QTR-Quadtone RIP

--- In QuadtoneRIP@yahoogroups.com, Roy Harrington <roy@...> wrote:
>
> Hi Jeff,
> 
> If you are seeing a drop in dMax using the ICC profile, the profile 
> conversion is probably being
> done incorrectly.  It sounds like RelCol conversion instead of 
> Perceptual.
> 
> I can't tell whether you are on a PC or Mac but Photoshop often 
> defaults to
> Relative Colormetric conversion rather than Perceptual Intent.   Be 
> sure to double check that you
> are using Perceptual and using Black Point Compensation.

I'll have to do some more testing to confirm what I saw, I suppose 
it's possible I got the test files mixed up but I don't think so. 
What I did was print the 21-step test file in QImage three times, 
once using "Gray Lab RGB" as the  output profile, once using the QTR-
Create-ICC-RGB generated profile with rel-col, and once using the 
profile with perceptual intent. Then I measured all three with Eye 
One and graphed the results:

http://www.pbase.com/image/65715318

If you think about it, this makes sense. I see no reason for rel-col 
to boost the shadows because it's basically going to do a direct 
mapping and then just clip any "out-of-gamut" colors, which in this 
case means dark tones that can't repro on matte paper. It's 
perceptual intent that will try to "fit" the image into the target 
profile by compressing the gamut.

This test was done using a custom curve I created for HPR, and the 
curve had been linearized. 

What's interesting, is that looking at the chart it would appear that 
rel-col is the clear choice. But what I found when printing a real 
image was that the shadows were too blocked up, due to the fact that 
anything below L*=17 was getting mapped to the same color. So the 
gray-lab approach seems to work best for me at the moment (with matte 
papers at least), although it does lighten the image just a bit.

Like I said, I plan to do some more investigation on this. I'll 
repeat the test and make sure I didn't get the samples mixed up. I 
also plan to do the same test with photo-black on Innova F-Type paper.