colord - Frequently Asked Questions

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# Frequently asked questions

## Table Of Contents

- Can I type colord with leading upper case 'C'?
- What do the version numbers mean?
- How does colord interface with the session?
- Are XRandR 1.2 devices still supported?
- Does colord or GNOME Color Manager convert my image?
- Why are the mappings stored in a database, not readable XML files?
- Does calibration work with the NVIDIA binary driver?
- What is the difference between a CMM, CMF and a CMS?
- What is a native sensor?
- How would an application like Simple Scan use colord?
- What is a qualifier used for?
- How do system daemons add devices and profiles?
- How would a system daemon like CUPS use colord?
- How do I translate colord?
- Is there an organization sponsoring development of colord?
- What's the difference between colord and Oyranos?
- Does the daemon run as root?
- What desktop applications support color management?
- >How can I help??

### Can I type colord with leading upper case 'C'

No.

### What do the version numbers mean

Version numbers are of the form ${major}.${minor}.${micro}.

Git Branch | Version Numbers | Maintained Status
--- | --- | ---
`colord-0-1` | `0.1.*` | `obsolete`
`colord-1-0` | `1.0.*` | `stable`, getting backported fixes
`master` | `1.1.*` | `unstable`, getting fixes and new features

Branches with the status `stable` are suitable for distributions like SLED and RHEL and are 100% ABI and API stable. Translatable source strings will not be added or modified and dependancy versions will not be updated. Tarball releases from this branch should be considered safe to push to users as normal updates.

Branches with the status `unstable` are suitable for distributions like Fedora or jhbuild and are potentially ABI and API unstable and will contain new features, refactoring or new translatable strings where required. Tarball releases from this branch should be QA'd before pushing to users as test updates.

Branches with the status `obsolete` do not get backported fixes, although may get security fixes backported if it is practical to do so.

Every 6 months (roughly aligned to the GNOME release schedule) we will branch a new minor version from master, so the new stable version is 1.(n+1).0 and the new unstable version is 1.(n+2).0. This means that even minor versions are stable, odd are unstable.

### How does colord interface with the session?

At start-up either gnome-settings-daemon or colord-kde has to create profiles and devices and otherwise act as the helper agent for colord in the user session. The session client has to do the following 6 things:

- Call `CreateDevice` for each connected XRandR screen and watch for changes to the screen.
- Create an ICC profile file for each XRandR device using the EDID primary information and store it in the home directory.
- Call `CreateProfile` for each profile found in the home directory.
- For each `::profile-added` event check if the `EDID_md5` metadata matches any XRandR devices. If it does, then it needs to call `Device.AddProfile()`
- For each `::device-added` event check if the device `modified` property is beyond a set threshold. If it is, show a notification that the user should calibrate the device.
- For each `::device-added` event from a XRandR device, get the default profile and push the gamma ramp to the X output.

### Are XRandR 1.2 devices still supported?

No. XRandR 1.3 is required to do per-output gamma correction.

### Does colord or GNOME Color Manager convert my image?

You can use the CdTransform object in libcolord for simple RGB->RGB transforms. You need to use a pixel conversion library, for instance lcms2 for anything more complicated.

### Why are the mappings stored in a database, not readable XML files?

If you have a registry that more than one program is changing, you need locking. Locking with plain files is neither fun nor reliable. From a raw performance point of view, it's two orders of magnitude faster to mmap a single 20k sqlite .db file than it is to read 200 small XML files from the file system.

The session needs to query things like give me all the profiles for device $foo and find all the devices that use profile $bar which is why we should be using a database, rather than searching discrete files on the file system every time the session asks for data.

If we're exchanging files between users, then single files do start to make sense. I don't personally see a use case where I want to exchange low level device->profile settings data with somebody else. A chain of settings from document to printer would be a very useful thing, but this is what you generate for a process and ship, rather than copy from the file system unchanged.

Having power users tweaking the mapping is something that does not make good design. These people can use the `colormgr` command line client to add and remove mappings, and to append qualifiers to certain profiles in a safe abstracted way.

