Negatives are images with reversed lightness and colors, such as those produced by film cameras. RawTherapee (version 4.2 at the time of writing) does not yet have a flawless single-click solution for dealing with them, so this page serves to inform you of the possible workaround solutions:
- Invert a diagonal tone curve either in the Exposure tool, or all of the curves in the RGB Curves tool. The downside is that there are tonal shifts when doing this.
- Use a negative Hald CLUT via the Film Simulation tool. The "RawTherapee Film Simulation Collection" contains one, get it from the Film Simulation page. The downside is that some controls might operate in reverse, such as the Exposure slider, and you may experience clipping in the shadows and/or highlights as these tools are not designed to work with negatives.
- In addition to using the neutral negative Hald CLUT as described above, if you have a successful workflow of not only inverting negatives but also toning them to your liking in RawTherapee or in other software, you could make your own negative Hald CLUT which reproduces the whole look including negative inversion. To do that, apply the same steps to the "identity Hald CLUT image" shipped with the RawTherapee Film Simulation Collection as you would to a photo negative, save it under a new name, then open a photo negative in RawTherapee and apply that new Hald CLUT image. This lets you instantly achieve not only negative inversion but also your own toning with the click of a button, leaving you only needing to expand the histogram by adjusting the Exposure slider or using curves.
- Currently the best method is to use the DCP (DNG Camera Profile) for your camera model but edited in DNG Profile Editor so that the diagonal tone curve is inverted, then manually loading this DCP in RawTherapee for all negative shots. Method outlined below.
Creating a DCP for Negatives
- Get DNG Profile Editor. It runs fine in Linux through wine.
- Convert one of your camera's or scanner's raw photos (it can be the photo of the negative) to DNG by following the guide "How to convert raw formats to DNG".
- Open the DNG image in DNG Profile Editor.
- In the Color Tables tab, see whether the Base Profile called "Adobe Standard (<your camera model>)" is available. If it is, then select it. If it is not, then select "Choose external profile" and find the file titled "<your camera model> Adobe Standard.dcp". The guide How to get LCP and DCP profiles explains how to get them and where to find them.
- In the Tone Curve tab invert the diagonal so that it goes from top-left to bottom-right (move the top point to the bottom).
- Still in the Tone Curve tab, there are three "Base Tone Curves" for you to choose from. "Base Profile" and "Camera Raw Default" are usually identical and have more contrast, while "Linear" makes the image look flat. We recommend you save one DCP which uses "Base Profile" and another which uses "Linear", and see for yourself which one better suits your needs in RawTherapee. Both DCPs will require further image tweaking in RawTherapee, but the "Linear" one will require more tweaking than the "Base Curve" one, though the latter might over-saturate colors - keep an eye on that.
- To create the DCP, click "File > Export <your camera model> profile...". As recommended in the previous step, save two versions.
To use this new DCP for negatives, once you have your negative raw opened in RawTherapee go to the Color tab > Color Management section > Input Profile, and select Custom, then find this new DCP file. Enable "Use DCP's tone curve".
When tweaking images in RawTherapee using these DCPs, remember that using tone curves which operate on RGB channels (Tone Curve 1 and 2 in the Exposure tool) will change not only the lightness but also the color saturation the stronger your curves. You can control color saturation by using the "Weighted Standard" and "Saturation and Value Blending" modes, or you can avoid the problem by working in L*a*b* space. The advantage of working in L*a*b* space as the L* curve does is that colors are not changed while you modify the lightness, so you can fix the lightness first using as strong an L* curve as you need, and then fix the color saturation using for example the CC curve.