QTR-Quadtone RIP

I was confused about this when I was first trying to do my own profiles,  
also. The reason you compare each ink to the next darker shade and then  
multiply to get the percentage relative to the darkest shade (K), is that  
with more ink shades the lightest ones fall into the last few percent of  
the K ink where it would be extremely hard to make comparison judgments  
visually or even through measuring devices. With two or three inks it's  
not so difficult, but when you get four, six, or seven inks, the lightest  
shades are so light that you can hardly see them. To try to compare them  
directly with the darkest ink would mean that the darkest ink would not be  
covering the paper. You would have ink dots with lots of white space  
around them to get an apparent density equivalent. That is the way "black  
only" printing works. You get the lightest shades by printing very sparse  
dark black ink dots. It makes for a grainy appearance in the lighter tones  
of the picture. Some people like the look, others opt for more shades of  
inks so that at the lighter picture tones, you are still covering the  
paper and this results in a much smoother tonality. The printer driver  
used, such as QTR, still has to figure out how much of each ink shade to  
use at any given picture tone. The algorithms use each ink's density  
relative to the darkest ink to calculate that. Each printer/paper/ink  
combination reacts differently so you have to develop separate curves for  
each combination. The process starts with you inputting the maximum amount  
of each shade of ink to be used and the relative density of each ink. QTR  
then calculates the amounts of each ink to use at each of the possible 256  
picture values. Because of all the variables involved, this result is  
hardly ever perfect. That is why you then have to print out a step wedge,  
measure the results, and then feed the data back into the program where it  
goes through another process to linearize the output. The internals of how  
it does all this are unknown to me. Jon Cone of InkJetMall has his own  
proprietary way of creating these curves which, he says, results in even  
smoother transitions. I don't know how his process works either.  This  
turn out to be a longer post than I had intended. I hope it might be of  
some help for folks trying to understand what is going on. I am also still  
in the process of trying to learn about it all.

Steve

On Sun, 01 Feb 2009 10:32:29 -0800, jorge caruso <rabsanito@...>  
wrote:

> Why don\u2019t we just look at the matching
> patches in K and LLK directly (like we do w/ K and LK) instead of doing
> first mathematics w/ LK and LLK? Is there a difference?



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