Removing Orange Mask from Colour Negative Scans

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carllooper
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Re: Removing Orange Mask from Colour Negative Scans

Post by carllooper » Fri Jan 29, 2016 3:12 am

tomfrh wrote:Well, the other "spectral" colours really do have corresponding physical reality, and even though we know the impression of "red" is in fact in our brains, we can at least take solace in the fact that there is a unique frequency out there in the REAL WORLD that it corresponds to.

but with these non spectral colour, e.g. magenta, our brain is totally misleading us, telling us there is a colour, but in fact there is no corresponding colour in the real world!

White/grey/black too! There's no such thing as "white".

This is perfectly valid of course, and the standard idea about it all, ie. that colours are just effects in our mind - but the philosophy I use these days is that the real world is an image (visual, sonic, tactile, etc), and that what goes on in our head is a reproduction or extension of that image, plus any theories we can entertain about that, eg. that red is an "impression in our brain" that otherwise corresponds to some unique frequency (or wavelength) of light out there in a "real world". Or alternatively (and no less theoretical) - that the concept of light and wavelengths, are a particularly useful way of describing colours such as red. In other words (according to this theory) it is red which belongs to the real world, and our description of red (by means of concepts such as light, etc) which belongs to what goes on in our head, or more so, within language (knowledge, etc). According to this approach we can therefore use the idea of a composite signal (prior to decomposition into more fundamental signals), to describe magenta. But the magenta itself is actually what is real, ie. not in our heads - or rather not just in our heads.

In simple terms: colours are real. And magenta is real. And we are not being misled at all. What we see is what is actually there. We're not imagining it. Or not just imagining it. Rather it becomes our descriptions of magenta that are a little less real. And our description of some reality behind magenta even less so. The so called "real world" behind what we see becomes the more "fictional" one (if often very useful one).

That's the theory I use anyway. And it makes sense to me. Why should light or colour be defined in terms of fundamental signals? Why should magenta be any less real than cyan or yellow, just because it doesn't correspond to a single wavelength of light? Why should single wavelengths of light be any more real than complex combinations of such? The only reason for reducing light into more simpler terms, it seems to me, is so that we might take advantage of that way of describing it. It is more economical. It is easier to describe light if we can simplify it that way. It's more to do with language and technological reasons than anything to do with that is how it is in reality. In reality, magenta would be there. It will only be for technological purposes that we assume it's not there, and assume that magenta (or more generally colour) is some sort of psychological effect in our heads.

The analogy I'd use would be a piano. One can decompose all the music that has been written (and yet to be written) into a set of key presses on a keyboard (into single notes) and suggest the keyboard is reality (or an analogous to what reality would be) and the music played on such a keyboard becomes an effect in our head. Or one can otherwise propose that music (in all it's complexity and simplicity) is that which is actually real, (outside of our head as much as in it) and the keyboard (and it's notes) are just a particularly nifty way of formalising a system of description for such music, because it allows for the reproduction of a whole range of different musical sounds, from complex compositions to single notes. Pursuing this idea it becomes a certain kind of domestication which treats the system of description as more real than that which is being described, and through such we can inadvertently come to assume that single notes are more real than the more complex "compositions" of those notes. For I doubt any musician actually composes music by simply rearranging individual notes. Although doing it that way is not out of the question (and not any less music). Rather I fully expect musicians start with music they can already hear (in some parallel universe) and decompose such into individual notes. They write it down as a score. It's then in the reproduction of that music (such as playing a piano) where the music is actually "composed" (from the notes). Or a better way of saying this is that during a performance they recompose the original music from the notes played on the keyboard. They put the music back together again. Reproduce or extend all the complexity and simplicity of the original reality.

But as mentioned, that's just the theory I use. Could be complete bollocks for all I know, but who cares? I certainly don't. I just find it very useful. Especially in creative work. But also very much in technical work.

C
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Re: Removing Orange Mask from Colour Negative Scans

Post by carllooper » Fri Jan 29, 2016 8:02 am

Ok, I've checked the After Effects (and corresponding Photoshop) method, and I'm pretty sure that the reason it might be giving more desireable results is as follows:

I'm thinking out loud here but from what I've been able to reconstruct from what I was doing there it seems to possess the following logic

The red signal is a perfectly fine signal. This is the same in the computer model. But due to dye bias in magenta, there would be an echo of this perfectly fine red signal, in the green channel, which would need to be removed from the green channel. However in theory (as implemented in the computer model) the green channel shouldn't really have any echo, as this is what an orange mask is supposed to neutralise. The green channel (as discussed in recent posts) should be carrying a correct green signal, other than it would require a simple level correction.

However this assumes an orange mask is actually doing it's job properly (ie. assumes it is conforming to the theoretical model) - but if it's not doing so, then there would still be an echo of the red signal in the green channel, however small or large.

