Illustrator 10: transparency on white not the same as transparency on background?

A rectangle, fill is #541C87 (purple), 50% opaque.

This transparency against the background is a pretty color.
I want to be able to use this color in other places, on top of other color rects.
So, put a white rectangle (#FFFFFF) behind the purple rect.
You would think this would result in the same color as the purple rect with background behind it!
But no, it is slightly diferrent color.

Why? How to fix? Is there a simpler way to use transparencies without having to put a white rect behind them?

Thank you.
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David BruggeConnect With a Mentor Commented:

Here is the word from Adobe by way of the forum that I referenced above.

A number of people mentioned that on different monitors, the effect cold be seen much more clearly than on others, and that it depended on what color mode and what preview setting that you were using as to how much difference there is

Jean P had observed that the effect went away when overprint preview was on (the overprint setting tells AI to print one color on top of another which on the press will let the underlying ink show through. Overprint preview (under the view menu) recalculates the view to take into consideration how the inks will blend together.

The same happens when you “flatten transparency” which tells AI to do the calculations to determine how the transparent objects will actually interact with their background.
The forum went on as follows:
 [b]JEAN P - 8:41am May 20, 06 PST (#16 of 19)[/b]

[snip] I would say that Illustrator is not fully calculating the display of the interaction of transparent objects with non-filled portions of the page until overprint preview is enabled. Illustrator is fully calculating the display of transparent objects with white fills below them.

Another way to test this is to go to the page set-up dialog, and enable "simulate colored paper" in the transparency settings. Now you will get the same rendering of the partially transparent object against the white page as you get against the white ink block.

I believe the background interaction is simply not being fully determined until you instruct Illustrator on how to do so.

[b]PAUL A GEORGE - 10:29am May 20, 06 PST (#17 of 19)[/b]

You are essentially correct. There are various tradeoffs involved in trying to calculate the final, true color of transparent objects that interact with the background (paper or whatever). When OPP (overprint preview) is on Illustrator tries to give an exact rendering of the final result including spot color simulation. When its off it tries to strike a balance between efficiency and accuracy.

Paul George is an engineer with Adobe. What he is saying is that the regular working mode in illustrator makes trade offs. In order for it to respond quickly, if gives up some of its accuracy.

Bottom line is this. The colors that you are seeing are a fluke. What AI is showing on screen is a mistake. Any attempt to export the image, or print the image will “correct” the mistake and the colors will disappear.

I was incorrect when I told you that you are able to export them as a TIFF. (I should have tested my answer more thoroughly.) As you already discovered, you cannot sample the colors and get a reading of what appears on the screen, because what appears, in Adobe Illustrator’s mind is wrong. It will simply report what “should” be there.

The only way to reproduce the colors as they appear, is to do a screen grab and sample the colors of the grab. THAT I have tested, and it does work.

Sorry for the wrong advice in the other question. Feel free to take back your points.

- David
First question I have is what is the intended final product? Web Graphic? Print Graphic?

mtnrAuthor Commented:
web, thanks.
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David BruggeCommented:

I have been aware of this phenomena since illustrator introduced transparency, but don’t have an answer. When I first saw your question I thought to myself, “I’ve always wondered that myself.” So I did a thorough search of the Adobe Knowledge Base and came up with nothing.

Then I posted a question in the Illustrator (Macintosh) forum [the mac forum is where the majority of adobe experts are.] After two days, all that I have gotten is noise and static. I will give it one more day and then post in the Windows forum.

I hope that I can get us both an answer.
mtnrAuthor Commented:
thanks for pulling out all the resources D_Brugge!

well thanks to your excellent solution on how to convert a RGB+opacity to an RGB, now I have more options for this problem.

but out of curiosity I'll leave this open for a few days to see what crops up.  Thanks!
David BruggeCommented:
There are not answers as yet in Adobe Forums as yet, but a few observations. If you would like to read the comments, here is a link:
David BruggeCommented:
not answers as yet in Adobe forums as yet....holy redundancy Batman!
The difference (at least as I know it) results from the way illustrator treats transparency in CMYK mode.
Just convert your document to RGB (after all, if the images are for some web design, it's better to work in RGB!) and the differences will disapear.
If CMYK is needed, just select the problematic objects (after the composition is ready and all adjustments made) and use Object->Flatten Trasparency.
mtnrAuthor Commented:
hmm, thats all very interesting. its true I didn't notice this until I got a much better monitor!
By the way, I didn't test it extensively but for my purposes I found the tiff trick worked acceptably well.
Have always liked Pixie for screen grabs, should have thought of doing that before....
Thanks again!
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