Random Thoughts on Photography [9] ~ the horror of "Curves"

Recently I took a quick look at color management.  

 

 

The science of human perception in colors is vast and by now pretty well understood.  You can read various studies and findings here on the internet.  You can also find standards that have been developed based on human perception models of colors and luminescence, so I won't try to bore anyone with the details.  

Of course there are many ways of "getting there from here."  There are algorithms that implement various aspects of color science.  Unfortunately these are the kinds of things that are for better or worse hidden from consumers to a certain level.  On the other hand camera manufacturers and RentWare do their best to present us with "pleasing images" by making early decisions for us.

Back when dinosaurs roamed the earth and film was what we used to make a photograph, we were completely dependent on what manufacturers produced.  It was their job, the manufacturers that is, to find ways of pleasing us the consumer so that they could sell more product. Now we have software that does similar things, trying to simplify as much as can be simplified, while offering the consumer some sense of control over the final "look" of their images.  

Rarely do we have access to something that offers us complete control, from the yet to be demosaiced RAW to fully color managed and ready for additional user input stages.  It's rather like watching an alchemist mix up egg white/ether/gunpowder/silver nitrate on their way to coating a glass plate in the wet-plate collodion process.  When dry plate and film were introduced all the "dirty work" is taken care of for us and we didn't have to be alchemists to make a photograph.

In trying to transfer image processing settings from one system to another I noted that camera manufacturers and RentWare companies tend toward "closed" products.  That is, they can "bake" into their image processing software their own "special sauce" and it is very difficult to understand what's going on behind the scenes.  I can imagine this is confusing and might have led to the rise of on-line discussion forum "color science" wars.  No one really knows much of anything, but people seem to have a lot of ideas.

RawTherapee is a software for image processing that doesn't hide much, if anything.  I'll bet this is one of the reasons people tend to avoid it.  Yes, it's Open Source.  Yes, it's free (as in liberated).  Yes, it can be very very complex.  But, if one understands what they're doing, many of the "icky bits" can be streamlined into a simple one button process.  In fact, the application provides many pre-configured processes.  I've found it a feature rich software.  The engineers seem to really know what they're doing.

One of the things I noticed while trying to understand the demosaicing and color management process was that once I got through these stages that there are several different kinds of "Curve" mode algorithms.  Ooph.  More to learn and understand.  Will this never end? Possibly not.

Girding the loins, I set out to fill in my knowledge gaps about "Curve" modes.  Here is what RawTherapee's man page has to say.

Curves ~

Standard ~ ....the values of each RGB channel are modified by the curve in a basic "correspondence" method, that is the same curve is applied to all channels...

What I've experienced in using this "Curve" is that manipulating the luminosity curve equally manipulates, and quite strongly, the RGB channels.    Changes in luminosity and contrast directly effect color saturation.  Strongly increased contrast yields strongly increased color saturation.  This is "un-natural" in terms of human persception of color.

Strong "Curves" manipulation can distort information and I have to be very careful to guard against loosing subtle tonal gradations in shadow and highlight regions.  Interestingly, this is the classic "Curves" algorithm that I'm sure everyone knows and loves. We've all gotten rather used to these effects and have learned to either work around them or embrace them as part of our images "look."

Weighted Standard ~ ...use this method to limit the color shift of the standard curve...

The algorithm decouples by using a different algorithm the behavior of the RGB channels in relationship to the luminosity curve.  Color saturation changes are much more subtle, in fact.  More subtle than the following "Curve" type, "Film-Like".  Weighted Standard looks and behaves more similarly to "Luminance" and "Perception."

Film-Like ~ The film-like curve provides a result highly similar to the standard type (that is strong saturation increase with increased contrast), but the RGB-HSV hue is kept constant - that is, there are less color-shift problems. ..

This is pretty interesting, actually.  Color saturation shifts remind me very much of film.  I still have to guard against subtle tonal loss in the highlights and shadows, but I've found "Film-Like" to be a little more manageable than "Standard."

Saturation and Value Blending ~ This mode is typically better suited for high-key shots...

This mode is a strange one to me.  I read the description and can't make heads nor tales of what's going on, which means yet another learning opportunity is presenting itself.  I guess I just don't shoot enough high-key to know how best to use this "Curve."

Luminance ~ Each component of the pixel is boosted by the same factor so color and saturation is kept stable, that is the result is very true to the original color...

And here is something that I'm still coming to grips with.  Using  "Luminance" or the next mode "Perceptual" which show "true to the original color" illustrate for me just how bland the real world is.  I begin to see just how much color distortion we've become accustomed to.

Coupling "Luminance" with LAB color space functions can bring an image more in line with "Standard" and "Film-Like" saturated color images.  But there's a subtle twist.  Highlight and shadow areas are controlled in a LAB color space and can retain more/different information.  This is an interesting option for potentially unique image processing.

Perceptual ~ This mode will keep the original color appearance concerning hue and saturation, that is if you for example apply an S-curve the image will indeed get increased contrast, but the hues will stay the same and the image doesn't look more or less saturated than the original...

If I really need saturation, there's always either the traditional RGB saturation slider or the LAB colorspace "chromicity" slider.  This mode looks similar to "Luminance."

This is a lot of information to digest, particularly if one isn't used to so many options.  What I've taken to doing is working with an image and trying each "Curves" mode to see what it does, and as a learning exercise to try to match "Standard" and "Film-Like" output.

For instance, I recently re-worked an image that I'd first processed using "Standard".  This time I worked it using "Luminance" curves mode.  I pushed hard on the "Chromaticity" LAB color space slider to get a similar looking image.  But with "Luminance" I was able to retain good highlight and shadow detail that I found to be more properly rendered.

In another instance, I worked with "Film-Like" and used gentle curves manipulations.  The output of that process is fairly "natural".  Using "Luminance" on the same image, again using subtle curves manipulations yielded what felt like a color-drained scene.  Again, it illustrated to me just how much we've come to accept wildly saturated images as "normal."

I have a few more things to share about curves and such.  But I'll save them for another time.  Until then, I will continue to work with RawTherapee's curves modes and see if I can begin to match the various proprietary software outputs.  As a further learning exercise, of course.