QTR-Quadtone RIP

Roy Harrington wrote:
> On Monday, September 4, 2006, at 02:03  PM, Ernst Dinkla wrote:
> 
>> Roy Harrington wrote:
>>
>>> I think Ernst may have pinpointed the issue.
>>
>> Roy,
>>
>> I'm not so sure about that right now.  I have done this before
>> and I think I made the same mistake again. Qimage actually
>> does exactly the same as Photoshop does when CM is off in both
>> applications. When CM is off in PS the Greyscale one will
>> convert to the same RGB numbers the Qimage conversion makes
>> when CM is off. So Qimage and Photoshop behave the same when
>> CM is off. The plain TIFF converter of my website that has no
>> CM at all produces the same numbers.
>>
>> When the Qimage Print to File RGB file is brought back into
>> Photoshop  and converted to greyscale (with CM on) to compare
>> it to the original that was loaded to Qimage then there's a
>> difference but that is because the CM in Qimage was off and
>> the CM in Photoshop was on. With CM off in Photoshop the
>> Qimage produced RGB file will convert back to the same
>> greyscale percentages on the steps.
>>
>> I guess QTR has the same conversion for RGB>Greyscale as
>> Qimage and Photoshop have with CM off. In that case my
>> workflow with greyscale converted to the QTR grey printer
>> profile, then loaded in Qimage with CM off and the print to
>> file RGB sent to QTR which converts it back to greyscale is
>> still valid and there is no fault in Qimage.
>>
>> Ernst
>>
> 
> Photoshop doesn't really have a CM off mode when you do
> conversions.   What you have set in Color Settings -- the working
> spaces determines what happens.
> 
> I'm not sure what Qimage does in these cases.
> 
> Roy
> 

Roy,

Let me start from another point then. Using Photoshop to 
simulate what goes on in Qimage is tricky. I have to admit 
that. Now I just do what my B&W workflow has been so far:

For this test only: I assign, with Photoshop CM on, the Gray 
Lab profile to the 21 step Greyscale target, convert the
assigned Greyscale target to the QTR Gray Matte paper profile, 
save that file.

Load that saved greyscale file in Qimage with CM off and after 
that load the Qimage processed RGB file in Photoshop with CM 
off, convert to Greyscale, and it gets the same %K numbers 
again as the original converted and saved greyscale target has 
in PS (minus or plus some 1% shifts as a result of rounding 
offs on RGB numbers). Could be that the RGB to greyscale 
conversion with CM off in PS is going along my custom monitor 
profile that pops in at the workspace line as a replacement 
for AdobeRGB when I switch CM off.

Even less PS influence: If I pull the same saved greyscale 
through the RGB conversion in TIFFconvert (libTIFF based) I 
get the same RGB numbers in Photoshop as the Qimage processed 
file gives. I expect QTR to behave the same as Qimage with CM 
off or behave like the libTIFF application that knows no CM 
like QTR knows no CM. I see some references to libTIFF in 
CUPS/GIMP/LCMS docs so it may well be the same conversion in 
QTR/TIFFconvert/Qimage. Whether the greyscale to RGB 
conversion defaults to a sRGB or another space relation in all 
applications without CM doesn't matter to me anymore if the 
conversion the other way around is the same again.

The test is like the way I have printed B&W with 
Photoshop>Qimage>QTR since QTR profiles became available. A 
scanned B&W file gets Gamma 2.2 assigned in Vuescan, that file 
is converted to Gray Lab in Photoshop, edited then on a 
calibrated monitor so I like the tones (soft proofing too), 
converted to Gray Matte paper profile (or the custom ones I 
made) and then brought to Qimage with CM off, processed to RGB 
and that dropped in QTR. That worked best for me and I see no 
flaw in this or it has to be the conversion from RGB to 
Greyscale in QTR that isn't transparent to me.

For the time being I stick to this method as I trust Qimage's 
CM conversions on B&W files less but have not tested them lately.


Ernst


-- 

                    --
           Ernst Dinkla


www.pigment-print.com
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