QTR-Quadtone RIP

Joost Horsten wrote:
> --- In QuadtoneRIP@yahoogroups.com, Ernst Dinkla <E.Dinkla@...> wrote:
>> Could someone explain to me then how extra curves and extra 
>> blending isn't touching color mixing ?  I can understand it 
>> for split toning but there one hue is replaced by another one 
>> along the tonal scale.
>>
>> Joost indicates it more or less in his first message:
>> "Since I can only interpolate between two curves I can only 
>> create tones at the boundary of the color space".
>>
>> Either the new blendings do not cooperate with one another 
>> (adding just another straight line) or they together describe 
>> a slice of the color space in a crude way (an area) or you go 
>> the whole 3D way and use a color engine.
>>
> 
> Ernst,
> 
> I'm not a color space expert. I'm physicist by training. I have given 
> color spaces some thought in the past, but my statements below are 
> not definite ones. Just a way to try to proceed the discussion. I 
> discussed this with a colleague of mine (same background). I address 
> single tone curves only. 
> 
> I do not understand your problem exactly. The blending IS of course 
> touching color mixing, since that's to me the whole point of blending 
> curves. But perhaps I misunderstand what you mean. I regard the three 
> curves (warm, cool, lab a/selenium) as the three primaries of the 
> color space, which is a subspace (a triangle near the origin) of the 
> LAB color space. The exact gamut / subcolor space is determined by 
> the combination of inkset and paper. Currently, QTR allows to 
> mix/blend two ink curves, which is the equivalent of interpolating 
> between two primaries in the LAB space. The color "space" one can get 
> with two curves/primaries only is actually a line (is that what you 
> call a slice?).  Mixing/blending 3 curves allows actually 
> interpolating within the 2D area of the color space. Any color point 
> within that triangular space is achievable with three curves.
> 
> There are many subtleties to the above. I am aware of at least the 
> following issues that might arise, at least in theory: 1) It actually 
> might be the case that the subcolor space is not exactly a triangle. 
> The actual physical mixing is done with inks on paper. This a 
> subtractive process that can give a color point that is not on a 
> straight line in the LAB space. It resembles more the behaviour of a 
> CMYK space. The boundaries in teh LAB space might be curved. 2) Even 
> if the interpolation in the LAB space gives a straight line, the 
> interpolation is not necessarily linear. For instance, a mix 50/50 
> mix of cool and warm inks, will give a different result than first 
> printing a 50% cool layer and overprint that with a 50% warm layer . 
> 3) In theory, it even might be that mixing two colors (with different 
> a and b coordinates) with same density (L value) might give a mix 
> with different L value, thus affecting the linearity of the curve. 4) 
> In reality, the inks are no pure primaries, but having each a color 
> spectrum of themselves, allowing for complicated spectral changes.
> 
> I'm not sure how important these issues are in reality. I expect 
> issue 3) not happen, since that would already undermine the current 2 
> curve blending in QTR. I might test this with two curves\ufffd  But I 
> think the above issues point out, that one can NOT expect that a 
> simple interpolation of 3 curves is sufficient to predict the color 
> point of the result. In other words, there's no guarantee that let's 
> ay a 33/33/33 mix of three curves with lab a and b values (0,9), (0,-
> 9) and (9,0) gives a color point of (3,0). So the simple mixing model 
> has clearly it's limitations. If one wants to achieve a certain tone, 
> one has to find the mix empirically. In order to predict a result or 
> to use softproofing one needs a full-fledged color model/engine. 
> 
> But, even with the limitations pointed out above, I'd really 
> appreciate the simple mixing model with more than two curves.
> 
> Am I addressing your questions or am I still way out?
> 
> Joost

In general you are close to what I think may cause problems: 
getting other results than expected.
The lines are not straight in that color space is just one 
thing. A good example is where Paul mixes ink toners and has 
to change the hue between the light and the dark toner 
slightly just to get it perceptually right in the print. All 
adaptions that show there's complexity that can't be 
translated into simple models.



Similar things have been discussed before on the digital B&W 
list. What makes a good digital B&W print ... is for example a 
thread that goes into the two streams of thinking in B&W 
printing right now. I'm no advocate of both but I think 
there's a solution in between that has much of the ABW modes 
of the latest OEM drivers but with the calibration etc of QTR 
+ the profiles. The most futuristic view is an ABW system with 
an underlying general B&W ICC profile for that paper and 
inkset and the flexibility and ease of use of ABW that both 
QTR with its sliders + semi-ICC profiles and David Tobie's 
solution with his B&W ICC profiles can't deliver.


Met vriendelijke groeten,Ernst


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