actual measurement results is too
   large, go to step 3.

What is wrong with this general idea?

You can use it for all of calibrating, profiling, and verifying.

Just that it is not characterising only the monitor, it will optimize
the output profile for a particular compositor in case the compositor
does something unexpected. It is Case 2.

However, if you leave out the iterative refinement, you'd get more or
less Case 1.

> > Are you trying to avoid having to tag the test color with a color
> > profile?
> >  
> No, although making that optionally possible might be nice for verification.

I think any verification should be done through the normal color
pipeline, as in, the public color management protocol interface used by
normal apps. But that requires using a wl_surface.

> > To me it would make most sense if you picked one or the other: either
> > pass the test color values to the wire directly, which means you do not
> > play with the "GPU curve" as it will be identity; or use the full
> > existing color correction pipeline just like normal apps do, tag your
> > test color with a suitably manufactured color profile which results in
> > an identity transformation on the first iteration, and iterate on the
> > output profile you set.
> >  
> But the calibration curve is strictly speaking not part of the profile
> and some screens really need that to create an at least somewhat
> usable profile, after setting the videocard LUT we of course need to
> characterize the output but as I said for that we don't need an input
> profile (just raw RGB triplets to the screen).

Here is something very fundamental I just do not seem to understand.

> > I think this is something Graeme tried to explain to me: using a
> > special path that does half of the operations is prone to bugs because
> > it does non-trivial things, affects the result of measurements, but is
> > normally not used. Although I think he would also object to the "pass
> > directly to the wire" mode as well, because that also is a special path
> > not used normally.
> >  
> Graemme also said the following as author of Argyll:
> https://www.argyllcms.com/doc/calvschar.html

Ok, I understand that difference between calibration and
characterization, and I also note that calibration is often
additionally stored in a profile file that characterizes a device.

> Another good description comes from Elle Stone
> https://ninedegreesbelow.com/photography/monitor-profile-calibrate-confuse.html

This one raises my point: "Altering the video card LUTs doesn't do
anything at all to the monitor itself."

Therefore I do not consider the "GPU curve" as calibration. It is
something the computer software stack chooses, not something inherent
to or adjustable in the monitor hardware.

In Wayland, the video card LUT is just a tool for the compositor to
change and use any way it wants at any time to achieve whatever the
compositor wants to do. It is not fixed by "calibration". Instead, it
will be used as part of the color transformation computed from input
and output color profiles. This idea is supported by video cards no
longer having a single LUT, but more complex color processing
pipelines, e.g. LUT-matrix-LUT. We want compositors to be able to use
that hardware instead of reserve it for "monitor calibration" or for
users to explicitly set.

Elle's section A4 point 2 is relevant here.

Is this idea of mine wrong?

Is a monitor color profile, VCGT tag excluded, not capable of
describing a monitor in such way that one can compute the necessary
"GPU curve" just from the input and output color profiles and pixel

Why is the VCGT tag not simply a part of the color transformation
between PCS and the device color space? It seems like it is some
strange side-track that is arbitrarily used to mangle the color pipeline
output. A short-hand for something I'd expect would be computed from
the color profiles instead.

> So yes we do need a special direct pass and pass via video card LUT
> modes, and is another reason I prefer to not use a wl_surface for
> this, that would be to easily abused.

I don't think it would not be any more easily "abused" that anything
else. The more complexity there is in a compositor, the more likely it
is to have bugs, if that is the "abuse". Adding a special pixel path in
addition to a normal pixel path is strictly increasing complexity.

> > > > > > Is the reason to use the "GPU curve" that you assume there is a
> > > > > > 8 bits per channel framebuffer and you need to use the hardware
> > > > > > LUT to choose which 8 bits wide range of the possibly 14 bits
> > > > > > channel you want to address? (Currently a client cannot know if
> > > > > > the framebuffer is 8 bits or less or more.)
> > > > > >  
> > > > >
> > > > > Currently not assuming anything about the curves bitnes, so not
> > > > > sure where you got that from? The curve should be set with the
> > > > > ICC vcgt tag I am pretty sure that supports higher bit depths
> > > > > then 8, if you have a better idea to be able to set the video
> > > > > card LUT I would like to hear that.  
> > > >
> > > > If you do not assume anything about bitness, why does the protocol
> > > > spec have bitdepth? Is it the per-channel bitdepth of the
> > > > framebuffer, the wire, the monitor, the minimum for the whole pixel
> > > > pipeline, or?  
> > >
> > > Maximum of whole pixel pipeline  
> >
> > I don't see how that could be useful.
> >  
> I think we have a bit of a misunderstanding going on here, and
> actually meant the same thing namely MIN( MAX(screen), MAX(GPU),
> MAX(communication) ), is that right?

Right, so the minimum bit depth over the currently established pixel
path. Not sure taking the max of what each component might support is
more useful compared to what's actually used.

> > > I tried to specified that the with a n-bit pipeline only the n least
> > > significant bits of the uint should be used. The intent is to only
> > > send 8 bit data over an 8 bit piline and the same for 10, 12 or 14
> > > bits  
> >
> > Oh, so the bitdepth value in the event is really just a scaling factor
> > for the test color values?
> >  
> Sort of? 8bit would be sent as 0...0xxxxxxxx (so 24 zeroes followed by
> the actual 8 bit value), 10 bit would be 0...0xxxxxxxxxx (22 zeroes
> followed by 10 bit value), etc. I don't think that is scalling?

Numerically it's just scaling, but let's call it encoding then.

> > Ok, that explains why it is so vaguely defined and quite arbitrary.
> >
> > Why not use the full 32 bits mapping to [0.0, 1.0] always, let the
> > compositor convert it to whatever format the framebuffer uses, and
> > remove the event?
> >  
> Will need to inspect the code of Argyll or get an answer from Graemme
> because this is a bit of an assumption of mine that we would need the
> actual display bitdepth, if we don't we can just as well do what you
> propose here.

Right. If it's only a value encoding thing for the protocol, just use
32 bits. If it is actually needed for the algorithms aside from simple
value encoding, then it will be needed for the algorithms, but the
protocol could still use just 32 bits. That would make the protocol
simpler to specify and understand.

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