QTR-Quadtone RIP

On Monday, September 4, 2006, at 12:58  AM, Ernst Dinkla wrote:

> Jeff Kohn wrote:
>> 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
>
> Jeff,
>
> You are correct. With CM off in Qimage and an "RGB" greyscale
> target without embedded profile loaded there is no change in
> the step values according to Photoshop. It is different
> however with a true greyscale target like you discovered. It
> doesn't have to be a profile related shift, it could well be a
> shift that happens when Qimage translates greyscale to RGB.
> I think it is a new problem as I have tested this in the past
> and  then there were no problems with the CM off (enough with
> CM on though). Doesn't look like a BPC  related issue either
> as the whole range shifts, even up to 4 or 5% at some points
> along the range. Most likely a shift to sRGB + BPC as far as I
> can simulate it with other Qimage settings.
>
> This has to discussed again with Mike Chaney. The problem is
> that Qimage is upgrading often and bugs that are only revealed
> in odd printing methods are not always discovered right away.
>
> 3 possibilities:
> The greyscale is getting a profile treatment when it is
> converted to RGB on loading (Qimage internally is RGB only)
> The "Honor EXIF color space tag when no profile is embedded"
> is still working on greyscales when CM is off, it is alright
> with RGB files though when it is switched off :-). I have
> discussed this feature with Mike in July 2005 on the Qimage
> list. There are more dangers in that feature.
> The greyscale/RGB file gets an sRGB profile treatment when it
> is saved as Print to File. The Web version of Print to File
> may interfere and that one converts to sRGB..
>
>            Ernst Dinkla
>

I think Ernst may have pinpointed the issue.

Grayscale to RGB conversions that happen implicitly behind the
scenes aren't well documented.  If they go though CM all the intent
and bpc options are hidden, which is a bit scary.

Even in Photoshop I'm not completely sure of all the implicit 
conversions,
but I think they all go through CM -- e.g. I know the eyedropper does 
and
this throws people off quite regularly.

It looks like a little sleuthing is called for.

Roy

-
Roy Harrington
roy@...
Black & White Photo Gallery
http://www.harrington.com