Excerpt from Transparency in Adobe Applications: A Print Production Guide -
During the flattening process, the flattener replaces transparent objects with objects that are visually equivalent to the transparent originals, but contain no transparency.
When the Flattener flattens transparency, it may use overprinting commands in the resulting file to create transparent effects using opaque objects.
Basically, the flattening creates tints of color where a transparence occurs and if the effect is in the same coordinate as another object, an overprint attribute is invloved such that both objects appear, neither knocking out the other.
An ink set with 50% transparency can be replaced with a 50% tint of the ink; and overprint allows visage of an object beneath it.
Correct.You should not see difference on screen if all the elements were handled correctly. The interaction of destination color space, color mode, blending mode, spot colors, all have to be allowed for.
But - you will see a difference at times if you disable overprint preview in Acrobat (it's preference on/off/auto dependent to your local set-up changes in AA8, AA9, AA X) ; use Mac Preview (possibly), or use any of a slew of other viewers.
Sooner or later, if you flatten transparency in pdf, you are going to see Stitching Lines, an on-screen anomaly of the drawn elements interaction; they do not appear when a flattened pdf passes through a RIP for commercial print, but are annoying/undesirable for a pdf document intended for monitor viewing use. (There is also a Yucky Discolored Box syndrome which happens; Spot colors, Transparency and RGB don't play well together)
PdfX-1a was a development which solved transparency handling in RIP's prior to the current releases. Modern RIP's can natively execute the conversion of flattening and RGB space, handled by the X-1a standard. It limits the occasional necessary fix-ups in a fast moving workflow and does not allow for all (any) color space profiles. X4 pdf is more fluent and compact (?) and allows more precise control downstream.
I may have slight errors in these statements, don't quote me on this excessively.
Yes, Ive never had a problem with files Ive released as X1a's w/transparencies. Was just checking up again. I have repeatedly told people that the white lines/stitching do not print that way, but to no avail. They see them, they freak. I have read of the numerous ways to eliminate seeing them on these forums and have passed the info along, but some ppl just dont want to deal with clicking another button, or unchecking a prefs box. Whatever, as long as I am confident about what the final output is going to be, Im fine with that. I have always resisted making X4's bc I have read that some printers do not accept them. I wish that I had the luxury of checking w/the printer, but, ususally, I do not. I am usually prepping fles for agencies who are too lazy to check with their printer, or are afraid to ask their client for the printer's requirements that they are releasing the file to. They feel they are inconveniencing their client. Seriously? How inconvenienced do you they think the client is going to feel when they've paid 5 grand for a lousy print run!