trying to understand colour management...

1 Calibration things
I have a monitor which is profiled with a Spyder colorimeter and the Colorvision software. I believe that the calibration software does two things: (1) it sets up the video card with some look-up-table data; (2) it creates a profile (*.icm) which it sets as the default profile for the monitor. I don't use the nVidia controls, Adobe Gamma or anything else to mess with the video card: the Spyder is in charge.

2 PC/CS3 Colour
So if I then run Photoshop, everything works fine. I'm confident in the settings there. I can read and write documents in a number of RGB and CMYK spaces and that's all good. CS3's "Save for web" works a treat, and I can convert output on the fly to sRGB and preview the output in a number of formats. Most interesting are:
- uncompensated colour (this is how the images look in my un-colour-managed broser Firefox [or IE7]
- standard windows colour (this is about how the image would look if I converted it to sRGB - pretty much the same as the original)
- use document colour profile (this is identical to the original).

3 un-managed colour
Ok, so I save my image and then I look at it in the browser. It looks like "uncompensated colour" from the CS3. This is what I would expect.


4 So what's the problem?
Well, maybe there isn't a problem, I'm not sure.

My monitor's "uncompensated colour" is some way from what I'd like it to be on this monitor. Specifically there's one of the Pantone reds which looks way wrong on the un-managed browser. So what I'd really like to do is to tweak the monitor so it looks "more standard" - specifically so that this Pantone colour looks the way I know it should look.

I know I could do that by using a colour managed browser (ok, Safari), but why can't I do this with the graphics card look-up-table and the colorimeter...? I'm missing something here. Why doesn't the colorimeter set the graphics card look up table to emulate sRGB, then just bung a standard sRGB profile into the windows settings. Why do I need both the LUT adjustments and the custom profile to describe the results? Couldn't it be done "one way or the other". That is: if you create a profile to characterize the monitor, why do you need the LUT at all? Alternatively, if you just cranked the LUT values to match sRGB, why do you need any profile other than sRGB?

Obviously I'm forgetting something, as they don't do it this way for fun. Can anyone put me right on this please? The more I understand, the more control I have...

{ replies to owm post... how sad is that? }

why both LUT and ICC?
Ok, there's more confusion and bad information on this than there is good. These chaps seem to have a grasp of the issue which makes sense.

The argument is...

(a) The graphics card look up table (LUT) sets the gamma and colour temperature, but it's apparently a relatively crude tool. The Spyder thing uses it to set the monitor as close to sRGB as it can get, but it's not precise.

(b) The profile matches sRGB precisely, providing data for colour-aware applications to get from (a) to true sRGB.


Which implies that the LUT has basically fewer entries than the ICC profile. The nVidia control panel [when used with my 8800GTS and 30" LCD] sets that up and it's got only a handful of sliders: just five numbers then. For sure the ICC profile has more data in it than this. QED.

PS reads the profile assigned to the monitor, and renders the image precisely based on the look up table in that profile.


which leaves my monitor...
Not as close to sRGB as I'd like it to be. Colours are good when viewed through CS3, but the LUT adjustment alone isn't close enough to sRGB for me not to worry when I look at my [non-colour-aware] browser.

I know that common PC browsers don't read embedded image profiles: that's not the issue here (these images don't have embedded profiles but are in sRGB).


I'm slightly surprised that the OS doesn't automagically apply the conversions from the ICC profile it knows is assigned to the monitor. Although the browser wouldn't be profile aware, the OS could be. I guess I have to go and read about WCS to understand why that isn't done... unless anyone can think of a reason?

Hi Phil, I'll try to tackle your questions!

The color gamut of your display doesn't necessarily match sRGB. It may be bigger in some areas, small than others. As such, it isn't always possible to create a profile that matches your display to sRGB, because it is possible that there are some colors in sRGB that just can't be produced on your display.

Another issue as you've mentioned above is that the video card LUT isn't the best place to encode the color space transformations because of the imprecision. The idea is to get the monitor relatively close to a known state (e.g., white point of 6500 K, 100 cd/m2) and then use a high-quality profile to describe how the monitor actually behaves. In other words, calibration gets your device close to a known state, then a profile is just a description of how that device really works. The better the calibration, the less artifacts you'll get as a result of applying a profile.

The problem is that Windows doesn't automatically do color-space conversions. It's true that if the OS provided routines to automatically handle conversions, then we wouldn't have to worry about all the various ways in which images can be viewed (uncompensated color, etc.). Unfortunately this wasn't a priority ...

Thanks - sounds about right. It would seem sensible to me that the OS should render accurate colour based on what it's fed (image, image profile and output device profile), but I can see that's not how it works for monitors or printers.

So I made some progress this week:
Previously I thought that I didn't need a colour managed browser because no one embeds profiles in images for web display.... Now I realize that's not true, as if your monitor is not precisely sRGB you need the browser to be colour-managed just to ensure it's using your carefully-built monitor profile.

thanks.

That's right, Phil. Even if your monitor is close to sRGB, it will never be exact. There will be some color or set of colors that's a little off, one way or another. Hence the need for a profile to describe the way your monitor actually behaves when fed various RGB values.

Thanks... I have a particular "pantone" colour in a client's logo which is hard to represent on my monitor without the profile, which is what lead me to this. Still, as the Canadians would say "it's all good" - I learnt some more stuff

The calibration LUT in the video card is using 3 1-D LUTs (one for each color). Those LUTs can be used to linearize each of the colors, but you can't adjust the primaries that way (you can only adjust color temperature and gamma). This means for instance, if you have a wide gamut display that can show aRGB, you can't calibrate it to show sRGB using the video card LUT.

In terms of ICC profiles, there are two types, matrix-based and LUT-based. For both, you can have a set of linearization LUTs that are 1-D like the LUTs in the video card.

For matrix profiles, you now have a matrix transformation (the input color vector is multiplied by a 3x3 matrix), this transformation can adjust the primaries.

For LUT profiles, you now use a 3-D LUT to do a color transformation, this can adjust the primaries as well. (The 3-D LUTs require huge amounts of space, thus you use the 1-D LUTs first to distribute the colors evenly across the 3-D LUT space so that you can get away with a smaller 3-D LUT without loosing accuracy.)

Bottom line, if your monitor primaries are close to the sRGB primaries, you can get a good color match with the video LUT calibration alone, if the primaries are not close, you are out of luck.

At least one monitor (the NEC Spectraview Reference) appears to be able to do the matrix transformation internally, it has aRGB gamut but also has an accurate sRGB mode.

(The video card LUT is in almost all cases 8-bit or less; the ICC color transformations can be either 8-bit or 16-bit.)

-- Tilo