QTR-Quadtone RIP

I'm running the latest version of QImage, 2007.208. What I don't get is that
even with Printer ICC disabled (in which case no profile conversion at all
should be taking place), there was still a change to the L* values in the
step chart image. It makes me wonder if when printing to file (instead of to
a printer) there's some intermediate profile conversion taking place that
shouldn't be (maybe to sRGB?).
 
Jeff


  _____  

From: QuadtoneRIP@yahoogroups.com [mailto:QuadtoneRIP@yahoogroups.com] On
Behalf Of Roy Harrington
Sent: Saturday, September 02, 2006 7:48 PM
To: QuadtoneRIP@yahoogroups.com
Subject: [QuadtoneRIP] Re: QImage, ICC Profiles, and some surprising results
(long)



Hi Jeff,

Nice analysis. It looks like you have covered a lot of bases.
I must confess I'm not a regular Qimage user -- I use PS on a Mac for all my
work.
But a little over a year ago I did coordinate with QImage and this looks
surprising
like some of the issues then. Any chance you are using a relatively old
version of
Qimage? The issue at the time was that Black Point Compensation was working
differently in Qimage than Photoshop. Internally to Qimage is a color
management
called LittleCMS which is actually doing these conversions.

Anyway I wonder if you have an old version or the issue has re-cropped up.

Roy

--- In QuadtoneRIP@ <mailto:QuadtoneRIP%40yahoogroups.com> yahoogroups.com,
"jkohn_home" <jkohn@...> wrote:
>
> 
> Roy (and other interested parties),
> 
> You may recall my discussion about whether or not it was wise to 
> convert to the QTR-CreateICC generated profiles using perceptual intent
> or not before printing, and I speculated that this could be a source of
> "double profiling" that would compromise the DMAX of the print. I
> posted this graph comparing 3 different methods of printing (using
> graylab, using ICC with perceptual, and using ICC with rel- col). It
> shows the behavior that led me to believe using the profile with
> perceptual intent might not be such a good idea:
> 
> http://www.pbase. <http://www.pbase.com/jkohn/image/65715318>
com/jkohn/image/65715318
> <http://www.pbase. <http://www.pbase.com/jkohn/image/65715318>
com/jkohn/image/65715318>
> 
> Well I decided to do some further testing since you didn't think the 
> results I was getting were correct/expected. This time I decided to 
> take QImage out of the loop by doing any profile conversions directly in
> Photoshop. The results were very different this time:
> 
> http://www.pbase. <http://www.pbase.com/jkohn/image/65974462>
com/jkohn/image/65974462
> <http://www.pbase. <http://www.pbase.com/jkohn/image/65974462>
com/jkohn/image/65974462>
> 
> As you can see, using the profile with perceptual intent now yields the
> most accurate results. I did a little more digging and it sure seems to
> me that the ICC handling in QImage is broken, at least as it relates to
> printing to file with grayscale images. I found that you can take the
> 21x4 random test file, print it to file in QImage with Printer ICC
> disabled, and the gray steps in the resulting file will have changed
> from what they should be. Likewise, enabling Printer ICC and printing
> to file using one of the QTR profiles (either gray-lab or
> curve-specific) will yield completely different results from doing the
> conversion in Photoshop.
> 
> I haven't determined exactly what's going on here and whether it just
> affects printing to file, or just happens with grayscale files. But
> right now, I do not recommend QTR users to use QImage for
> layout/interpolation if you care about getting accurate tonality in your
> prints.
> 
> As the second chart above shows, the ICC profile with perceptual intent
> seemed to yield the most accurate results when printing a grayscale
> step chart. But when I repeated the test using a real world image, I
> found that the deep shadows were too murky and didn't have the detail
> in them that I wanted. Conversely, printing directly from gray-lab gave
> better shadow separation, but also lightens the midtones too much. So I
> did one final test using gray- lab instead of the profile, but boosting
> the gamma setting in QTR to +4 in order to darken the midtones some.
> This proved to be "just right", as the real-world image now showed
> excellent shadow detail and the midtone values were closer to where
> they should be:
> 
> http://www.pbase. <http://www.pbase.com/image/66139883> com/image/66139883
> <http://www.pbase. <http://www.pbase.com/image/66139883>
com/image/66139883>
> 
> So in the case of my HPR curves for the 2400, it looks like I'm going 
> to be printing directly from gray-lab using gamma=+4, unless the image
> has no shadows to be concerned with (which is rare for me). The
> resulting prints look amazing, it was definitely worth all the work (I
> can't tell you how many step charts I've printed in the last few
> weeks). My HPR curves are finished, I'll try to write up some notes on
> their usage and post to the group tomorrow. The I guess I'll have to
> start testing with QImage's ICC handling is also broken for printing
> color directly to the print...
>