Retaining highlight details in Raw Therapee

Highlight recovery is essential for large dynamic range scenes and large prints. This article compares strategies for highlight recovery when processing raw photos in Raw Therapee (RT, version 4.0.8). A good strategy is to implement highlight recovery in a matrix transform from device RGB to the profile connection space.

Why highlight recovery?

The dynamic range of real world scenes can be very large and 16 to 17 stops is not unusual. Our eyes can see 10 to 14 stops of instantaneous dynamic range and large-sensor digital cameras can record 8 to 11 stops. A decent graphics display has around 10 stops of dynamic range, although high contrast ratios are achieved with dark blacks rather than bright whites. A typical photographic print has a dynamic range of about 8 stops.

Highlight recovery attempts to produce realistic-looking highlights in a more limited dynamic range output. Without highlight recovery, high brightness areas in a scene are rendered as featureless white.

Highlight recovery example 1

My first example is an afternoon photo from the Australian outback. Direct sunlight is reflecting strongly off the tree trunks. Exposure is as-metered and I selected the photo that looked best on the LCD of my camera.

Parachilna Gorge, South Australia. Taken with Canon 400D (Digital Rebel XTi) digital SLR. Processed with Raw Therapee.

First, I processed the image with my custom profile, based on an overexposed image (“hot matrix”). Beware: these highlight examples might look ugly on an uncalibrated display.

Highlight detail at 200%, processed with “hot matrix” profile.

Second, I processed the image with an alternative matrix profile, based on an image exposed as-metered off an 18% grey card (“standard matrix”). Compared to the “hot matrix”, more highlights are blown. The “standard matrix” clips at lightness (L*) = 100 (see the xicclu plots in my camera profiling blog) whereas specular highlights can exceed L* = 100.

Highlight detail at 200%, processed with “standard matrix” profile. Highlights are “blown”.

To recover highlights with the “standard matrix”, I applied a negative exposure adjustment to the raw RGB data and then corrected the exposure in the RGB working colour space. RGB curves are helpful for increasing exposure and preserving highlights. Exposure compensation (RT Exposure tab) simply scales the RGB data and positive exposure compensation can easily result in clipping.

Highlight detail at 200%, processed with “standard matrix” profile, 0.5 raw white point linear correction (-1 stop) and +1 stop RGB curve. Reducing raw exposure is safe when the raw photo RGB channels are not clipped.

Highlight detail at 200%, processed with “standard matrix” profile, 0.5 raw white point linear correction (-1 stop) and +1 stop exposure compensation. Scaling RGB in the working colour space has resulted in clipping.

The next image shows highlight recovery with the Canon EOS 400D DCP profile distributed with RT. The highlight detail is much the same as my “hot matrix” profile.

Highlight detail at 200%, processed with DCP profile.

Canon Digital Photo Professional (DPP) does a good job of highlight recovery and has likely applied more complex and additional tools.

Highlight detail at 200%, processed with Canon DPP, Standard picture style.

Highlight recovery example 2

My second example is a catch of tropical reef fish. The mid-day tropical sun was intense, there are strong specular reflections and the raw photo RGB channels were clipping. Exposure is as-metered.

Coral trout, red-throated emperors, leather jacket and one reef cod. Taken with Canon 400D (Digital Rebel XTi) digital SLR. Processed with Raw Therapee.

The “hot matrix”, RT DCP and Canon DPP results showed satisfactory highlight detail with this photo. The “standard matrix” profile clipped badly. Negative exposure adjustments to clipped raw photo RGB channels resulted in false magenta highlights.

Highlight detail at 100%, processed with “hot matrix” profile.

Highlight detail at 100%, processed with DCP profile.

Highlight detail at 100%, processed with Canon DPP, Standard picture style.

Highlight detail at 100%, processed with “standard matrix” profile. Highlights are clipped

Highlight detail at 200%, processed with “standard matrix” profile, 0.5 raw white point linear correction (-1 stop) and +1 stop RGB curve. There is purple fringing around  specular highlights. Reducing raw exposure risks false highlight colours when the raw photo RGB channels are clipped.

Highlight reconstruction by colour propagation recovered most, but not all, of the false magenta highlights.

Highlight detail 200%, processed with “standard matrix” profile, 0.5 raw white point linear correction (-1 stop), +1 stop RGB curve and highlight reconstruction by colour propagation. Most of the purple fringing has been removed.

Highlight detail 200%, processed with “hot matrix” profile. Specular highlights are almost entirely white.

I also tried the RT highlight recovery sliders (Exposure tab) with the “standard matrix”. Strong highlight recovery (amount 100, threshold 100) produced a flat looking image and did not increase highlight detail.

Highlight detail at 100%, processed with “standard matrix” profile, highlight recovery amount 100, highlight recovery threshold 100. The RT highlight recovery tool was not effective in this example

Conclusions

A good strategy for RT is to implement highlight recovery in a matrix transform from device RGB to the profile connection space, like in my “hot matrix” profiles. The Camera standard and DCP profiles in RT appear to use the same trick. With linear transforms, results are colour consistent. The alternative of scaling raw photo RGB channels often comes with unpleasant side-effects when the raw RGB channels are clipped.

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