Can this be done in Photoshop or GIMP ?

Non-destructive "toning" of monochrome images.

Start out with a greyscale step-wedge that goes from 0 to 100 in 11 steps. In other words, the Luminance or Brightness values should be exactly 0, 10, 20...100.

Using any method you like, convert the stepwedge to "sepia" - or any other color you want.

After conversion, each of the steps should still be exactly the same Luminosity as they started out, namely 0, 10, 20... 100. If we start with a step wedge that contains 100 tones, they should all be the same as they started. In other words, all the original Luminosity values should be maintained.

I have tried a variety of methods that are provided by Photoshop and GIMP, and which are discussed in tutorials on the web. So far, every method I have tried, results in a change to the luminosity values - including the Photoshop tools which provide the explicit option called "Preserve Luminosity".

(After using the standard techniques, one could always apply some kind of correction curve to undo the damage done, and manually "force" the Luminosity values back to where they started - but I'm looking for a procedure that actually does this automatically: a non-destructive conversion).

Please don't ask me general questions like: "Have you tried try LAB color ?", "Have you tried X ?" or whatever. I am looking for a method that works. If you have one, I bow to your feet in gratitude. I confess, I'm a beginner !

Thanks in advance.

Okay, the only way to preserve a Step Tablet would be to use Duotone, and apply no Curve (just a straight line). Use Black for one colour, then choose a Pantone that matches the Sepia look you want. Then this exists as a PSD file, or can be exported as an EPS. If you have a Postscript printer, you can then print this. Use this link to save to disk a PSD; right click or control click as appropriate.

I don't think there is a quick and easy way to make this a JPEG. If you tried to convert this to an RGB or CMYK image, then the values would change.

Ciao!

Gordon Moat
A G Studio

I just tried this, and the resulting step-wedge is completely different after having the Duotone curve applied.

Unless you adjust one of the curves within the duotone, the resulting image will, by definition, be different in Luminosity. Getting multi-tone curves balanced manually with respect to one another is a tedious process, and gives an approximate result at best. When software does things, it can get quite close to perfect.

I know a bit about these duotones, having fiddled around a bit in the past. In fact, I am looking for something to replace my existing Quadtone curves, since I am unable to get them perfectly balanced, and I was hoping that someone already solved this problem programatically.

Perhaps there is something I have missed ?

Another problem with Dutones, Tritones, and Quadtones, is that one must first convert from RGB to 8-bit Greyscale. In the process, we are tossing out 1/2 the tonal values from the image. This subject has been discussed here and elsewhere, and experts have told us that while most printers are 8-bit, it's best to perform conversion from 16 to 8 bit as the last step in the workflow.

Otherwise, we can end up with gaps or banding in the tonal curve. Subsequent adjustments result in additional loss of tones. It is for just this reason that I am trying to find an automatic non-destructive process.

The only other thing you might have missed is doing a monotone. In other words, rather than combine a colour with black, simply start with the final colour you want. So if you want an overall Sepia, just pick one colour that is near Sepia. Create your Step Wedge from that. Anyway, the coffee must not have been working this morning, because it probably would have been better to use a monotone.

I am not sure why you are converting from RGB to Greyscale for making a Step Wedge. Why not just start out in Greyscale? Then select Duotone from the menu, but only do a Monotone. After that, you could try converting to RGB or CMYK (or CcMmYK, or CMYKog, etc.) to run it on your inkjet printer.

Interesting observation for you for PhotoShop. RGB contains in each Channel a component of the K (Black) Channel of CMYK. If you had a Greyscale image, then converted to RGB, and then went to CMYK, any 100% black in your Greyscale image goes through a conversion. The final CMYK values might be Cyan 100%, Magenta 100%, Yellow 60%, and Black 100%. You could have 400% Total Ink, but only on some digital presses and laser printers. With a press run, you risk warping the paper. When printing to an inkjet printer, GCR or UCR will clip whenever too much ink is present, so you cannot print 100% of each ink.

What that means in practical terms is that some colours of inks will dominate over other colours, and under some lighting might be visible as a colour cast. That colour cast might be more apparent in lighter or darker tones, though that depends upon the ink. So you could create a set of curves to adjust around this, though a practical matter makes it tough to map enough points, though with practice you might get close enough.

While a monotone, duotone, tritone, or quadtone does indeed work very well in commercial printing, translating those to CMYK creates a different set of problems. This is the same issue with Pantone colours, in that a commercial printer can know what to expect with one Pantone ink, but when that desired Pantone colour is translated to CMYK, the end result can appear noticeably different.

So out of curiousity, are you trying to set this up for a particular inkjet printer? Are you doing this to a CcMmYK device? Something else? Also, if you want images done for on-demand printing off a digital press (Lulu, Blurb, et al), then the best you could hope would be simulating the effect.

Ciao!

Gordon Moat
A G Studio

Ken, I know you asked us not to wander from your very particular question but what Gordon is writing is likely relevant. So now that he's broken the ice ....... what is the purpose of this linear conversion you are seeking?

...what is the purpose of this linear conversion you are seeking?

I generally prefer the look of warm-toned images. I have been using my Quadtone curves for a few years, printing on my Epson printer. Using the standard Ultrachrome inks, each photo can get its own treatment, since the entire range of colors are available.

Having recently become aware of the benefits of a 16-bit workflow, I have explored some other ways of producing warm-toned images - and have discovered their shortcomings. The biggest shock for me was to discover that the GIMP only supports 8-bit operations.

I would like to be able to make tonal conversions in 16-bit mode, introducing minimal loss. Perhaps the problem is harder than I have realized, and the existing products come as close as practically possible. In that case, it's best to just accept the existing 16-bit methods, and correct for the minor flaws that they introduce, while still in 16-bit mode. There ought to be plenty of tonal values left, before we drop to 8-bit for printing.

http://www.kenleegallery.com/images/tech/rosestoned.jpg

For those whose workflow is 100% Analog, this is probably a laughable conundrum. But then, they discard negatives whenever a big scratch or spots appear. Either way, it seems, there is a price to pay in the pursuit of Beauty.

In Photoshop; convert picture to Lab colour mode. ”Tone” with the curves command by adjusting the a and b channels, but leave the L channel unchanged. Convert back to RGB if desired.

Please see the attachment below: I adjusted only the A channel, by changing the curve. As you can see the Luminosity values have changed. While the top portion goes 0, 25, 50, 75, 100, the lower portion goes 0, 29, 55, 80, 100.

Can you kindly show me a sample of how this works ? I am new to LAB mode.