The p-Code Machine

Note: Since GNOME Color Manager has been released, I highly recommend using that, instead of my manual simplified procedure below.

In the past I’ve written about color profiling your own DSLR, but I’ve never really talked about profiling your screen on an open source operating system like Ubuntu. When talking about a screen, I’m generally talking about a TFT flatscreen. Though it’s true old CRT tubes often produce a wider color range, I can’t bear to look at a CRT anymore, so I’ll focus on TFTs instead.

First you’ll need to check a few things about your screen. It’s critical your screen does static contrast. In the past this was the norm, but newer screens have something called dynamic contrast, and this can wreak havoc on profiling software. Dynamic contrast basically means your screen adjust contrast based on the image it is outputting at that moment, so contrast can theoretically change from second to second, really advanced versions of this could in theory even change contrast locally in some sections of your screen. If your screen has dynamic contrast, and you can’t turn it off, you’re basically screwed, although many TFTs either don’t have dynamic contrast, or allow you to turn it off. Please beware that dynamic contrast could be cleverly disguised under any number of trademarks…

Next you’ll need to buy a screen colorimeter, there are currently two colorimeter of which I’ve read reports to work properly on Linux, these are the X-Rite Huey (also sold under the Pantone brand), and DataColor’s Spyder 2. I highly recommend against buying a Spyder 2, since it requires firmware to be loaded, which may not be redistributed, which means you need to find it on the CD yourself. X-Rite’s Huey just works, without any fiddling. Also note that there are two versions of the Huey, the plain one, and the Pro version, both devices are identical, only the software that comes with it, is any different. So we basically don’t care, since we’re not using the original software anyways. We’re going to use ArgyllCMS, which you can get from my PPA for Ubuntu. Before investing in a colorimeter please consider the fact that color profiling will not make your crappy screen better, at best it compensates somewhat for it’s shortcomings.  Invest in a good screen before investing in color management!

Assuming your screen does static contrast, and you’ve bought yourself a colorimeter, we are ready to roll. First reset your screen to it’s factory defaults. Then disable it’s dynamic contrast (if any). Then make sure your screen color setting is set to equal R-G-B values, on most screen this involves choosing Custom Color, and setting the invidivual RGB channels to values of 255,255,255. I can also recommend you change the OSD position away from the centre of your screen. Leave all other settings be for now.

Now you can start dispcal from a terminal (use -y c for CRTs):

# dispcal -v -y l hp

First dispcal will popup a square in the centre of your screen, attach the colorimeter to screen with it’s measuring area over the square. Press enter to confirm, and you’ll be presented with a menu, select option 3 “White level” from this menu, and adjust your screen brightness using the OSD (this is why I previously instructed you to move the OSD away from the centre) to a value which makes the white easy on your eyes below 100%. My HP screen does 300cd/m2 at 68% brightness, cheaper screens usually need 90% brightness or so. Please note you don’t need to use 300cd/m2, but that works well for me. I do recommend to take a set cd/m2 value and stick with it. You need to reprofile your screen at regular intervals depending on usage, usually monthly or bi-monthly, when reprofiling you can recheck your brightness as well, starting below 100% gives you headroom for when your backlight ages and darkens. Once your brightness is set to a value of your liking you can quit dispcal.

Once you’re done with dispcal, you can use targen to generate a set of colors to test your panel with. Please note that when generating a matrix profile (-a S with colprof), measuring 99 patches seems enough to get excellent accuracy:

# targen -v -d 3 -f 99 hp

Generating a test set won’t take long, once targen is done, you can start the actual profiling process with dispread (use -y c for CRTs):

# dispread -v -y l hp

Depending on the amount of patches specified with targen, the profiling can be either slow or fast, using the previous specified 99 patches, the measuring process should usually take about 2 minutes to fully complete. The dispread utility is the last step in the actual profiling process, but it only generates raw data, to be analyzed and processed later on by a utility called colprof:

# colprof -v -A "HP" -M "LP2475w"  -D "HP LP2475w (`date ‘+%-e %b %Y’`)" -C "Public Domain" -q h -a S -n c hp

The above commandline will result in the creation of a ICC color profile, you can copy this profile to your /usr/share/color/icc directory, where most color management enabled software will find it, others need to be explicitly pointed to the profile.

Loading an ICC profile into a color management aware application does not provide maximum accuracy, the profile has a table which can be loaded into the display driver of your system. You can use the xcalib utility to do this. In my case, the difference is minimal, so I’m choosing not to do this.