Simulating color reversal films - DIR couplers

[AbsurdePhoton]

Hi there, I had posted things only in the B&W section, now it is the turn of color films.

For those who don't know, I am trying to simulate analog films in a software which is for the moment not publicly published, but i will surely do that when I consider the simulation good enough.
The good news is I am also simulating the printing process now (only paper for the moment). All works pretty well for B&W, I obtain good negatives and I can simulate regular and multi-grade B&W papers.

But I had a hard time understanding how color negative films work, you can count several weeks, and it is not finished. I have attained a satisfactory simulation for color negatives : I combine the spectral sensitivity curves, H&D curves, dye curves, for the films themselves and the printing part uses color compensating filters in a virtual enlarger (Kodak Wratten filters).

But --- something is still wrong. I obtain what seems to be good negatives (overall orange hue), they look convincing. But when I virtually "print" them, the colors are good but a LOT faded, no matter how I play with the virtual color compensating filters (which in fact mostly modify the color balance, nothing else).

I think I found what I missed, an essential part : the DIR couplers.

What I understand is that when a dye is formed, at a certain level there is something that also forms, the DIR couplers, that prevent in a certain extent the other dyes to form. And this is chemical. So this is a system of "dominant" dye that "boosts" the colors. My result is good, but it misses saturation. If I boost the saturation in Gimp, the photo looks well printed.

Could someone here explain the process in details, so that I could imagine a way to simulate the DIR couplers ?
Another question : are there also DIR couplers in printing papers ?

Thanks in advance.


( here is an example with Kodak Gold 200 + Kodak Ektacolor Edge 7 : original, negative, print without color filters - I used a pretty much saturated source photo for the demo )

source image : https://wallpapers.com/wallpapers/english-longhorn-beef-cattle-breed-h6zojezohim1ld7e.html

 

[koraks]

The dominant factor seems to be a contrast/gamma mismatch, and that doesn't look like a DIR coupler problem.

If I boost the saturation in Gimp, the photo looks well printed.

?

I see a gamma problem, not a saturation problem?

Another question : are there also DIR couplers in printing papers ?

I don't think so, no. The simple reason is that they're not necessary since the problem of crosstalk between the color layers is avoided in other, simpler ways that are not possible in a recording material.

Your question should be answerable on the basis of publications in the IS&T domain. Some of that may be accessible, although much/most of it is behind institutional paywalls. Have you done a literature search yet?

 

[Rudeofus]

DIR couplers do not directly inhibit dye formation, they inhibit photographic development. Unlike restrainers they don't do this all the time, but only after reacting with an oxidized color developer. This whole thing happens in three steps:

Ag+ + CD-4 <===> Ag + CD-4OX
CD-4OX + DIR <===> CD-4-phenol + powerful restrainer
Ag+ + powerful restrainer <===> undevelopable Ag+

As silver gets developed, more and more restrainer is released and development in this region slows down. Dependent of how color layers are arranged on your film, this development inhibitor can diffuse into adjacent layers, e.g. as more green/magenta is developed, blue/yellow and red/cyan are also more restrained, giving you more saturated colors. This is how film makers create "vivid colors" versus "natural colors".

 

[Lachlan Young]

Couplers etc aren't the problem here, it's understanding how the mask gets automatically (and properly) corrected out in a darkroom situation. If you think about how the mask works, what the paper is effectively 'seeing' would equate to the areas of greatest mask density (i.e. negative rebate) being the white point pre-inversion.

The DIR/ DIAR couplers affect not just the colour correction, but aspects like sharpness (and achieving 100%+ contrast response at very low frequencies is something digital really cannot do), apparent 'granularity' from the dye clouds and highlight density control.

 

[koraks]

it's understanding how the mask gets automatically (and properly) corrected out in a darkroom situation.

I wonder how the mask was modeled here in the first place. It's kind of abstract to me since the mask in reality will have a significant role in terms of the spectra of the primary dyes and the (density-dependent!) correction that the mask applies over this, but since we're in digital space here, there's also the interaction with the (hypothetical, simulated?) scanner which has a certain spectral sensitivity. So I wonder if I'm looking at the simulated color negative, what it really is I'm looking at.

Taking it for a spin for a manual inversion exercise, the only thing I can tell for sure is that the image does not respond like any scanned color negative I've ever seen. I don't know what's wrong with it exactly, only that it's not representative for how a scanned color negative looks like in reality. The clipping in the blue channel doesn't help either, but that's not the main problem. There's something about the curve shape that's very odd.

This is what a real color negative does if you take a scan of the negative and apply a simple linear curve to it:

As you can see, it doesn't go totally wonky-bananas. You also typically end up with a rather flat positive this way, which makes sense due to the curve shape of a typical RA4 paper, which is both steep and has a pronounced toe & shoulder. If you apply a toe + shoulder to the digital curves, things start to look more like what you'd expect a print to come out like:

 

[AbsurdePhoton]

Thanks for these answers !

@koraks : if I boost the saturation OF THE PRINT the result looks better, not the developed state. The developed state is just a visualization, the real values are kept as Transmittance of the film, to be fed to the printing part.

Somewhere in my workflow, for each discretized wavelength value, I do :
densityTotal = densityYellow + densityMagenta + densityCyan + densityMin (the last one is what gives the developed film its orange hue, is this the "mask" you're all talking about ?

densityTotal multiplied by an output spectrum (for each discretized wavelength value) is what gives the "negative" image, it is just for visualization, for example with daylight spectrum, after converting the discretized spectrum to the XYZ color space then converted back to RGB. I think that should give a good hint at what a negative looks like. So there's a a gamma problem at this step.

@koraks again : could you post the negative scan of the village scene ? I'd like to see what you mean by "the image does not respond like any scanned color negative I've ever seen", so I can test it myself.

About the different curves : I am keeping all the original values, without distorsion. The only thing i do to the HD curve is transforming it to relative exposure (just a translation to 0). But I have the exposure and time compensation values to get back to the "real" output density values (i.e. I don't use "real, physical" exposure and time compensation values)

About the inhibiting couplers : I think I will have to include that for at least the saturated colors effect. I can use the MTF curve to simulate the sharpening (and inversely blurring) effect.

Thanks again to all of you.