### Does calibration work with the NVIDIA binary driver?

The binary driver from NVIDIA only supports XRandR 1.2, which means that it does not support per-monitor gamma tables. This means we can only send gamma correction tables to the screen, which means multiple monitor would not be supported.

As the driver has no source code, we can't fix this and have to wait for NVIDIA to release a fixed version of their driver. Nowadays the open source nouveau driver provides a much better experience for NVIDIA hardware, and gnome-color-manager of course works flawlessly with all of the open source drivers.

### What is the difference between a CMM, CMF and a CMS?

A CMM (Color Management Module) is the software component that takes the pixels in an image and converts them from one colorspace to another. There are several CMM engines in existence, both free and proprietary. Free CMM engines include ArgyllCMS and lcms2, of which the latter is used heavily by colord and both are used by gnome-color-manager.

A CMF (Color Management Framework) is a software component that allows applications to register devices and profiles, and also allows applications to query what profiles to use for a device. A CMF is a higher logical layer component to a CMM engine -- in this way, colord aims to be primarily a CMF, and is not a full featured CMM.

It is the authors opinion that the abbreviation CMS is a heavily overloaded term (in that ArgyllCMS and L[ittle]CMS include them in their name), but do not actually provide a system framework for other applications to use. In casual usage, both LCMS and colord exhibit some parts of a complete CMS, and in this way CMS is often used as a general term for both a CMM and a CMF.

### What is a native sensor?

A native sensor is a calibration device such as a colorimeter or photospectrometer that colord knows how to interface with.

Many sensor makes and models are recognised, and are available to be used by programs such as ArgyllCMS. The following sensors have native drivers and do not need to spawn ArgyllCMS to take readings:

- Pantone Huey
- Hughski ColorHug
- XRite DTP94

A native driver allows colord to lock the device, read a sample from the sensors (or from the ambient light sensor) and unlock the device. A native device allows client programs to get samples from the device trivially using either raw D-Bus or libcolord without relying on external programs.

### What is a qualifier used for?

A qualifier is a single string with three sub-strings joined with dots. For example, `RGB.Glossy.300dpi` would be the qualifier on a profile for a RGB device, printing on glossy paper and at 300 dots-per-inch.

Qualifiers are assigned to a profile either by CUPS itself or by the user and this allows CUPS to choose the correct profile for a given print job. The sub-strings are tried in preference order, so that for a print job of type `RGB.Glossy.300dpi` the following fallbacks would be used:

1. `RGB.Glossy.300dpi`
2. `RGB.Glossy.*`
3. `RGB.*.300dpi`
4. `RGB.*.*`
5. `*`

The fallback scheme allows one profile to potentially match lots of different print conditions, as very few users will have more than a couple of different printer profiles.

The tri-dotted notation was chosen in preference to JSON or XML as it is already being used by CUPS in the `cupsICCProfile` PPD Extensions format.

### How would an application like Simple Scan use colord?

It's actually really easy. Simple Scan already knows the `SANE_Device` object of the scanner that it wants to use. From the `SANE_Device` object we can easily get the device_id for the color managed device in colord. To do this, refer to the device and profile specification for scanner devices. This specifies the device_id for a sane device is made up from `"sane_" + sane_device->model`.

We can now query colord using libcolord for the device that matches this ID:

        CdClient *client = NULL;
        CdDevice *device = NULL;
        CdProfile *profile = NULL;
        GError *error = NULL;

        /* create a connection to colord */
        client = cd_client_new ();

        /* find the device for a specific ID */
        device = cd_client_find_device_sync (client, device_id, NULL, &error);
        if (device == NULL) {
            g_warning ("failed to find a device in colord: %s,
                       error->message);
            g_error_free (error);
            goto out;
        }

        /* get the default profile for this device */
        profile = cd_device_get_default_profile (device);
        if (profile == NULL)
            goto out;

        /* TODO: use lcms to convert the scanned image */
        g_message ("need to use: %s", cd_profile_get_filename (profile));
    out:
        if (profile != NULL)
            g_object_unref (profile);
        if (device != NULL)
            g_object_unref (device);
        g_object_unref (client);

If you're not happy gaining an additional dependency of libcolord, then it's also pretty easy to use raw D-Bus calls to do the same thing. See the examples folder for some code samples.

