Loose Ends

A few followups from this past week.

Chuck Holst updates us about the fire:

The Pagami Creek fire is especially poignant for me. Seventeen years ago next week my wife and I honeymooned in the Boundary Waters, canoeing and portaging a 60-mile loop just north of the current fire area, and exiting at Lake One. Most of our route was devastated in 1999 by a windstorm that blew down many if not most of the trees, and now the fire is threatening the southern part of our route that was not affected. Only a week or two ago we were talking about returning to Lake One this month if we could get away.

In reply to some of the other comments, so far the fire has not consumed any buildings that I know of. Most of the fire has remained within the Boundary Waters Canoe Area Wilderness Area, where no roads or buildings are permitted, except for a few ranger cabins. Though there are many lakes in the area, which provide a source of water with which to fight the fire, the northeastern corner of Minnesota, known as the Arrowhead, has suffered a drought this summer, which has dried out the vegetation. With the wind shifting to the south today, there is concern that the fire will explode into new life if it reaches the dried-out tinder in the 1999 blowdown area. Then it may indeed threaten the resorts and other private property along the Gunfint Trail. 

Access to the affected area is difficult. Transportation into the area for most of the firefighters and their gear is by boot and canoe.

Response curves at the extremes: Dave Polaschek, who Ctein described as "the printing engineer on Photoshop," wrote to say that he thought Royce Howland's comment about the Printer Mystery was on the mark.

First, what Royce wrote:

I wonder if this situation could be debugged a bit further by working through some analysis similar to what Norman Koren describes here.

Using Koren's Gamutvision tool, I took a look at the generic profile for Ilford Galerie Gold Fibre Silk (as I read it, Ctein's paper for larger printers), specifically the version for my Epson 4880 printer. There are some things about this profile that look odd to me. For example the B&W density response visualization shows a noticeable flattening of L* in the bottom few tones which could reduce contrast down there. The same visualization also shows some very sharp discontinuities in the a*, b* and c* curves, again in the bottom tones where L* < 6 or so. This is true for both perceptual and relative colorimetric rendering intents, though RC is marginally less skewed.

In comparison, the generic 4880 profile for one of my favorite Harman papers shows nothing similar to these effects.

Now, Ctein certainly isn't using the generic Ilford profile for a 4880. But potentially there is something funky going on in the bottom tones / greatest densities with this paper, even with custom profiles. Gamutvision may be able to show it, if so. Besides the density response curves display, there are many other profile visualizations. Plus there's a useful false-color soft-proofing display, more comprehensive than what soft-proofing in Photoshop shows....

And then Dave's response:

Royce, you've hit on exactly what I think is the problem.

When I was working on printers back in the day, one of the toughest nuts for us to crack was the way the response curve of the ink got highly non-linear near the ends of the curve. It's almost the inverse of the typical film response curve, getting steeper at the ends until it slams into either paper white or ink running off the print and suddenly makes a right-angle-turn and goes flat.

The typical ICC profile has a bunch of curve points, and the CMM interpolates between those points. That's all well and good when you're in the smooth part of the response curve for each ink, but when the curve changes slope abruptly (as it does when you hit the ends), where your points are relative to that abrupt change in the curve becomes critically important. Pick the wrong points, and the (usually linear) interpolation will miss the abrupt change, and you start getting problems.

The software inside the printer (or driver) knows better the shape of the response curve for the printer, sometimes even knowing about that specific printer in printers that monitor themselves, and it can shape the curve to play tricks to extend the apparent gamut a little outside the actual gamut of the printer. It also can speak to the printer in its native color-space of CCYMMMKKKRGBASDF or whatever inks the printer has, rather than being forced to express the colors in an RGB space (all inkjet drivers I'm aware of today take RGB input).

Having seen the software inside the various pieces involved, it doesn't surprise me at all that the printer can sometimes do a better job than any application software does. If anything, I'm more surprised by how frequently application-managed color can "beat" the printer's internal color-matching which has something of an unfair advantage.

tl;dr: This stuff is hard, and the physical world is messy.

(Yr. Hmbl. Ed. had to go look up "tl;dr"—it means "too long; didn't read," which, according to the Urban Dictionary, is "said whenever a nerd makes a post that is too long to bother reading." Not so in this case, or else maybe I'm a nerd too.)

The Milwaukee Job: Six TOP readers responded to the ad for my interior designer neighbor (in the post "Jobs") who needs help. Three of them have professional websites (shown in the order in which I received their emails):

Brad Bell Architectural Photography

Brian Fussell, Rangeline Real Estate Photography

(Brian has an extensive selection of pictures on his site, although there's no navigation. I think I looked at hundreds.)

John Herbst, Arch Photography

Of course I passed all six along to my neighbor, who says she is grateful. Our thanks to all of you.

Mike

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Original contents copyright 2011 by Michael C. Johnston and/or the bylined author. All Rights Reserved.

Comments

[fjf]

nle;wrm

This is not yet in the urban dictionary. It stands for Not Long Enough; Want to Read More.

This level of detail is great. It is like back in the day when messing around with D-76 or Pyro and throwing in a few grains of this or that to alter the response curve.

The more I understand about a process the better I can exploit it to achieve the outcomes I desire. So many thanks to Dave, Royce and Ctein. More please.

[Riley]

The variety in the web sites is quite interesting.

[Dave Polaschek]

fjf, there's a lot more that could be said about this. The science behind how and why printers make the dots they do (for all types of printers - most notably for us offset, laser and inkjet, but even woodblock, letterpress and other ways of getting ink on paper) plus the way film and photographic paper turn light into pigments, well, it's complex.

In a perfect world, a printer that accepts input in the range 0-255 for each input channel would have the right-angles in that response curve right at 0 and 255. That is, at 0 you would have as much ink as possible on the paper without getting drips, and at 255 you would have no ink. More importantly, at 1, you'd have slightly less than total ink coverage, and at 254, you'd have as little ink as the printer could possibly put down. 16-bit printing would work the same way, only with finer steps.

For a number of reasons, it doesn't happen that way in the real world. Ulichney's "Digital Halftoning", ISBN 978-0262210096 covers a lot of the technology and tricks up to 1987 when it was published, and I strongly recommend it if you're interested in the history and science of turning images into ink, but it's very much a reference / text. Not light reading at all, and there's been more than 20 years of art since then.

I'd love to be able to write more on the subject, but I need to balance that with writing code for Photoshop, finding time for my (many) other hobbies, and having time to eat and sleep. So will there be more? Probably at some point, but any such writing will be done in my copious free time.

[fjf]

Dave:
Thanks for responding. I'll try and locate the book and I'm sure Mike would jack up his already generous per word pay rates to obtain more on this topic and compensate you for family and hobby time etc :-)

When Jobs introduced the NexT he ran Display Postscript for screen output with standard Postscript for print output. Using the same render engine for both seems an elegant solution provided there is accurate screen calibration.

Thanks for your work on PS. Great product. Like Mike, I find it makes sleepless nights most enjoyable.

Cheers!