QTR-Quadtone RIP

Perceptual Linearity is a bit of a misnomer. What the approach of darkening the shadows actually does is help to match the print contrast to the monitor assuming the monitor has a set DARK value that is pretty dark and light value that is pretty light and the paper has some specific dMax value that is lighter than the monitor’s dark value. So, in effect, the printed values are changed to simulate the monitor. Both monitor and print are now no-longer linear but they kinda look the same.

This perceptual non-linear icc curve outlined in the blog post linked below is dependent on dMax values. Perceived contrast is much higher when print dMax is lower (L 2.5 etc) and there is less of a need to simulate a higher contrast by pulling quarter-tone shadows down. This curve becomes stronger when dealing with dMax in the range of L 24 (Platinum prints). 

What we’ve done with Piezography (for the past decade) is two things. We’ve encoded our targets in gamma 2.2 and we’ve calibrated our monitors (good monitors) in gamma 2.2. We’ve also made sure to set the contrast ratio of the monitors to a specific target of between 180:1 and 250:1 by adjusting both the white and black levels. We also build an RGB create-icc to soft-proof with. The result of the above steps, gives an actual linear monitor image that matches to a print that is actually linear. There is no need for the ICC. The reason for using a linear workflow is that fine-adjustments of the shadows become much easier and predictable (even with monitors that aren’t super awesome).

However, there-in lies the rub. When dealing with images created by non-linear monitors not using the Piezography monitor setup described above, the shadows have been modified to print a bit too open when just printed. Often people like this, often people don’t. In our print-testing with create-icc, we’ve found that the shadows tend to block just a bit too much. They become a little un-stable to work-with. This is the case for paper-printed shadows; however, we’ve found that when using a Create-ICC profile to print negatives that are then printed in the darkroom, Create-ICC works really really well as a final perceptual match-calibration to the monitor.

As a “practitioner” of piezography in the past, I’ve used create-icc to great effect. It’s enabled me to anticipate the contrast of my client’s images and quickly match previous print processes. The tonal values of Piezography are linear, the tonal values of Create-ICC are variable based on dMax that sometimes pulls everything a bit too dark, the tonal values of ABW are a combination of the two and are also just weird sometimes (less shadow gamma is pulled down and the midtones to highlights are usually pulled just a bit up).

If someone is using the create-icc profile as-is to print with, I would suggest calibrating their monitor and workflow to match a print created using it. If someone is using the Piezography linear .quads with Gray Gamma 2.2 images, I would suggest calibrating their monitor and workflow to match a print using that. What we are working on internally at InkjetMall is a way to modify the "perceptual" linearity of either a .quad directly or a Create-ICC profile (indirectly) to enable more flexible tonal matches in real-world environments. That, however, is a hack only used when one can’t properly calibrate a monitor to a defined environment. In the flow of work, one really should decide on a print standard and keep it that way. Otherwise everything gets uber complex.

best,
Walker








> On Apr 15, 2016, at 11:18 PM, brian_downunda@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:
> 
> I've hesitated to respond to this, because this is an issue that I've struggled with over the years, and I'm not sure whether my view is shared around here.  At the risk of being seen as a shameless self-promoter, I posted my thoughts on this issue in another blog post <http://www.cyberhalides.com/piezography-printing/the-piezography-heretic-to-convert-or-not-to-convert/>, written to answer a slight different question, but covering essentially the same material as your question.
> 
> If you've already seen this post, there is some new material that I inserted today in light of your question, particularly what happens if you convert the standard 21x4 to an ICC generated by QTR-Create-ICC, print and measure.  Since Roy's objective in designing the ICC generator was to create prints that were perceptually linear, this new graph is in effect his answer.  The article also includes a link to Roy's most recent statement on perceptual linearity.
> 
> Another practitioner on this forum has hinted to me that he has developed an approach that combines the open shadows from printing in GG22 with the perceptual linearity of converting to an ICC for printing.  If I've understood this correctly and if this approach gets released, then there will be another option.
> 
> 
> 
> ---In QuadtoneRIP@yahoogroups.com <mailto:QuadtoneRIP@yahoogroups.com>, <artodd138@...> wrote :
> 
> On a side note - does anyone have a graph of L* values that would appear linear to a standard observer?  Looking quickly at the cyberhalides link and other references it seems strange that a perfectly linear response in ink density is desirable.  Even though it is measured as linear wouldn't it not necessarily be perceived as linear because of the nature of standard human vision?  What seems to make piezo curves so nice is that they appear linear, and in fact when measured and plotted are not perfectly straight; the slope increasing in the deep shadows to increase separation.  Am I incorrect in thinking that human vision is not as good at separating dark values as compared to light and therefore an increase in slope at the shadow end of a curve would be required to produce a perceptually linear grayscale?
>  
> 
>