There isn’t some kind of secret or overly-complicated math (ok, it is some complicated math) that needs to happen to relinearize a quad file, but it is a lot of work, and it took me many hours (six months?) and more than a few boxes of paper to get to something I feel is a “perfected prototype”.
I made a long post on my site that has some overlap with what I wrote here: http://www.bwmastery.com/blog/2015/relinearize-quad-curves
To answer Brian’s question and to clear up what I’ve been able to do through testing and refining a number of different methods:
- For creating QTR profiles I do have a way of somewhat automatically filling in the ink limit and overlaps for partitioning k6/k7 inks using the standard QTR Curve Creation tools and workflow (as well as some additional gray gamma gray overlap settings). This is information that can be found and pieced together from various sources online. I just did it in a more logical, repeatable, and refined manner, and this is what will make up some of the upcoming book. Some of that is in the post here:
http://www.bwmastery.com/blog/2015/in-search-of-the-perfect-qtr-profile
- I also have a way of parsing out the Piezography master curves to 21 control points so ACV curves can be created and edited in Photoshop and then assigned to each channel in a QTR ink descriptor file (doing this with the ink\_curve= input bypasses the standard QTR gray ink partitioning functions and creates new master quad curves based on each of the ACV curves). You can see how this looks in an ink descriptor file in one of the screenshot illustrations in the post about relinearizing)
- When I have made my own profiles from the master curves I use a K curve that was created with an ACV and lowering the ink limit so there is not as much ink being loaded up at the very end of the scale. - Those new master curves can then be linearized with my spreadsheet template/prototype that takes in an existing .quad file and a 51x3 measurement data file and then automatically creates a correction curve and arrives at a new linearized set of quad curves.
- If all you had were those ACV curves for each ink in the QIDF, you could print and measure the 21x4 step wedge, place it in the linearize= line and with a bit of luck you could create your own custom “piezography-style” curves with just the tools available in QTR. The problem is that linearizing with the QTR tools doesn’t work consistently and you get more errors than it is worth. Ask me how I know…)
- Alternatively, with my method I can introduce a slight compression in the shadows and a different shaped gray curve the new quad curves are mapped to, but that does not block up like using the color managed workflow for “relinearizing”
- Since the my method works directly from an existing .quad file it means that I can also relinearize existing Piezography curves for papers aging or out of spec printers, or to map them to a different shaped gray curve (I’ve done this as a test for myself and a few different people now).
- The new linearized quad values are then pasted into a text file template with the correct header information for the printer and number of inks being used and is then saved with the .quad extension and installed like usual.
- It is also possible for me to create custom Quad Profiles from the existing Piezography master curves (although the higher limit of \~60% in the K channel is a problem and can cause reversals from 96%-100%.) That is why I think there is something about the ink limits for the custom curves being set for whatever the black point or dmax is and then the other steps are scaled up/down (or left/right depending on how you look at it).
Some of the maths involved with the piecewise curve fitting and spline functions are over my head, but the steps of arriving at an accurate correction curve and interpolating new quad values aren’t. Once I got the correction curve correct it was then just a matter of applying different spline interpolation functions to arrive at the correct quad value for every 256 points on the scale.
I apologize for being somewhat cagey about the details of how exactly this all works, but it does represent several months of work and now I am working on it possibly being developed into a standalone application. Until that is finished, I am offering a relinearization service through my bwmastery site.
I also created an updated single 51 step target and i1 profiler workflow for measuring the target single 51-step target 3 times (rather than printing the same target 3 time and measuring each target once).
You can find that here: http://www.bwmastery.com/blog/2015/a-new-i1-profiler-workflow-for-qtr
Richard Boutwell