Does a mirroring / flipping CTM such as "-1 0 0 1 420 0" also implicitly reverse the colorant layer in the pdf?
Typically a mirrored CTM would be used when printing on the back of a transparency.
In this case the viewing order of colorant layers is the oposite of the printing order on the substrate. If a page is prepared in viewing order, the desired result would be printed in reverse of the viewing order.
Or would you need 2 ColorantControls with the respective ColorantOrder elements/attributes (JDF/XJDF) in reverse. One in the order of the page for all pre-imposition pages and one in reverse for post-imposition.
Currently the specifications ar silent and that will give us random implementations.
I'd suggest adding something like this to PlacedObject/@CTM but am unsure of the "NOT".
If @CTM contains components that turn a page upside down (see figure Table 2.1: Matrices and Orientation values for describing the orientation of a Component) the order of colors SHALL / SHALL NOT be reversed.
Let me rephrase, there may be situations where implementations use it for a different reason than described and since the original documentation didn't suggest flipping, it's reasonable to assume the command was interpreted by developers without flipping.
In most implementations, you will not have a CTM that flips in a Layout.
All rotations, resizing and shifts will not cause a reverse of color orientations.
You have a point that there are probably quite a few implementations where hardware mirroring needs to be accounted for without reversing any color orientation.
The order of colors cannot be reversed from the ctm keyword since there are many situations where a CTM is used to orientate for a totally different reason than you described, especially when digital printing.
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