Tweaking color...

We have a system that converts PDF to AFP.  Since we any commercial tool to convert the PDF to a raster some of them produce different results for PDF gray.

In general we require that customers always create CMYK images for projects destined to AFP because some AFP devices only support required image features, such as transparency, in CMYK.  Now customers don't always do what they are told so sometimes people do things like create a PDF Gray in a file instead of a CMYK Gray.

So what's the difference?  If you create a document in CMYK then you can generally rely on the creation tools to put the colors you define as CMYK colors into the file in a predictable way.  This means that if I say I want CMYK(23%, 10%, 5%, 6%) that's what I will get.  So if I say I want CMYK(0,0,0,50%) then I should get 50% black and no other colors.

Gray, on the other hand, is different.  While you might imagine that Gray(50%) is the same as CMYK(0,0,0,50%) it may not be - depending on the tools you are using.  Further, the AFP IOCA model does not have direct support for gray-only or black and white images (it only supports RGB, CMYK, YCrCb and YCbCr). 

Without getting into very complex color issues we can say that there are various methods for producing gray on a CMYK device.  One method is to use only CMY in approximately equal amounts to simulate gray.  Another method is to use CMYK and to vary the proportion of K inversely to C, M, and Y in order to render part of the image with K and part with CMY.  A third option is to use only K as if it where Gray and ignore CMY.

Additionally device ICC profiling further affects this by doing its own conversions between gray and various gray representations.

So rendering gray as an AFP color is driven, in this situation, by a number of factors:

1. Device registration.
2. Choice of representation for gray during AFP conversion.
3. Device representation of an AFP gray representation.

So for my current application #1 is likely to take priority because when printing very small type errors in device registration between the colors create low-quality output.

So what we do is create a color transformation for AFP Raster Pro that tells it to convert shades of CMY gray to K black which eliminates the need for registration issues.