Monitor calibration and Linux

How I set up color management on my Linux system
Posted in: photography.

Introduction

Not all monitors are alike. If you are one of the many people who have an external monitor sitting next to their laptop, you might have noticed at some point that colors on one monitor look slightly (or sometimes wildly) different than on the other monitor. When you view a picture on your smartphone, colors may be more saturated than on your computer, and vice versa.

This is not a big problem when I am programming or reading, but it becomes an issue when you are doing any visual stuff – including HTML/CSS. For example, when I am editing a picture on my external monitor, I might notice that the reds look a bit too orange-like. But on my laptop screen, they look fine. Does this mean I should shift them or not? Which monitor should I “trust”?

Generating ICC profiles

The answer is simple: for things like this, you can only trust calibrated monitors. There are many tools to calibrate a monitor, and most systems provide some sort of calibration wizard that asks you a bunch of questions. However, if you take digital photography (or any other hobby involving digital visuals) seriously, the only answer is a hardware calibrator. Without one, editing pictures is guesswork at best.

Unfortunately, these are not free. I picked up a DataColor Spyder4Express for around $150. I’m not sure if that particular model is still available, but there are newer models now for similar prices. It is not cheap, but it is still a lot less than the average lens, and I think this benefits you more than buying another lens in a lot of cases. Of course, you can also consider borrowing one. Calibrating once every month or so is more than enough.

In general, these calibrators (or wizards) generate ICC profiles (.icm or .icc files). Such a file is basically a mapping between color spaces, in this case for a specific monitor.

Sometimes, generating and finding these files is a bit tricky. For example, the calibrator I have does not support Linux (but I dual boot Windows).

Additionally, it has the arbitrary restriction that you can only calibrate a single monitor when using the software directly (it is almost like they really want you to get the expensive model!). The problem, however, is easily solved by calibrating one monitor, copying the ICC profile (usually in /Windows/System32/spool/drivers/color/) somewhere and then calibrating other monitors. Now, what remains is loading these profiles in Linux.

Linux scripting

While there are a few solutions available, I decided to implement my own in a simple script to learn a bit more about these things – simply because stuff like this is always interesting.

It turns out that monitors can be identified by reading their EDID block. This is (usually) 128 bytes of binary data, that contains, among other things, human-readable names for most monitors. The whole EDID block can be extracted by a tool like xrandr, but we need some code to parse the data. This Python snippet extracts the human-readable parts from an EDID block. These can appear at various offsets but the tags that precede them are known.

EDID_NAME_TAGS = [
    bytearray.fromhex('000000fc00'),
    bytearray.fromhex('000000fe00')
]

def name_from_edid(edid):
  names = []
  for i in range(0, 6):
    offset = 36 + i * 18
    tag = edid[offset : offset + 5]
    val = edid[offset + 5 : offset + 18]
    if tag in EDID_NAME_TAGS:
      names += [val.decode('utf-8').strip()]
  return '-'.join(names)

For my ThinkPad’s monitor, this returns LG-Display-LP140QH1-SPB1.

The rest is fairly trivial. The complete script reads human-readable names from the EDID blocks and then finds the corresponding files in $HOME/.color/icc. If such a file is found, it uses dispwin from argyllcms to set the profile for that monitor.

$ colorprof
[DP1] checking: /home/jasper/.color/icc/SyncMaster.icc
[DP1] running: dispwin -d 2 /home/jasper/.color/icc/SyncMaster.icc
[eDP1] checking: /home/jasper/.color/icc/LG-Display-LP140QH1-SPB1.icc
[eDP1] running: dispwin -d 1 /home/jasper/.color/icc/LG-Display-LP140QH1-SPB1.icc
ce0f13b2-4a83-4c1c-b2b9-b6d18f4ee6d2