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Easily Edit QuadTone Rip .quad Files

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And how to edit them without setting the house on fire.

Quad Tone Rip, generally requires that you need to first create an ink descriptor file, and compile the curves with either the install script in OSX or with the Windows QTRGui. Roy Harrington's behind-the-scenes disney magic turns all those input values in the .qdif/.txt files into something the printer can understand. The formulas behind the partitioning, dot gain, and contrast adjustments are a bit of a black box, but basically you get a series of numbers in a text file and you leave the rest up to the QuadToneRIP driver. 

This little workflow doesn't explain how to recreate the Piezography profiles, which is proprietary process, nor does it allow you to edit the shape or linearization for different papers. If you are interested in that: please consider my QuadToneProfiler tools.

The rest of this post will simply instruct you how to transpose the values for each separate ink curve to another ink using a text editor. This is not rocket science but it might be helpful if you are trying to create a custom setup (or accidentally loaded the inks into the wrong cartridge. Been there, done that...) 

Backstory

I was digging around trying to get the Piezography .quad profiles to work with a custom setup that works with the digital negative inkset, and the K6 P2 curves all installed in one printer (with the ability to print on matte and glossy papers—it can be done on an 8-head 3800, but you need to spray the glossy prints with print shield rather than using the supplied gloss optimizer).

The .quad file open in the default CurveView tool, but if you open them with a text editor, you see long strings of numbers. At first it looked kinda confusing, but after moving them to a spreadsheet and graphing the values it all made sense—the graphs looks exactly the same as in CurveView.

I "discovered" that each line of the .quad file represents a luminance value between 0-255 (sounds like an 8-bit single channel grayscale to me). Then it clicked when I realized that each number on those lines is "just" a 16-bit value (really a 16-bit unsigned integer) that represents the amount of ink from each channel at each luminance value on the 0-255 scale. This really not much of a discovery if you have any computer science or engineering background, but I haven't come across anyone explain this in all the hours reading and trying to figure out how QTR does what it does (damn it Jim, I'm a photographer not an engineer).

But what that realization allowed me to do was copy and paste the Piezography curves made for the standard setup so that it matched where I had installed the shades I was actually using.

If you load the ink shades in a position that doesn't match with what is specified with the piezography instructions it will not work. It will print, but with extreme and unacceptable problems. The good news is that you can duplicate the .quad file for the paper you want, and then replace the 256 values from one ink channel to another. 

Here are the 6 easy steps

1: First, you will need to duplicate the original .quad file and rename it to designate a modified .quad file. I do it like this " original-name-mod.quad " you just need to make sure you don't use anything other than letters, numbers, underscores or dashes in the file name (and don't mix them up with the original curves).

2: List the ink positions as they are installed from darkest to lightest as instructed for your inkset and printer.

It is usually something like this:
Shade 1 - K
Shade 2 - C
Shade 3 - LC
Shade 4 - M
Shade 5 - LM
Shade 6 - LK
Shade 7 - Y
Gloss Optimizer- LLK

4: You should end up with something that looks like this:

3: Then list the inks as they are installed in your printer (from darkest to lightest) next to the the first list

*The following is only an example
Shade 1 - K
Shade 2 - LK
Shade 3 - C
Shade 4 - M
Shade 5 - LC
Shade 6 - LM
Gloss Optimizer - Y

This gives you an easy to chart that shows you the curves you need to copy to the modified .quad file

Copy from Original  --> Paste to Modified
K    --    No Change
C    -->  LK
LC  -->  C
M   --    No Change
LM -->  LC
LK  -->  LM
Y    --    No change

5: Copy the values from the origin file to the modified one. Ex: #C Curve (Everything between #C Curve and  #M Curve) in the original .quad file and paste them over the values below #LK Curve in the modified file.

There are 256 lines that need to be copied and pasted for each ink/shade and it can be easy to lose track of where you are getting and putting things. Alternatively, you can delete all the original curve values in the modified .quad file prior to pasting in the new values, which helps cut down on any copy/paste errors.

You do this for each of the remaining ink positions. Just make sure you don't miss any lines, or get the values out of order.

6: When that copying and pasting is all done, you save the modified .quad file and place it in the folder with all the other QTR profiles and install the curves like normal. You can then open the original and modified .quad files in curve view to make sure the shape of each shade is the same. The only change should be the color representation of the inks in different positions. I've done this for the method-3 digital negative curves, as well as the K6 P2 matte and glossy curves, and they all work flawlessly. 

I would still suggest to run a few test images, and check the linearity before printing your next show... 

Do you want to learn more about QuadTone RIP (QTR)? Check out these posts from the blog

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