Part 3: The Residual Image

A more than interesting part of the wavelet tools can be found in the section called Residual image. It offers the possibility to adapt the shadows and highlights but in such a way, that you have the impression that you are sculpting with light!

A photo decomposed using wavelet algorithms results in several wavelet layers and a residual image. These wavelet layers contain the different detail levels, the residual image contains ‘the rest’. Otherwise stated: wavelet levels + residual image = original image. When you decompose an image into three levels, the residual image is relatively close to the original image. The more levels you use, the more blurred the residual image will be. Here are three examples of the residual image at five, seven and nine wavelet levels, respectively.

residual.five.levels

residual.seven.levels

residual.nine.levels

By touching just some sliders in the Residual entry of the wavelet tool, we can do very special things to a photo. Jacques Desmis, the author of the wavelet tool in RawTherapee, said it this way:

“Modification of the residual image is one of the key aspects of wavelet processing”.

I cannot agree more. It feels as if we have a brand new tool at hand.

I’ll show you. This raw photo is opened with the neutral profile.

original.neutral

Click on the wavelet tab, activate the tool by clicking on the little icon left of its name and open the entry Residual image. Leave all the other settings at their defaults.

residual.entry

Many things there, but don’t worry, we are only going to use a couple of sliders to modify the photo. I start with the highlights. First question is: what are those highlights? There’s a little trick to find out. Check the box before Toning and color balance at the bottom of the Residual image entry and drag one of the sliders in the Highlights section to plus or minus 100. I choose yellow to indicate the highlights, that looks so.

original.neutral.yellow

I reset that color slider to 0 and I give the Highlights slider above a value of 50. This action whitens the yellow area from above a tiny little bit. Now I take the Highlights threshold slider and move it slightly to the left, value 60.

highlights.threshold.60

Now things are starting to change, look at the buildings on the right, they start to ‘shine‘ a bit, as does the white traffic triangle on the ground.

I drag the Highlights threshold slider more to the left, to the value of 50.

highlights.threshold.50

As you can see, the shining increases and the building on the left starts to join the light game. There’s virtual no overexposure yet.

A threshold value of 42 gives this.

highlights.threshold.42

See how the asphalt lightens significantly, as well as a part of the sky. Notice a kind of vignetting in the sky (kind of, as this is no vignetting). There’s also a dark halo around the boy in the middle. That halo disappears when the contrast slider is set to minus 50, but the ‘glowing’ gets less important as well.

Shadows

The Shadows slider works as expected: drag it to the right and the shadows are lifted, to the left and the shadows get darker. The Shadows threshold slider can be considered as a ‘strength’ slider. At 0 the shadow slider has no effect at all, at 100 the effect is maximal. The threshold slider only works when the shadows slider has a value other than 0.

The first five sliders apparently interact, so try different settings. In fact, you can alter the highlights with the shadows threshold slider! It can be instructive to colorize the highlights again, as I showed earlier. When I found a combination of settings that pleased me, the highlight area was like so (and much larger than before).

original.result.yellow

Here’s the final image, treated with the tools in the residual section only.

original.result.1600px

Some more examples. First the original raw file with the neutral profile applied, then the (deliberately overexposed) result. Photo taken in 2014 on the Museumplein in Amsterdam, near the Rijksmuseum. Click on the result to see it bigger.

example2.neutral

example2.result.1600px

Another one, same time, same place.

example3.neutral

example3.result.1600px

Remember that all the photos here were processed by using only the residual image tools and nothing else. So no sharpening, no final touchup, niente.

And two last ones.

example4.neutral

example4.result.1600px

example5.original

example5.result.1600px

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pm|jul15