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.
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.
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.
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.
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.
Canon Digital Photo Professional (DPP) does a good job of highlight recovery and has likely applied more complex and additional tools.
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.
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 reconstruction by colour propagation recovered most, but not all, of the false magenta highlights.
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.
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.