### How do system daemons add devices and profiles?

Like this:

        for device in self.devices:
            o = CreateDevice(device.name)
            for profile in device.profiles:
                p = AddProfile(profile.id)
                if profile.icc_filename:
                    p.SetFilename(profile.icc_filename)
                p.SetQualifier(profile.qualifier)
                o.AddProfile(p)

### How would a system daemon like CUPS use colord?

Like this:

        d = FindDeviceById(device.name)
        if d:
            p = d.GetProfileForQualifier(qualifier)
            if p:
                p.Use()

### How do I translate colord?

See the colord Transifex pages for how to contribute to localisation.

### Is there an organization or corporation sponsoring development of colord?

colord is not sponsored by anyone, although Red Hat gives me the time to work on random open source projects. Thanks!

### What's the difference between colord and Oyranos?

On the surface, Oyranos and colord look like similar projects. I'm not going to be able to write an unbiased comparison against colord and Oyranos (as the author of the former) but I've supplied the table below which hopefully helps.

| Oyranos | colord
--- | --- | ---
Portability | Windows, Linux, OSX | Linux, FreeBSD[1]
Desktop support | FLTK (native) and KDE | GNOME, KDE and XFCE, LXDE, MATE
Client access | C | C, DBus (native), all others via GObject Introspection
Client library | Synchronous, and not MT-safe | Synchronous, asynchronous and MT safe
Lines of code | 151k | 46k
Memory allocation | custom | GLib
Object system | custom | GObject
Build system | custom | automake
Settings store | elektra [2] | sqlite
Session presentation | photographer, prepress and designer | None [3]
Full screen color management | Compiz, using X atoms as IPC | Wayland with subsurfaces used as opt-out areas (WIP)
Project age | 9 years | 3 years
Required by | icc_examin Kolor Manager CinePaint (obsolete) | CUPS Foomatic GTK+ colord-kde gnome-control-center gnome-settings-daemon xiccd GNOME Color Manager Simple Scan redshift colorhug-client

Notes:

1. In my opinion, it is better to leverage the native color management stack for each operating system, for instance, using Windows Color System on win32, and ColorSync on OSX rather than try to install alongside the existing frameworks.
2. Elektra could be replaced by another configuration engine.
3. This no user profile system in colord as users vary rarely sit into well defined groups and putting profiles in the UI abstracts the settings one stage further from what the desktop user is trying to achieve.

### Does the daemon run as root?

Since version 0.1.12 (released August 2011) colord runs as the colord user, not root — if you're still running this or an older version please file a bug with your distribution to get it to upgrade the package to something more modern!

The original reason for running as the root user was to check the PolicyKit authorisation request, but PolicyKit 0.104 (released October 2011) added functionality to allow non-root daemons to do this as well.

### What desktop applications support color management

Applications can query colord directly, or can use the per-session `_ICC_PROFILE` X atom.

Software | colord support | _ICC_PROFILE support | Notes
--- | --- | --- | ---
GTK+ | Yes | No | Only the print dialog
Simple Scan | Yes | No |
GIMP | No | Yes | Need to enable it before use
Firefox | No | Yes | Need to enable it before use
Eye of GNOME | No | Yes |
Darktable | Yes | Yes |
Krita | No | Yes |
Calligra | No | Yes |
Digikam | No | Yes | Need to enable it before use
UFRaw | No | Yes |
Inkscape | No | Yes |
Entangle | No | Yes |

### How can I help?

Anyone interested in colord development is invited to join the channel `#gnome-color-manager` on `irc.gnome.org`. We are a friendly bunch, and can answer questions on D-Bus interfaces, general lcms questions or anything else colord related.

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