Friday, September 16, 2011

PatchTool Review PART I: From Target Image to CGATS file and vice versa.

Danny Pascale's PatchTool

Nutshell Review: If you need to hand-measure profiling targets or generate custom target grids, or wish to convert the patch layout output by one program to that needed by another, then you might do well to get a copy of PatchTool . But in addition to its invaluable patch conversion functions, this utility now also provides a sophisticated quality assurance toolset that can be employed for display, printer, and instrument verification.

PatchTool is one of those small specialized color utilities which can pay for themselves in a single hour of use, by quickly solving a problem that could otherwise be done by hand, were there but world enough and time. 

I'll talk about the simple and basic patch stuff today, the rest I will deal with in Part II of this review in a few days. Let me tell you about the simple tasks in which PatchTool can help a color consultant or color management expert, and why they're essential.

PatchTool's basic usage is the conversion of target images to CGATS files and vice versa, and also changing the layout of the image corresponding to a CGATS file. It can switch columns and rows, rearrange the array to a predetermined number of rows and columns,  etc, It can also acquire data from an EyeOne spectro to measure a set of patches and generate a file, thereby allowing for the creation of custom targets and Photoshop swatch sets,  and measure on-screen. All of this is useful to  color consultants and power-users:

Every user of color management software knows that to profile a device one must print or photograph a target. Then, the colors of the target are described in the computer by a so-called CGATS file. In fact, it is common to speak of the target's CGATS reference file, which describes known values supplied by a manufacturer,  and the corresponding CGATS measurement file which is written out by the user's instrument driver software. Of course a reference file, eg for a Colorchecker  chart can itself be viewed as a file written out by the chart-measuring technician of the manufacturer who supplied the chart. 

To a certain point, CGATS files are compatible between pieces of software, but as always the devil lurks in the details. One piece of software writes measurements of a grid row by row; then a profiling package by a different manufacturer expects its inputs column by column. What now? Well it's time to find some conversion helper.  Which is where PatchTool comes and solves the issue by reading the first file and spitting out another with the columns and rows flipped ! 

PatchTool is a an utility that is there to fix little annoying details, and what is impressive is that it really helps to gets all these little annoying details just right so you don't have to write custom scripts. In the image above you can see the options provided for writing values into a CGATS file, line by line — and of course if you read in a file, and spit it out with different options, you've done a conversion. 

Another typical use of this type of software for me is custom-measuring my ColorChecker and ColorChecker SG targets. I find that my own chart data —from my personal measured targets —deviate enough from the generic published images and reference files, that it is worth to measure my targets myself and create my own reference files for camera profiling, and before doing a chart-to-print comparison or even a chart-to-screen comparison. Here PatchTool can help out by driving a spectro and then spitting out a measured CGATS file, and even an image of the core of the measured target.What more could one wish for? Well, just maybe templates of the cardboard target borders to overlay, or even  comparison "holey" Photoshop layered templates …

This completes my quick description of the basic functions of PatchTool; it's not really an in-depth review because I've had a need, and used the software for two days, and it's done everything I needed without forcing me to pause and think. I'll have an exploratory look and address the "power user" stuff in the next installment. 

By the way, I like to leave my readers with a pointer to some useful net freebie as a reward for getting this far. Today's resource is Danny Pascale's fan page for the Xrite Colorchecker target, which has Colorchecker images in every possible colorspace.  Download one of these images, get out your physical target, and check out your printer and monitor profiles !


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