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GamSat's new Ambiance feature

We have introduced a powerful new feature to GamSat called Ambiance in version 1.02a. To illustrate what this is about let’s take a look at a slide depicting a kingfisher shot under less than ideal conditions in winter. The image seems to be beyond repair at first glance. What you are seeing is not the expected darkness of a linear scan but rather that of a regular system gamma encoded slide scan. The slide was very dense in certain areas while being almost transparent in others like the bird’s white collar or the little fish’s head. The image was shot at a mostly frozen lake and by the time the bird finally showed up the sun had vanished and it had started to snow a bit. So please don’t expect miracles as this particular image will naturally never become that of a kingfisher on a sunny afternoon.

Let's do an initial correction using ColorPos that fully maintains the image’s color integrity. The resulting image is still quite dark. As we do not want to drive any parts of the image to saturation in one or more color channels we have to resort to other means of image editing as we go on. We shall therefore explore using a Gamma adjustment for artistic purposes. As we have pointed out in numerous other documents doing this kind of adjustment to a color Image in RGB mode in Photoshop will inevitably damage color integrity and introduce unnatural murky colors.

To illustrate this we have applied a gamma adjustment by the factor of 1/γ = 1.25 to the bird in Photoshop utilizing the Levels dialogue. An equal adjustment using GamSat will not disintegrate color integrity and will be visibly more natural. The value used in GamSat is γ itself rather than 1/γ. In numerical terms this means γ=0.8 (1 / 1.25). The adjustment is not being applied to the RGB channels directly in this case but instead to the luminance part of a CIE L*a*b* representation of the image. This kind of adjustment will leave color completely untouched.
An adjustment in luminance is not the same as a change in exposure though. Exposure changes are equal to changes in overall intensity. The careful mathematical analysis of such overall intensity changes will show that this kind of change is not being represented by a change in luminance alone but is being accompanied by a change in L*a*b* saturation as well. By exposure change we refer to the use of a different f-stop / shutter speed combination as well as to equivalent procedures in image editing such as the highlights slider in ColorPos.
What this means is that if any pixel’s luminance changes this change alone will not match a change in exposure. To obtain such a match the pixel’s saturation has to be adjusted by a factor that can be derived precisely from the definition of CIE L*a*b*.

The new Ambiance feature allows you to specify whether a compensation of this kind is supposed to be applied to the image as luminance changes occur and to what percentage you want the compensation to be applied. A value of 0 represents no saturation compensation while a value of 100 means full compensation. The effect of choosing an Ambiance setting of 100 instead of 0 while applying the Gamma value of 0.8 to the image is subtle but visible. The kingfisher becomes more vivid in color. You may want to compare this adjustment to the original Gamma adjustment without Ambiance.
Ambiance also applies to Highlight changes brought about in GamSat. We can brighten the bird some more using a slight raise in Highlights. Had we left Ambiance at 0 during all adjustments our adjusted image would look less natural in color. The advantage of using Ambiance over applying a saturation change via the global saturation value is that Ambiance will control the increase or decrease in saturation on a pixel basis depending on actual luminance changes. The luminance changes can so be brought to match changes in intensity like they would be brought about by very elaborate application of burning / dodging in the darkroom.
Please go ahead and compare the simple RGB Gamma change in Photoshop against the same Gamma adjustment paired with the new Ambiance feature of GamSat to see in detail what we mean by murky colors due to lost color integrity.