QTR-Quadtone RIP

Roy Harrington wrote:
> --- 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

Roy,

After my initial mistake with the CM of Photoshop interfering, 
even when switched off, there wasn't a problem anymore later 
on. I kept the conversions to a minimum and checked the same 
conversions in other applications too and everywhere on the 
right units per file type.
The file that is exported from Qimage doesn't have an embedded 
profile if the right settings are used in Qimage. My last 
message was more a warning to be sceptic about the next one 
who reports that Qimage mangles the numbers with CM off. First 
thing to ask is whether it was seen in Photoshop. I made the 
mistake myself 2x now.  But I'm a bit paranoia on Qimage's 
behaviour because I discovered abnormalities in the past with 
Qimage CM on and the use of the QTR RGB profiles (BPC 
interpretation probably). The Gray ones didn't work at all at 
that time. You are a bit reserved on what happens in Qimage 
too if I read between the lines. It would be wonderful if 
Qimage had a greyscale bypass, asked for it, but that will not 
happen.

AdobeRGB and Gamma 2.2 are the ones I get from Vuescan. 
Vuescan defaults greyscale embedded profiles always to Gamma 
2.2 when AdobeRGB is chosen as the embedded profile for color 
file export, no other choice possible. Logical relation. When 
I load the Gamma 2.2 in Photoshop I convert it to Gray Lab as 
that is the profile I like to use. Not much difference in 
practice but let's say it has the theoretical approach that I 
like. Been thinking of that concept at the time you developed 
it. More human than dotgain or gamma :-)

The files I get from others are AdobeRGB or Gamma 2.2, as 
requested. As long as there are profiles embedded one may 
expect a logical CM past. May not be so obvious though. 
Sometimes I get files without embedded profiles and that's 
more of a p**a as it usually means there's more wrong.

I'm aware of the matches based on Gamma but the use of both 
QTR versions RGBLab and GrayLab has no practical sense to me, 
I prefer to keep the greyscale images grey all the time and 
AdobeRGB is more or less the common workhorse right now so 
that's the other standard. Conversions to one another hardly 
happen and CM should take care of that. To exaggerate; number 
checking may have been the topic this week, it isn't the rest 
of the time.

Have to use QTR for two canvasses tomorrow. Nice to know I did 
it right so far.

Ernst







-- 

                    --
           Ernst Dinkla


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