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understanding the QTR linearization function

understanding the QTR linearization function

2014-10-31 by amog19@...

Hi All,

as mentioned, I have started to make experience with QTR and progressing with experiments. I want to better understand how the Curve Creator / Linearization function works. Here is my scenario and question:

After measuring the 21-step grey chart with spyder print I have the 3 column chart (as text file) with LAB L, a, and b values as output. Dropping that file on QTR-Linearize-Data.exe seems to extend those LAB values with a simple graph and at the bottom of the file a line reading: Linearize="value array" - basically listing the LAB L values corresponding with the luminiscence values of the grey patches for easy copy & paste into the QTR Curve Creator / Linearization tab. There is no change to the LAB values. This way I have told QTR what the native L value vor each patch is - not any correction value - right? This means that QTR creates its own correction values based on those L values - right?


Another question: QTR comes not only with a 21-step grey chart but also with a 51-step chart. Does the QTR Curve Creator / Linearization function use these 51 values - thus resulting in a more differenciated linearization? All tutorials I have come across only mention the 21-step chart.


Thanks for your feedback,

Amin

Re: understanding the QTR linearization function

2014-11-01 by richard@...

I tried responding to this yesterday, but sometimes these replies don't show up at all. I really wish yahoo wasn't such a yahoo company. If my first response shows up in the next week it will explain this a little more clearly.

You are correct in that the linearization droplet is a script that strips out all the irrelevant data from the output file from the photospectrometer, graphs the density increase, and gives you a string that you paste into the linearization="...." in the qdif file

The install script then takes that input data and creates a curve (on a mac you can acually see in the terminal window the lienarization curve that is created 0,0, 5,2.14 10,7.15 ...,... etc.

I have used those to rough in a acv curve in the grey_curve=" " area when the linearization is really off and then reprint a second stepwedge with the curve, no learization, and then fine tune with the rough curve and final linearization.

It is much easier to just get the grey_val overlaps set right and the linearization should just be one step process.

Richard Boutwell


Re: understanding the QTR linearization function

2014-11-09 by amog19@...

Hi Richard,


thanks for your input. You pointed me to some important aspects of QTR linearization. I believe your sketched value array represents the grey curve linearization, not the main linearization curve. The interesting thing about the grey curve array field is that it can take more than 21 or 51 value pairs - it should take 100 value pairs, to be precise. With that it is possible to manipulate very exactly the grey curve. I transformed the LAB L values from SpyderPrint to grey curve values and did that way the whole linearization. Especially in the lights this seems to have some benefit since you can exactly determine for each % density input a corresponding correction factor. I applied this for the first 11 values (0% to 10%) then in increments of 10%.


I still have to understand what the relationship between the linearization curve and the grey curve is. Does the linearization curve "rule" the other curves - i.e. is it applied to the grey and toner curves?


Regards,

Amin

Re: understanding the QTR linearization function

2014-11-14 by richard@...

I have been messing a lot with the finer points of the .txt file creation process for the last few days perfecting the process of making 6-grey ink profiles to get of the banding that occurs with the shape of the QTR overlaps.

It seems like you are able to linearize first with just the 21-step target and then fine tune the shape of the curve afterward with gray_curve="0;0 x;x x;x ... x;x x;x 100;100"

If you are slicing the scale to 1/100 I would recommend using an average of multiple measurements. You would be surprised with the variation in the reading (at least i was). I use an average of 6 measurements for setting the overlaps and linearization (only 21 steps). Usually that is fine, but I had one that was a little too dense in the shadows and I brought the highlights down and the shadows up with the gray_curve=" " line. Not much, but enough to match the contrast of a different paper I profiled earlier in the week that had a little more pop.

All that being said, there are a lot of people out there that are much smarter with programming, engineering, and math. I don't actually know how the behind the scenes curve creation functions are doing the math (and I never really bothered to ask specifically), but just that I have messed with it enough over the years to find out what works to make the best print.

Richard Boutwell

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