Thanks for the reply! I will post some pdf maps on our site as soon as we can solve this color problem. These last couple of days have been spent reading the pdf ref manual (1.5) to try to come to grips with the transparency, transparency groups and blending colorspace issues. I think I am starting to understand a bit more what is going on.
One thing I noticed about our color scheme is that two of the elevation colors give an "out of printer gamut" warning when using photoshop to sample the rgb values of a screen capture of our original color scheme. Photshop suggests alternate colors which look exactly like what Acrobat reader is rendering to screen. So I guess we should change those colors anyway. If all the colors in our map were specified as CMYK insted of RGB, then it would seem that there is no need for a CMYK->RGB->CMYK conversion. Would this eliminate the need for specifying a blending color space and are there any potential issues for rendering to a display?
Assuming it is necessary to set the blending color space, can someone point me in the right direction for using the low-level API to effect this?
Lastly, I have tried using the SetOutputIntents method to embed and ICC profile in the pdf doc. Is this the right method to use? As I understand an ouput intent is for controlling how the document is printed. Does this also affect how the document is displayed?
Unfortunately I just get a "Object reference not set to an instance of an object." error in the itextsharp public PdfIndirectObject AddToBody(PdfObject objecta). I will post my code at a later date if we decide to go with ICC profiles.
Thanks much,
Eliot Cline
----- Original Message ----- Cc: Date: Saturday, January 20 2007 7:17 PM Subject: Re: [iText-questions] Blending colorspace On Jan 19, 2007, at 6:13 PM, Eliot M Cline wrote:
> I have been developing an itextsharp app that produces high-
> quality, atlas-style maps.
Excellent! I'd love to see some of the results...
> As previously mentioned on this list, setting any transparency in a
> pdf doc will potentially alter the display/printing of the
> originally defined colors.
Yes and no. let me clarify...(I keep meaning to write a blog entry
on this...)
Acrobat 7 and later ALWAYS use color management to render pages in
order to produce the highest quality color for your documents.
Unless a document has an "output intent" present it is ASSUMED that
you wish to view it using RGB/Monitor space, and so Acrobat sets that
as the "implicit output intent" for the document. And if the
document is entirely in RGB anyway, no conversions necessary. Again,
this is ON THE FLY rendering stuff - the document contents are NOT
affected.
When transparency is used, things get a bit more complex as there is
a need for a "blending space", which defines what colorspace to use
for blending the colors in conjunction with their alpha values. If
the colors in the document aren't already in that blending space,
then they need to be transformed into that space and then processed.
The result of the blending, which is, again in "blending space" then
has to be transformed back to "Monitor Space" in order to be rendered
ot the monitor. The default blending space, when none is specified,
is CMYK. Therefore, if you have RGB colors w/o an explicit blending
space they are going RGB->CMYK->RGB a VERY lossy operation which
causes the 'dulling" of the colors.
> This is happening with our maps and we would very much like to
> figure out how to stop this.
> I have seen from other postings and the pdf ref manual that this
> problem can be solved by setting the blending colorspace or the
> transparency group colorspace (maybe they are the same thing?).
Correct.
> It does not seem that itext can set these. Is this true, or have I
> just missed something?
>
You can indeed set them with iText - you just have to use low level
APIs. I don't recall there being high level ones...
> Other postings seem to indicate that embedding an ICC profile can
> also solve this problem. My knowledge of this subject is limited,
> so I would like to ask if this is actually the case before trying
> to chase down how to do it.
>
Yes, if you were to tag your data with ICC profiles, then Acrobat
can do a high quality color conversion and no quality would be lost
when going RGB->CMYK->RGB.
Leonard
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