This transformation represents the first deviation from a simple Sabattier effect. Instead of inverting the highlights, we invert a tonal range within the image – in this case, the sky which falls tonally between the highlights on the bike and the mid tone of the image.
If there is sky in the image it is often worth experimenting with that in isolation. The sky normally has a tonal gradient ranging from lightest at the horizon, darkening towards the top of the image. In fact some people use the higher regions of blue sky for metering as it represents a useful mid tone. Mixed in the tonal range are clouds and (less desireable) aircraft vapour trails.
In this image quite a lot of the bike falls in the same tonal range as the sky, this is affected as well, you can of course suppress this by masking, but it is not usually necessary and hasn’t been done here. Try creating an inversion similar to the example below and experiment with the effects of the transition points.
There are 2 transition points, a lower darker one and a higher, brighter one. The upper transition point is located within the range of the sky tones. To the left of the transition point (larger upper cyan arrow) is the range of sky tones that are inverted. To the right of the transition point (smaller upper cyan arrow) are the sky tones that were not inverted – but have been stretched (tonal range expanded) so that they range from almost black to about 15% brightness, overlapping the inverted tones near to them. This overlap isn’t visible in the final image, but the next example will show ways of controlling this for effect. The highlights, which occupy the top 80% of this image, have had their tonal range extended so that they range from almost black, to pure white. In the darker areas they also overlap with the inverted sky tones.
This stretching – The Stretch – is a powerful tool for enhancing effects and generally complements other manipulations within the image. Most inversions require The Stretch to become noticeable in the resultant image. Posterisation is a usual outcome, which adds to the effect. In a monochrome image, the stretch increases the contrast of the stretched tonal range. In a colour image, depending on which channel is used, hue, brightness & saturation can be stretched. When the H channel is stretched, a relatively narrow range of colours can be stretched to rainbow like effects (more on this later).
In this image two tonal ranges are stretched – the inverted sky range (cyan arrow)and the highlight range (red arrow). The partial inversion and stretching of the sky and other highlight tones have created a dark background for the bike, framing it nicely, and emphasising the white edges created around it brightening the mid-tones. A lucky accident this time is that the clouds and vapour trail have both moved to a similar darkness, forming and X shaped frame for the bike within the darkened sky. (Incidentally I deliberately framed the bike with these elements at exposure time, with hindsight this would have worked better if the camera had been a little higher off the ground.) Sometimes stretching causes other tonal ranges to be compressed, but with inversions, no compression is needed.
The grass has become lighter as the tone range that it is in has stretched into the lighter tones.
The stretch is a normal by product of an inversion. Sometimes it helps little, but often it dramatically improves our image, especially when we stretch a colour range in a colour image and rainbow effects are the result.