QTR-Quadtone RIP

--- In QuadtoneRIP@yahoogroups.com, Ernst Dinkla <E.Dinkla@...> wrote:
>
> Roy Harrington wrote:
> > On Wednesday, September 6, 2006, at 03:25  PM, Ernst Dinkla wrote:
> > 
> >> Roy Harrington wrote:
> >>
> >>> The only way to understand all this is break everything into 
> >>> individual
> >>> steps and
> >>> understanding what each does.
> >> Roy,
> >>
> >> One way to get some grip on this is by getting the knowledge
> >> how QTR does the RGB to Grayscale conversion.
> > 
> > Well, that's easy.   For a gray RGB file i.e. R=G=B, the grayscale 
> > values are the same as R, G, or B.
> > When it's actually a color image fixed weightings are used:
> > 
> >      Gray = .31*R + .61*G + .08*B
> > 
> >> Given the print results I get it must be compatible to Qimage
> > 
> > Qimage never does RGB to Grayscale, but Grayscale to RGB in the absence 
> > of profiles & CM (Ptr ICC Off)
> > I think it does R=G=B = Gray,  so I guess that's compatible.   In 
> > general these probably would be
> > the non-CM conversions for any non-ICC product.
> > 
> > All CM conversions will preserve L values as much as possible.  The 
> > difficulties are where K=100 is not L=0,
> > then BPC etc can give various results.
> > 
> > Roy
> 
> Roy,
> 
> So the main lesson learned is: avoid Photoshop to check Print 
> to File results, even with CM off. The other one: there's no 
> flaw in the Qimage transfer when CM in Qimage is off.
> 
> Ernst
> 

I wouldn't say avoid Photoshop.  Just realize what it does and use appropriately.

With Photoshop the key things are:

1) When you open a file do NOT do a conversion if the file has an embedded profile.

2) In Color Settings you should NOT have "Convert to Working Space" ON.
(I'd recommend these things all the time anyway)

3) When you use the Info palette to see what's there, look at K for grayscale files
and RGB for RGB files.  There's a tendency to look at wrong ones depending on
whether you want 0-100 or 0-255 values.  Looking at L-values always involves
CM conversion since they are not what's stored.   L-values are a good way to see
what is being preserved on CM conversions.

Because of (3), if you are using a workflow which bounces you back and forth with
Gray <--> RGB conversions,  I would stick with Working Spaces that have the same
Luminosity curves.   Common pairs are:
Gray Gamma 2.2  and  AdobeRGB
QTR GrayLab        and QTR RGBLab
Gray Gamma 1.8  and ColorMatch

If you always use these pairs then (3) is not an issue since conversions will yield
the same values.

Roy


> -- 
> 
>                     --
>            Ernst Dinkla
> 
> 
> www.pigment-print.com
> (         unvollendet         )
>