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, <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?