Matrix multiplication

[Elle (Elle Stone)]

Hi @Tynach - my apologies for taking so long to circle back around to your very thought-provoking post - I bet you thought I forgot about this post, but nope, not at all!

Edit: with my usual stellar inability to speak clearly, when I tried to rewrite my initial sentence to make it more clear, I left out the critical part of the sentence above, which is that I bet you thought I forgot about this thread, not true!

So to try again, your post was very thought-provoking, and I didn’t forget about it, in fact have been mulling over the points you’ve made. So I just edited the original sentence above to put in the missing phrase. Sigh.

@Tynach

It’s important to note that in both RGB colorspaces (both sRGB and the monitor’s colorspace), the RGB value for ‘white’ remains [1, 1, 1].

I never stopped to think about what RGB values the monitor profile might have for the color white near-white colors - thanks! for mentioning that.

I used ArgyllCMS xicclu to check several monitor profiles that I made at different times, using different algorithms, and sure enough “white” defined as Lab color (100, 0,0) was close to or exactly (1,1,1) in the monitor space, using relative colorimetric intent:

xicclu -ir -pL -fif file-name-for-monitor-profile

But “how close are the channel values for white and near-white” does depend on the type of monitor profile. For my LUT monitor profiles, R, G, and B are only approximately the same for grayscale values, being very close for white and near white, and progressively farther apart as the grays get darker. On the other hand, for my profile made using “-aS”, R=G=B up and down the gray axis.

I’m guessing that "how different are the channel values for white and gray also depends on what sort of prior calibration was done using the vcgt tag, before making the monitor profile.

@Tynach

Now, the reason I had gotten so confused while learning all this, was because I was wanting to figure this all out so that I could specifically use absolute colorimetric conversions between colorspaces; I didn’t want to use white point adaptation. Specifically, I wanted to make one image look identical on several different monitors, and make that look identical to the original object in real life. I had all displays in the same room as the object, too.

Your goal of making one image look identical on several different monitors, and also make the image look identical to the original object in real life, of course means that at some point you took a photograph of the real life object (I’m really good at figuring out the obvious ).

Recently I took a photograph of a painting and used RawTherapee’s CIECAM02 module to make the colors on the screen match the colors in the painting:

RT CIECAM02 to match colors in a photograph of a painting to colors in the painting

I’m probably not the first person to use RawTherapee’s wonderful CIECAM02 module in this way. But it was a nice illustration of what color appearance models are all about. So I thought I’d share what happened when I photographed a small painting and realized the colors in the photograph didn’t match the colors in the painting. A few days ago I painted a small “exercise in mixing foliage greens” (handprint : mixing green), in connection with topics discussed here: To mix and paint the green c…

Of course your situation - multiple monitors in the same room right along with the photographed object - might have the advantage that the entire room is evenly lit with the same color and brightness of light. In which case “what color of light” to calibrate all the monitors to might depend on the color of the ambient light. But then you’d need to consider whatever compromises might be required when calibrating any given monitor to a white point that’s too far away from its native white point.

I had been thinking about your quest to make the colors look the same on all your monitors, and thinking that the CIECAM02 modules might be a way to accomplish your goal (even without first calibrating and perhaps also profiling the monitors). Making images look the same on different display devices was @jdc 's motivation for RawTherapee’s CIECAM02 module existing in the first place.

@ggbutcher - the RawTherapee CIECAM02 module is something that might also work for displaying images on your projection screen, though it might mean making a solid white image (and perhaps also an 11-step grayscale wedge at L=100 down to L=0) using editing software, projecting that image onto your screen, taking a photograph of the projected image, and seeing what color of white the projected white actually is. There are sophisticated devices for meauring such things, but probably a photograph would get you “in the ballpark”.

@gwgill - now that @Tynach has a colorimeter and can calibrate and profile his various monitors, would this colprof switch allow him to accomplish his goal of making images look the same on all the monitors using ICC profile color management? Or (as I sort of suspect) am I missing something critical in how images are displayed on different monitors?

http://argyllcms.com/doc/colprof.html#ua

For input profiles, this flag forces the effective intent to be Absolute Colorimetric even when used with Relative Colorimetric intent selection in a CMM, by setting a D50 white point tag. This also has the effect of preserving the conversion of colors whiter than the white patch of the test chart without clipping them (similar to the -u flag), but does not hue correct white. This flag can be useful when an input profile is needed for using a scanner as a “poor mans” colorimeter.