How to create DCP color profiles

What are DCP profiles and why do I need them?

Technically, each photosite in a digital photography camera's image sensor outputs a certain current based on the number of photons of light that hit that photosite. The current is converted into a number. These numbers, along with some metadata, are stored in what is known as a "raw file". At this point there is no concept of color and the raw data looks nothing like an image. As in traditional photography, the image must be "developed" into a usable form. One of the steps of this development involves translating the numbers into accurate colors, and for that you need to profile the camera, to map the numbers to specific known colors.

Practically, you must use a color profile in order to attain accurate color reproduction, and currently the best way to go about this is using a "DNG camera profile" (DCP for short - note that this is entirely different to the other DCP which is the Digital Cinema Package). RawTherapee ships with profiles generated for D65 daylight color temperatures (derived from Adobe, from dcraw, made in-house, or from you, the dear reader) and these are the basis for developing each raw file you open. Furthermore, you can create your own DCP profiles tailored to specific lighting situations for ultimate color accuracy. Let's say you are shooting a wedding, or panoramas for a virtual tour. You need accurate and consistent colors. The color spectrum of the light outside the building is completely different to that inside, and even inside the light in each room will be different as the general consumer light bulbs produce light of different temperatures and spectrums, and there are usually a bunch of light bulbs illuminating a single room. If you come prepared with a color calibration chart, such as the X-Rite ColorChecker Passport 24-patch chart, all you need to do is to take a photo of this chart in the same locations as your normal photos, one shot per lighting situation (which usually translates to one shot per room and outside), then generate DCP profiles from these shots and use them in RawTherapee to achieve correct and consistent colors and white balance between all your shots.

Find out more:

• QPcard article on ICC and DCP color profiles: http://www.qpcard.com/dcp_icc_profile
• X-Rite ColorChecker Passport product page: http://xritephoto.com/ph_product_overview.aspx?id=1257
• X-Rite ColorChecker Passport PDF manual: http://www.xrite.com/documents/manuals/en/ColorCheckerPassport_User_Manual_en.pdf

Shooting the color chart

The industry-standard X-Rite ColorChecker Passport 24-patch chart, used to profile digital cameras.

Shoot the color target with the sun relative to you in such a way that there is no glare - no direct sunlight bouncing from the target right into your camera. Note that in this incorrectly simplified image the target is not facing the photographer, but in real life you would be standing almost above it and it would face you almost directly.

A clear, non-tinted incandescent tungsten light bulb. Use this type of light bulb for your StdA shot.

This guide assumes you use the X-Rite ColorChecker Passport 24-patch chart.

There are two scenarios when you will want to shoot a chart:

1. To provide RawTherapee with an accurate general-purpose D65 DCP profile for use by everyone with your camera model,
2. To handle specific non-D65 lighting situations for yourself only.

In the former case, you will want to take extra care to make sure your profile sticks to the requirements because many people will use it. In the latter case, you will probably only use that profile on that specific occasion, and it will be of no use to other people.

Shooting the color chart for inclusion in RawTherapee

• We need photos in two light conditions: daylight (properly called "CIE Standard Illuminant D65", or "D65" for short) and tungsten (properly called "CIE Standard Illuminant A", or "StdA" for short).
• Wait for a clear, sunny day when the sun is high above the horizon, near the zenith at midday. You don't want stray colors in the spectrum which appear when the sun is near the horizon.
• Regarding the D65 shot, find a spot far away from anything that could reflect light. Your balcony is not a good spot - the walls will reflect light and negatively influence the light spectrum, even if you can't see that with your bare eyes. The park is not a good spot. A large, empty-ish parking, far away from walls and trees and parked cars, is a great place. Anything other than clear sunlight is bad. Forget about using a flash. Nothing matches real sunlight.
• Regarding the StdA shot, you need a real tungsten incandescent light bulb, the kind that were used everywhere until the world moved to compact fluorescent lamps in the last decade. Don't use fluorescent lights - even if they have a warm color, the light spectrum is absolutely different to tungsten. Don't use halogen lights. Don't use tinted bulbs.
• Position your color target and yourself relative to the source of the light (the Sun for the D65 shot, the light bulb for the StdA shot) in such a way that the light does not bounce directly from the target into your lens causing glare, but also so that it does not come in at an extreme angle.
• Ensure that the face of the target is evenly lit and that the light reflecting from the ground and nearby objects does not fall on it.
• While the sun is far away and you could get away with tilting the chart not perpendicular to the sun when shooting D65, a light bulb will be close to you and due to the inverse-square law it could suffer from uneven lighting. Tilt it towards the light source to guarantee even lighting but in such a way as to avoid glare.
• Remove all filters from your lens.
• Set your lens to f/8, or to whichever aperture value is in your lens's sweet spot - it's usually around f/8. Don't go below f/5.6. This minimizes vignetting.
• Shoot at your camera's lowest base ISO, typically ISO100.
• Shoot raw. Exposure bracketing is acceptable and even recommended - it will help you/us find the best-exposed shot without clipping. Turn off anything which could influence the raw file, such as long exposure noise reduction.
• Make sure that the photos are sharp. The DCP generation software needs to be able to identify dust and scratches to isolate them, so do not shoot out-of-focus or with motion blur.
• Shoot the evenly-exposed chart so that it fills about a third of your frame - not more, not less. The center of the frame has the best optics and lowest vignetting.
• Open a new issue on the RawTherapee GitHub page, upload your D65 and StdA raw photos using Filebin and paste a link to them in your new GitHub issue. We are interested only in the raw photos, not in a ready-made DCP - we will make one ourselves. If you bracketed, you can upload a series of each and we will choose the best ones.

