Contrast and Chroma
One thing I have been wanting to write about is how sigmoid mapping affects both contrast and chroma. There is an interesting relationship between the sigmoid module in preserve hue mode and the color balance rgb module which can be exploited for some interesting results.
Increasing contrast in the sigmoid module
Here is a set of different contrast settings from an image with low contrast at capture. Notice how the chroma of the image follows along with the increased contrast.
| contrast = 1.2 (similar to the original linear picture) |
contrast = 1.6 |
contrast = 2.4 |
contrast = 3.2 |
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This is different compared to the color balance rgb where the contrast parameter only changes the intensity of the color and the chroma slider is orthogonal to the contrast. The “rgb ratio” preservation option in the sigmoid module (or any of the preserve color options in filmic) is on the other hand similar to the color balance rgb contrast option and only changes the intensity of the pixel without adding any chroma changes to the image. This is sometimes what you want and it is also what is used in the Reinhard tone mapper (achievable using the sigmoid module with contrast = 1, skew = 0, and rgb ratio as color mode). The effect is kind of weird when you try to add contrast to an image in the tone mapping stage:
| contrast = 1.2 (rgb ratio) |
contrast = 1.6 (rgb ratio) |
contrast = 2.4 (rgb ratio) |
contrast = 3.2 (rgb ratio) |
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Noticed how the image seems to desaturate when the contrast is increased? It didn’t from a mathematical standpoint. A good example of that the perceptual effect can be quite different from the mathematical intention. I think this might be one of the reasons why some users have struggled with the filmic module and why the middle tones saturation slider was added to that module. Many simply experienced that filmic desaturated their image in the same way as the slide deck above. I guess this effect has a name after some old dude that I just haven’t found out about just yet, if you know the name please share it!
Saturation from contrast!
Back to the preserve hue option which changes both intensity and chroma with increased contrast. This behavior can actually be exploited when combined with the orthogonal settings of the color balance rgb module. First reducing contrast and then increasing contrast actually leads to an overall increase in saturation!
| sigmoid contrast = 1.6, color balance, vibrance = +35, linear chroma = +35 |
sigmoid contrast = 2.6, color balance, contrast = -35 |
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Isn’t that interesting? They are almost identical but the processing path is totally opposite!
Why? Well, the sigmoid color change for the “preserve hue” mode can be explained as an exposure + linear chroma change. I.e. first a common factor for the rgb triplet and then a linear chroma scaling as in the color balance rgb module. The amount of chroma change is however not uniform but is instead a function of the original pixel intensity and chroma. This input-dependent amount has two very nice properties, it is smooth, and it approaches the gamut limit but always stays inside of it. Colors with high chroma are thus not changed as much as colors with low chroma, a bit like the vibrance slider in color balance rgb! This is also why colors desaturate to white as the intensity increases as has been shown in an earlier post. Also, remember that hue is kept constant throughout this operation in the same way as for “rgb ratio” and in the filmic module.
The same method can be used on pictures with high dynamic range, not only for low dynamic range pictures like above. Take for example this image from HDRI heaven:
| sigmoid contrast = 1.6 (default) |
sigmoid contrast = 1.2 |
sigmoid contrast = 1.5, color balance rgb contrast = -20 |
sigmoid contrast = 1.9, color balance rgb contrast = -40 |
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Compared to using rgb ratio (or filmic with preserve color) and increasing chroma in color balance rgb:
| sigmoid contrast = 1.2 (rgb ratio) |
sigmoid contrast = 1.2 (rgb ratio), color balance, vibrance = +25, linear chrome = +25 |
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So what do I want to say with this post? Increase your image saturation using the " contrast on contrast method"? No, definitely not, it will most certainly bite both you and me in the butt later on. No, I had liked to say these three things:
- A chroma increase is necessary when increasing contrast as the image otherwise will look desaturated.
- The preserve hue option, gives that for free, with robust properties, in the tone mapping.
- The relationship between contrast and chroma usually needs adjustment and that is where the color balance rgb shines the brightest! My current approach when editing using the sigmoid module is to find a sigmoid contrast setting that minimizes the needed work in the color balance rgb module.
All of the above of course also works for the opposite use case. You can decrease the saturation of an image by increasing contrast in color balance rgb and decrease the contrast in the sigmoid module. But again, probably not advisable as the main method of working with color in your image.
Now I’m interested in what you think about these observations!
It is sadly too limited because I can’t shift the points on the histogram sideways or create more points. I’ve been dreaming about a scene referred replacement for the rgb tone curve, but I have no idea if that is even possible. Having it in Sigmoid may be what I’m wishing for? 