So the way of removing any such red signal in the green channel would be to subtract the red signal (a proportion thereof) from the green signal. In other words - to neutralise it - which, if we're working with negative, means subtracting a proportion of cyan (= red) from magenta (= green). Which is exactly what the After Effects/Photoshop method does.

And once we've recovered a good green signal (= magenta in negative) we can then use the good green signal to correct for any echo of the good green signal in the blue channel. And just like subtracting red from green in the above, we would subtract green (proportion thereof) from blue, which in negative means subtracting a proportion of the magenta (= green) from the yellow (= blue). And again, this is exactly what the After Effects/Photoshop method does.

So I wouldn't necessarily abandon the After Effects/Photoshop method. In theory one should only have to adjust the green and blue levels of a neg scan but the After Effects/Photoshop method may very well be correcting for a mask that hasn't quite neutralised the echoes that a mask is otherwise designed to neutralise - and so the After Effects/Photoshop method is providing that extra tweakability on neutralisation that a simple green/blue level adjustment can't (because it is based on the idea that the unwanted signals are already neutralised).

And general colour balancing with a good eye would otherwise do the rest.

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Re: Removing Orange Mask from Colour Negative Scans

Post by carllooper » Sat Jan 30, 2016 2:13 am

Another possibility (theory) for why the After Effects/Photoshop method might be giving better data, is that the RGB filters used in digital capture may not be ideal. One can imagine that the impurities which fustrate cyan and magenta dyes might also fustrate RGB filters in digital cameras, if in a somewhat different way. Although I can't imagine how they would be anywhere near as problematic as CMY dyes, since the job of RGB filters is a lot simpler. One thing I've recently learnt (which I didn't otherwise know) is that Bayer filters on digital cameras have a certain amount of built-in cross channel leakage. This is to help improve the resolution of digital cameras but at the expense of a drop in colour saturation. So this could very well be inteferring with the operation of the orange mask - a theory that would need to be tested of course, as I'm just making this up now as I speak.

Thinking out loud.

Other factors might be non-linearities in the dye impurities, where a simple inverse proportionality model fails to account for such. Some cross talk between the operation of the orange mask, and the operation of the model might be allowing for some extra twiddle room.

What is ultimately required is a more sophisticated model, that not only takes into account the way film is supposed to work in theory, but the way it might also deviate from such in practice. A model which would account for more of the subtle variables involved, and a more sophisticated user interface for adjusting those variables, to compensate.

I'll be doing some more colour correction on neg in the near future (for a Super8 film), so I'll be writing up a new model for that, which factors in what has been learnt from this recent rethink on all of this.

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Re: Removing Orange Mask from Colour Negative Scans

Post by S8 Booster » Mon Feb 01, 2016 9:14 pm

Haven't read all posts here but is there a less academic approach to the problem?

Initial: The technique which converts neg film to film prints. It gets rid of the orange all together doesn't it? This should be possible digitally by using PS or any suitable digital tool shouldn't it?

http://photo.net/learn/darkroom/color-darkroom

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Re: Removing Orange Mask from Colour Negative Scans

Post by tomfrh » Mon Feb 08, 2016 7:29 am

Haven't read all posts here but is there a less academic approach to the problem?

Initial: The technique which converts neg film to film prints. It gets rid of the orange all together doesn't it?
That would appear to be correct!


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Re: Removing Orange Mask from Colour Negative Scans

Post by gdemonta » Sat May 14, 2016 12:18 pm

carllooper wrote:So here's an example of doing the orange mask removal in Photoshop.

For this to work you need to convert your image into a CMYK image without the K. You can do this by creating a custom CMYK profile in which the Black Ink limit is set to 0%. This will interpret your digital image in the same way that colour negative is created, ie. without a black channel.

So after loading a test image, select:

Edit > Convert To Profile ...
Profile: Custom CMYK

And then enter the following:

Ink Colors: SWOP (Coated)
Dot Gain: 0%
Separation Type: UCR
Black Ink Limit: 0%
Total Ink Limit: 300%

Image


And then follow this:

Image


Something similar can be done in After Effects, but (as far as I can tell) After Effects doesn't provide CMYK space in which to work, but it's easy enough to prepare comps that can do it. I've indicated one such approach in the first post, using Effect>Channel>Shift Channels.

If I get time I'll post an After Effects tutorial.
Thanks Carllooper for the great tutorial. Unfortunately the images are gone, do you think you could reupload them or put back more details on your photoshop method? thank you so much!


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Re: Removing Orange Mask from Colour Negative Scans

Post by flyinghigh69 » Thu Jun 15, 2017 9:56 pm

Best solution I found was to color correct during capture. This eliminates the need to fiddle around in Photoshop. To do this, you need a camera that can save a preset white balance. The Nikon D610 can do this for example. You set up the leader from your developed film on your light table, frame it so that it fills the entire frame, and save the white balance for that scene. Then go ahead and photograph your negatives using this preset white balance. This is basically the same technique as tweaking the white balance in Photoshop, setting white to a previously sampled image of the leader (as explained in another post in this thread). Good luck!


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