Shooting the color chart in specific lighting situations for yourself

• As above, you will want the chart to fill a third of your frame, f/8, ISO100, sharp, though the points about light do not apply here. If you're shooting at an event at which you will take many photos under the same lighting conditions - business portraits, real estate, a series of images for a 360° panorama in the forest, or a wedding - take a shot of the chart just before (or after) you take your actual photos.

Creating the DCP

There are several ways of creating a DCP. The simplest is using the software that came bundled with the test target. The higher quality and open-source alternative is using DCamProf.

Creating DCP profiles using X-Rite software

Simply install the X-Rite ColorChecker Passport software bundled with your chart (it works in Linux too using wine, just follow the same steps as outlined for installing Adobe DNG Converter in the How to convert raw formats to DNG article) and open your shot in it. It expects the shot to be in the DNG format, so you should first convert it to DNG. Using it is straightforward, it's intuitive.

Creating DCP profiles using DCamProf

DCamProf, written by Anders Torger, is a free and open-source command line tool for generating camera profiles and performing tasks related to camera profiles and profiling. Notably, it can generate profiles in both the ICC and DNG formats, and it is capable of generating high quality, smooth LUT profiles from CC24 shots or other targets.

The program's homepage has a full list of features and extensive, clear documentation. Use that as your primary source of information. This RawPedia article serves merely as a reference, to exemplify the process of compiling the program (not necessary under Windows and macOS) and using it to create a dual-illuminant DCP profile.

DCamProf homepage: http://www.ludd.ltu.se/~torger/dcamprof.html

Windows and macOS users can download a DCamProf binary and run it. Linux users may need to compile it. Before compiling, make sure you have the required dependencies: LCMS2 and libtiff Compiling is as simple as downloading and uncompressing the source code of the latest version of DCamProf, then going into the folder from a terminal and writing "make".

You will also need ArgyllCMS to be installed as we need to use some of its tools. In the example code below, its tools are prepended with "argyll-". This might not be so on your system, e.g. in Gentoo and Sabayon the "scanin" tool executable is called "argyll-scanin" while in Ubuntu its called "scanin". Adjust accordingly.

To generate a dual-illuminant DCP profile from two ColorChecker Passport shots:

A photo of an X-Rite ColorChecker Passport CC24 target is rotated and cropped in RawTherapee ready to be used by DCamProf to generate a DCP.

1. Take two photographs of the color target as described above in the Shooting the color chart section.
2. For each of the two photos, do the following:

   Open the photo in the latest version of RawTherapee,

   Apply the Neutral profile,

   Rotate it so that the brown patch is in the top-left corner and the black one in the bottom-right corner,

   Crop at the corner markers,

   In the Color Management tool click "Save reference image for profiling", and in the window that appears make sure "Apply white balance" is not checked. Save the one as "daylight.tif" and the other as "tungsten.tif" to the same folder which contains the compiled "dcamprof" executable.

3. Go into the folder with the compiled "dcamprof" executable and run:

argyll-scanin -v -dipn tungsten.tif /usr/share/argyllcms/ref/ColorChecker.cht data-examples/cc24_ref.cie tungsten-diag.tif
argyll-scanin -v -dipn daylight.tif /usr/share/argyllcms/ref/ColorChecker.cht data-examples/cc24_ref.cie daylight-diag.tif
./dcamprof make-profile -i StdA tungsten.ti3 tungsten.json
./dcamprof make-profile -i D50 -C daylight.ti3 daylight.json
./dcamprof make-dcp -n "Pentax K10D" -n "Pentax K10D" -t acr -o neutral tungsten.json daylight.json "PENTAX K10D.dcp"

It is very important that during the make-dcp step you specify tungsten.json first and daylight.json second!

In this example, the output file name is PENTAX K10D.dcp. You must use the name exactly as RawTherapee identifies it. Simply open the D65 or StdA photo in RawTherapee and toggle the "Quick info" panel (shortcut key "i") to see which name RawTherapee identifies your camera as.

Your dual-illuminant DCP profile is ready for use.

To have RawTherapee automatically use your new DCP, place the DCP file in the dcpprofiles folder. This folder will typically be inside the RawTherapee installation folder in Windows, or in /usr/share/rawtherapee/dcpprofiles/ in Linux and macOS. Alternatively, create a new default PP3 for raw files which uses your new DCP - see Creating processing profiles for general use.

Per default DCamProf will smooth the LUT to prioritize smoothness over accuracy. This makes the profile more robust for general-purpose use. If you wish you can control all smoothing parameters manually but it's generally not needed. See DCamProf's documentation for further information.

Inclusion of the color matrix in RawTherapee

The Adding Support for New Raw Formats article covers adding support for new raw formats and improving support of existing formats.